MCE Nondestructive Testing Methods

MCE 476 - Nondestructive Testing Methods
Instructor: Dr. Mostafa Ranjbar BSc, MSc, Ph.D. (Dr.-Ing.) from Technical University of Dresden, Germany Nondestructive Testing The field of Nondestructive Testing (NDT) is a very broad, interdisciplinary field that plays a critical role in assuring that structural components and systems perform their function in a reliable and cost effective fashion. NDT technicians and engineers define and implement tests that locate and characterize material conditions and flaws that might otherwise cause planes to crash, reactors to fail, trains to derail, pipelines to burst, and a variety of less visible, but equally troubling events. Theses tests are performed in a manner that does not affect the future usefulness of the object or material. In other words, NDT allows parts and materials to be inspected and measured without damaging them. Because it allows inspection without interfering with a product's final use, NDT provides an excellent balance between quality control and cost-effectiveness. Generally speaking, NDT applies to industrial inspections. While technologies are used in NDT that are similar to those used in the medical industry, typically nonliving objects are the subjects of the inspections.

References: “Introduction to Nondestructive Testing - A Training Guide”, P. E. Mix, 2005, John Wiley & Sons. “Handbook of Nondestructive Evaluation,” Hellier, Chuck, 2001, McGraw-Hill Professional.

Course Outline Week Topic 1 Introduction 2 Failure Detection 3
Selection of the NDT Method 4 Visual Inspection 5 Ultrasonic 6 Eddy Current 7 Magnetic Particle Testing 8 Midterm 9 Thermal Testing 10 Acoustic Emission 11 Optical interferometer 12 Structural Health Monitoring 13 Vibration Analysis 14 Final

Assessment criteria Percentage (%) Midterm exams 30 Homework and Projects 20 Final exam 50

Course Objectives Understanding the basic principles of various NDT methods Fundamentals, importance of NDT, applications, limitations of NDT methods and techniques and codes, standards and specifications related to non-destructive testing technology.

Definition of NDT (NDE)
The use of noninvasive techniques to determine the integrity of a material, component or structure or quantitatively measure some characteristic of an object. Nondestructive Evaluation Nondestructive Evaluation (NDE) is a term that is often used interchangeably with NDT. However, technically, NDE is used to describe measurements that are more quantitative in nature. For example, a NDE method would not only locate a defect, but it would also be used to measure something about that defect such as its size, shape, and orientation. NDE may be used to determine material properties such as fracture toughness, formability, and other physical characteristics. i.e. Inspect or measure without doing harm.

What are Some Uses of NDE Methods?
Flaw Detection and Evaluation Leak Detection Location Determination Dimensional Measurements Structure and Microstructure Characterization Estimation of Mechanical and Physical Properties Stress (Strain) and Dynamic Response Measurements Material Sorting and Chemical Composition Determination Fluorescent penetrant indication

Why Nondestructive? Test piece too precious to be destroyed
Test piece to be reuse after inspection Test piece is in service For quality control purpose Something you simply cannot do harm to, e.g. fetus in mother’s uterus

When are NDE Methods Used?
There are NDE application at almost any stage in the production or life cycle of a component. To assist in product development To screen or sort incoming materials To monitor, improve or control manufacturing processes To verify proper processing such as heat treating To verify proper assembly To inspect for in-service damage

Major types of NDT Detection of surface flaws Visual
Magnetic Particle Inspection Fluorescent Dye Penetrant Inspection Detection of internal flaws Radiography Ultrasonic Testing Eddy current Testing NDT/NDE Methods The number of NDT methods that can be used to inspect components and make measurements is large and continues to grow. Researchers continue to find new ways of applying physics and other scientific disciplines to develop better NDT methods. However, there are six NDT methods that are used most often. These methods are visual inspection, penetrant testing, magnetic particle testing, electromagnetic or eddy current testing, radiography, and ultrasonic testing. These methods and a few others are briefly described below. Background on Nondestructive Testing (NDT) and Nondestructive Evaluation (NDE) Nondestructive testing has been practiced for many decades. One of the earliest applications was the detection of surface cracks in railcar wheels and axles. The parts were dipped in oil, then cleaned and dusted with a powder. When a crack was present, the oil would seep from the defect and wet the oil providing visual indicating that the component was flawed. This eventually led to oils that were specifically formulated for performing these and other inspections and this inspection technique is now called penetrant testing. X-rays were discovered in 1895 by Wilhelm Conrad Roentgen( ) who was a Professor at Wuerzburg University in Germany. Soon after his discovery, Roentgen produced the first industrial radiograph when he imaged a set of weights in a box to show his colleagues. Other electronic inspection techniques such as ultrasonic and eddy current testing started with the initial rapid developments in instrumentation spurred by the technological advances, and subsequent defense and space efforts following World War II. In the early days, the primary purpose was the detection of defects. Critical parts were produced with a "safe life" design, and were intended to be defects during their useful life. The detection of a defects was automatically a cause for removal of the component from service. In the early 1970's, two events occurred which caused a major change in the way inspections were viewed. The continued improvement of inspection technology, in particular the ability to detect smaller and smaller flaws, led to more and more parts being rejected (even though the probability of part failure had not changed). At this time the discipline of fracture mechanics emerged, which enabled one to predict whether a crack of a given size would fail under a particular load if a particular material property or fracture toughness were known. Other laws were developed to predict the rate of growth of cracks under cyclic loading (fatigue). With the advent of these tools, it became possible to accept structures containing defects if the sizes of those defects were known. This formed the basis for a new design philosophy called "damage tolerant designs." Components having known defects could continue to be used as long as it could be established that those defects would not grow to a critical size that would result in catastrophic failure. A new challenge was thus presented to the nondestructive testing community. Mere detection of flaws was not enough. One needed to also obtain quantitative information about flaw size to serve as an input to fracture mechanics calculations to predict the remaining life of a component. These needs, which were particularly strongly in the defense and nuclear power industries, led to the creation of a number of research programs around the world and the emergence of nondestructive evaluation (NDE) as a new discipline.

What is Nondestructive Testing?
Nondestructive Testing (NDT) refers to technology that allows a component to be inspected for serviceability, without impairing its usefulness

Principle Excitation Source Signal / Image Recognition Display Result
Input transducer Measurement transducer Signal / Image Processing

Technologies Artificial Neural Nets Pattern Recognition Fuzzy Logic
Data Fusion Excitation Source Signal / Image Processing Recognition Display Result Input transducer Measurement transducer Hardware - Probe design Instrumentation Control Systems Communications Electromagnetics / mechanics Numerical Modeling Supercomputing Software Development GUIs Computer Graphics Virtual Reality Digital Filters Morphology Data Compression Wavelets

Received Signal / Image
Issues Excitation Source Forward Problem Inverse Problem Received Signal / Image

NDE - A Full Spectrum Technology
Materials Development Design NDE Technologies Processing Manufacturing In-Service Monitoring

Intelligent Synthesis Environment
NASA concept for engineering design of aerospace systems in the 21st century Technology benefit estimator NDE simulation in cost estimating NDE in simulated manufacturing NDE in repair simulation

1. Visual Inspection Most basic and common inspection method.
Portable video inspection unit with zoom allows inspection of large tanks and vessels, railroad tank cars, sewer lines. Most basic and common inspection method. Tools include fiberscopes, borescopes, magnifying glasses and mirrors. Robotic crawlers permit observation in hazardous or tight areas, such as air ducts, reactors, pipelines. Visual and Optical Testing (VT) Visual inspection involves using an inspector's eyes to look for defects. The inspector may also use special tools such as magnifying glasses, mirrors, or borescopes to gain access and more closely inspect the subject area. Visual examiners follow procedures that range from simple to very complex.

2. Magnetic Particle Inspection (MPI)
2.1 Introduction A nondestructive testing method used for defect detection. Fast and relatively easy to apply and part surface preparation is not as critical as for some other NDT methods. – MPI one of the most widely utilized nondestructive testing methods. MPI uses magnetic fields and small magnetic particles, such as iron filings to detect flaws in components. The only requirement from an inspectability standpoint is that the component being inspected must be made of a ferromagnetic material such as iron, nickel, cobalt, or some of their alloys. Ferromagnetic materials are materials that can be magnetized to a level that will allow the inspection to be affective. The method is used to inspect a variety of product forms such as castings, forgings, and weldments. Many different industries use magnetic particle inspection for determining a component's fitness-for-use. Some examples of industries that use magnetic particle inspection are the structural steel, automotive, petrochemical, power generation, and aerospace industries. Underwater inspection is another area where magnetic particle inspection may be used to test such things as offshore structures and underwater pipelines. Magnetic particle inspection is a nondestructive testing method used for defect detection. MPI is a fast and relatively easy to apply and part surface preparation is not as critical as it is for some other NDT methods. These characteristics make MPI one of the most widely utilized nondestructive testing methods. MPI uses magnetic fields and small magnetic particles, such as iron filings to detect flaws in components. The only requirement from an inspectability standpoint is that the component being inspected must be made of a ferromagnetic material such iron, nickel, cobalt, or some of their alloys. Ferromagnetic materials are materials that can be magnetized to a level that will allow the inspection to be affective. The method is used to inspect a variety of product forms such as castings, forgings, and weldments. Many different industries use magnetic particle inspection for determining a component's fitness-for-use. Some examples of industries that use magnetic particle inspection are the structural steel, automotive, petrochemical, power generation, and aerospace industries. Underwater inspection is another area where magnetic particle inspection may be used to test such things as offshore structures and underwater pipelines.

2.2 Basic Principles In theory, magnetic particle inspection (MPI) is a relatively simple concept. It can be considered as a combination of two nondestructive testing methods: magnetic flux leakage testing and visual testing. Consider a bar magnet. It has a magnetic field in and around the magnet. Any place that a magnetic line of force exits or enters the magnet is called a pole. A pole where a magnetic line of force exits the magnet is called a north pole and a pole where a line of force enters the magnet is called a south pole.

Interaction of materials with an external magnetic field
When a material is placed within a magnetic field, the magnetic forces of the material's electrons will be affected. This effect is known as Faraday's Law of Magnetic Induction. However, materials can react quite differently to the presence of an external magnetic field. This reaction is dependent on a number of factors such as the atomic and molecular structure of the material, and the net magnetic field associated with the atoms. The magnetic moments associated with atoms have three origins. These are the electron orbital motion, the change in orbital motion caused by an external magnetic field, and the spin of the electrons.

Diamagnetic, Paramagnetic, and Ferromagnetic Materials
Diamagnetic metals: very weak and negative susceptibility to magnetic fields. Diamagnetic materials are slightly repelled by a magnetic field and the material does not retain the magnetic properties when the external field is removed. Paramagnetic metals: small and positive susceptibility to magnetic fields. These materials are slightly attracted by a magnetic field and the material does not retain the magnetic properties when the external field is removed. Ferromagnetic materials: large and positive susceptibility to an external magnetic field. They exhibit a strong attraction to magnetic fields and are able to retain their magnetic properties after the external field has been removed. Diamagnetic materials are solids with all paired electron and, therefore, no permanent net magnetic moment per atom. Diamagnetic properties arise from the realignment of the electron orbits under the influence of an external magnetic field. Most elements in the periodic table, including copper, silver, and gold, are diamagnetic. Paramagnetic properties are due to the presence of some unpaired electrons and from the realignment of the electron orbits caused by the external magnetic field. Paramagnetic materials include Magnesium, molybdenum, lithium, and tantalum. Ferromagnetic materials have some unpaired electrons so their atoms have a net magnetic moment. They get their strong magnetic properties due to the presence of magnetic domains. In these domains, large numbers of atoms moments (10^12 to 10^15) are aligned parallel so that the magnetic force within the domain is strong. When a ferromagnetic material is in the unmagnitized state, the domains are nearly randomly organized and the net magnetic field for the part as a whole is zero. When a magnetizing force is applied, the domains become aligned to produce a strong magnetic field within the part. Iron, Nickel, and cobalt are examples of ferromagnetic materials. Components with these materials are commonly inspected using the magnetic particle method.

Ferromagnetic materials become magnetized when the magnetic domains within the material are aligned. This can be done by placing the material in a strong external magnetic field or by passes electrical current through the material. Some or all of the domains can become aligned. The more domains that are aligned, the stronger the magnetic field in the material. When all of the domains are aligned, the material is said to be magnetically saturated. When a material is magnetically saturated, no additional amount of external magnetization force will cause an increase in its internal level of magnetization. Unmagnetized material Magnetized material

General Properties of Magnetic Lines of Force
General Properties of Magnetic Lines of Force Follow the path of least resistance between opposite magnetic poles. Never cross one another. All have the same strength. Their density decreases (they spread out) when they move from an area of higher permeability to an area of lower permeability. Their density decreases with increasing distance from the poles. flow from the south pole to the north pole within the material and north pole to south pole in air. As discussed previously a magnetic field is a change in energy within a volume of space. The magnetic field surrounding a bar magnet can be seen in the magnetograph below. A magnetograph can be created by placing a piece of paper over a magnet and sprinkling the paper with iron filings. The particles align themselves with the lines of magnetic force produced by the magnet. The magnetic lines of force show where the magnetic field exits the material at one pole and reenters the material at another pole along the length of the magnet. It should be noted that the magnetic lines of force exist in three-dimensions but are only seen in two dimensions in the image.

When a bar magnet is broken in the center of its length, two complete bar magnets with magnetic poles on each end of each piece will result. If the magnet is just cracked but not broken completely in two, a north and south pole will form at each edge of the crack. The magnetic field exits the north pole and reenters the at the south pole. The magnetic field spreads out when it encounter the small air gap created by the crack because the air can not support as much magnetic field per unit volume as the magnet can. When the field spreads out, it appears to leak out of the material and, thus, it is called a flux leakage field.

If iron particles are sprinkled on a cracked magnet, the particles will be attracted to and cluster not only at the poles at the ends of the magnet but also at the poles at the edges of the crack. This cluster of particles is much easier to see than the actual crack and this is the basis for magnetic particle inspection.

Magnetic Particle Inspection
The magnetic flux line close to the surface of a ferromagnetic material tends to follow the surface profile of the material Discontinuities (cracks or voids) of the material perpendicular to the flux lines cause fringing of the magnetic flux lines, i.e. flux leakage The leakage field can attract other ferromagnetic particles

The magnetic particles form a ridge many times wider than the crack itself, thus making the otherwise invisible crack visible Cracks just below the surface can also be revealed

MPI is not sensitive to shallow and smooth surface defects
The effectiveness of MPI depends strongly on the orientation of the crack related to the flux lines

2.3 Testing Procedure of MPI
Cleaning Demagnetization Contrast dyes (e.g. white paint for dark particles) Magnetizing the object Addition of magnetic particles Illumination during inspection (e.g. UV lamp) Interpretation Demagnetization - prevent accumulation of iron particles or influence to sensitive instruments

Indirect magnetization: using a strong external magnetic field to establish a magnetic field within the component (a) permanent magnets The use of permanent magnets is a low cost method of establishing a magnetic field. However, their use is limited due to lack of control of the field strength and the difficulty of placing and removing strong permanent magnets from the component. Electromagnets in the form of an adjustable horseshoe magnet (called a yoke) eliminate the problems associated with permanent magnets and are used extensively in industry. Electromagnets only exhibit a magnetic flux when electric current is flowing around the soft iron core. When the magnet is placed on the component, a magnetic field is established between the north and south poles of the magnet. Another way of indirectly inducting a magnetic field in a material is by using the magnetic field of a current carrying conductor. A circular magnetic field can be established in cylindrical components by using a central conductors. Typically, one or more cylindrical components are hung from a solid copper bar running through the inside diameter. Current is passed through the copper bar and the resulting circular magnetic field established a magnetic field with the test components. The use of coils and solenoids is a third method of indirect magnetization. When the length of a component is several time larger than its diameter, a longitudinal magnetic field can be established in the component. The component is placed longitudinally in the concentrated magnetic field that fills the center of a coil or solenoid. This magnetization technique is often referred to as a "coil shot." (b) Electromagnets (c) coil shot

Some Standards for MPI Procedure
British Standards BS M.35: Aerospace Series: Magnetic Particle Flaw Detection of Materials and Components BS 4397: Methods for magnetic particle testing of welds ASTM Standards ASTM E : Standard Practice for Magnetic Particle Examination ASTM E : Standard reference photographs for magnetic particle indications on ferrous castings etc….

2.4 Advantages of MPI One of the most dependable and sensitive methods for surface defects fast, simple and inexpensive direct, visible indication on surface unaffected by possible deposits, e.g. oil, grease or other metals chips, in the cracks can be used on painted objects surface preparation not required results readily documented with photo or tape impression

2.5 Limitations of MPI Only good for ferromagnetic materials
sub-surface defects will not always be indicated relative direction between the magnetic field and the defect line is important objects must be demagnetized before and after the examination the current magnetization may cause burn scars on the item examined

Examples of visible dry magnetic particle indications
Examples of visible dry magnetic particle indications Indication of a crack in a saw blade Indication of cracks in a weldment One of the advantages that a magnetic particle inspection has over some of the other nondestructive evaluation methods is that flaw indications generally resemble the actual flaw. This is not the case with NDT methods such as ultrasonic and eddy current inspection, where an electronic signal must be interpreted. When magnetic particle inspection is used, cracks on the surface of the part appear as sharp lines that follow the path of the crack. Flaws that exist below the surface of the part are less defined and more difficult to detect. Below are some examples of magnetic particle indications produced using dry particles. Indication of cracks running between attachment holes in a hinge Before and after inspection pictures of cracks emanating from a hole

Examples of Fluorescent Wet Magnetic Particle Indications
Magnetic particle wet fluorescent indication of a cracks in a drive shaft Magnetic particle wet fluorescent indication of a crack in a bearing The indications produced using the wet magnetic particles are more sharp than dry particle indications formed on similar defects. When fluorescent particles are used, the visibility of the indications is greatly improved because the eye is drawn to the "glowing" regions in the dark setting. Below are a few examples of fluorescent wet magnetic particle indications. Magnetic particle wet fluorescent indication of a cracks at a fastener hole

3. Dye Penetrant Inspection
Liquid penetrant inspection (LPI) is one of the most widely used nondestructive evaluation (NDE) methods. Its popularity can be attributed to two main factors, which are its relative ease of use and its flexibility. LPI can be used to inspect almost any material provided that its surface is not extremely rough or porous. Materials that are commonly inspected using LPI include metals (aluminum, copper, steel, titanium, etc.), glass, many ceramic materials, rubber, and plastics. LPI offers flexibility in performing inspections because it can be applied in a large variety of applications ranging from automotive spark plugs to critical aircraft components. Penetrant material can be applied with a spray can or a cotton swab to inspect for flaws known to occur in a specific area or it can be applied by dipping or spraying to quickly inspect large areas. At right, visible dye penetrant being locally applied to a highly loaded connecting point to check for fatigue cracking.

3.1 Introduction Liquid penetration inspection is a method that is used to reveal surface breaking flaws by bleedout of a colored or fluorescent dye from the flaw. The technique is based on the ability of a liquid to be drawn into a "clean" surface breaking flaw by capillary action. After a period of time called the "dwell," excess surface penetrant is removed and a developer applied. This acts as a "blotter." It draws the penetrant from the flaw to reveal its presence. Colored (contrast) penetrants require good white light while fluorescent penetrants need to be used in darkened conditions with an ultraviolet "black light". Unlike MPI, this method can be used in non-ferromagnetic materials and even non-metals Modern methods can reveal cracks 2m wide Standard: ASTM E Liquid Penetrant Inspection Method Liquid penetration inspection is a method that is used to reveal surface breaking flaws by bleedout of a colored or fluorescent dye from the flaw. The technique is based on the ability of a liquid to be drawn into a "clean" surface breaking flaw by capillary action. After a period of time called the "dwell," excess surface penetrant is removed and a developer applied. This acts as a "blotter." It draws the penetrant from the flaw to reveal its presence. Colored (contrast) penetrants require good white light while fluorescent penetrants need to be used in darkened conditions with an ultraviolet "black light".

Why Liquid Penetrant Inspection?
To improves the detectability of flaws There are basically two ways that a penetrant inspection process makes flaws more easily seen. LPI produces a flaw indication that is much larger and easier for the eye to detect than the flaw itself. LPI produces a flaw indication with a high level of contrast between the indication and the background. The advantage that a liquid penetrant inspection (LPI) offers over an unaided visual inspection is that it makes defects easier to see for the inspector. There are basically two ways that a penetrant inspection process makes flaws more easily seen. First, LPI produces a flaw indication that is much larger and easier for the eye to detect than the flaw itself. Many flaws are so small or narrow that they are undetectable by the unaided eye. Due to the physical features of the eye, there is a threshold below which objects cannot be resolved. This threshold of visual acuity is around inch for a person with 20/20 vision. The second way that LPI improves the detectability of a flaw is that it produces a flaw indication with a high level of contrast between the indication and the background which also helps to make the indication more easily seen. When a visible dye penetrant inspection is performed, the penetrant materials are formulated using a bright red dye that provides for a high level of contrast between the white developer that serves as a background as well as to pull the trapped penetrant from the flaw. When a fluorescent penetrant inspection is performed, the penetrant materials are formulated to glow brightly and to give off light at a wavelength that the eye is most sensitive to under dim lighting conditions. The advantage that a liquid penetrant inspection (LPI) offers over an unaided visual inspection is that it makes defects easier to see for the inspector.

3.2 Basic processing steps of LPI
Surface Preparation: One of the most critical steps of a liquid penetrant inspection is the surface preparation. The surface must be free of oil, grease, water, or other contaminants that may prevent penetrant from entering flaws. The sample may also require etching if mechanical operations such as machining, sanding, or grit blasting have been performed. These and other mechanical operations can smear the surface of the sample, thus closing the defects. Penetrant Application: Once the surface has been thoroughly cleaned and dried, the penetrant material is applied by spraying, brushing, or immersing the parts in a penetrant bath. Penetrant Dwell: The penetrant is left on the surface for a sufficient time to allow as much penetrant as possible to be drawn from or to seep into a defect. The times vary depending on the application, penetrant materials used, the material, the form of the material being inspected, and the type of defect being inspected. Generally, there is no harm in using a longer penetrant dwell time as long as the penetrant is not allowed to dry. Surface Preparation: One of the most critical steps of a liquid penetrant inspection is the surface preparation. The surface must be free of oil, grease, water, or other contaminants that may prevent penetrant from entering flaws. The sample may also require etching if mechanical operations such as machining, sanding, or grit blasting have been performed. These and other mechanical operations can smear the surface of the sample, thus closing the defects. Penetrant Application: Once the surface has been thoroughly cleaned and dried, the penetrant material is applied by spraying, brushing, or immersing the parts in a penetrant bath. Penetrant Dwell: The penetrant is left on the surface for a sufficient time to allow as much penetrant as possible to be drawn from or to seep into a defect. Penetrant dwell time is the total time that the penetrant is in contact with the part surface. Dwell times are usually recommended by the penetrant producers or required by the specification being followed. The times vary depending on the application, penetrant materials used, the material, the form of the material being inspected, and the type of defect being inspected. Minimum dwell times typically range from 5 to 60 minutes. Generally, there is no harm in using a longer penetrant dwell time as long as the penetrant is not allowed to dry. The ideal dwell time is often determined by experimentation and is often very specific to a particular application. Excess Penetrant Removal: This is the most delicate part of the inspection procedure because the excess penetrant must be removed from the surface of the sample while removing as little penetrant as possible from defects. Depending on the penetrant system used, this step may involve cleaning with a solvent, direct rinsing with water, or first treated with an emulsifier and then rinsing with water. Developer Application: A thin layer of developer is then applied to the sample to draw penetrant trapped in flaws back to the surface where it will be visible. Developers come in a variety of forms that may be applied by dusting (dry powdered), dipping, or spraying (wet developers). Indication Development: The developer is allowed to stand on the part surface for a period of time sufficient to permit the extraction of the trapped penetrant out of any surface flaws. This development time is usually a minimum of 10 minutes and significantly longer times may be necessary for tight cracks. Inspection: Inspection is then performed under appropriate lighting to detect indications from any flaws which may be present. Clean Surface: The final step in the process is to thoroughly clean the part surface to remove the developer from the parts that were found to be acceptable.

Excess Penetrant Removal: This is the most delicate part of the inspection procedure because the excess penetrant must be removed from the surface of the sample while removing as little penetrant as possible from defects. Depending on the penetrant system used, this step may involve cleaning with a solvent, direct rinsing with water, or first treated with an emulsifier and then rinsing with water. Developer Application: A thin layer of developer is then applied to the sample to draw penetrant trapped in flaws back to the surface where it will be visible. Developers come in a variety of forms that may be applied by dusting (dry powdered), dipping, or spraying (wet developers). Indication Development: The developer is allowed to stand on the part surface for a period of time sufficient to permit the extraction of the trapped penetrant out of any surface flaws. This development time is usually a minimum of 10 minutes and significantly longer times may be necessary for tight cracks.

Inspection: Inspection is then performed under appropriate lighting to detect indications from any flaws which may be present. Clean Surface: The final step in the process is to thoroughly clean the part surface to remove the developer from the parts that were found to be acceptable.

Penetrant testing materials
A penetrant must possess a number of important characteristics. A penetrant must spread easily over the surface of the material being inspected to provide complete and even coverage. be drawn into surface breaking defects by capillary action. remain in the defect but remove easily from the surface of the part. remain fluid so it can be drawn back to the surface of the part through the drying and developing steps. be highly visible or fluoresce brightly to produce easy to see indications. must not be harmful to the material being tested or the inspector.

Penetrant Types Dye penetrants Fluorescent penetrants
The liquids are coloured so that they provide good contrast against the developer Usually red liquid against white developer Observation performed in ordinary daylight or good indoor illumination Fluorescent penetrants Liquid contain additives to give fluorescence under UV Object should be shielded from visible light during inspection Fluorescent indications are easy to see in the dark Standard: Aerospace Material Specification (AMS) 2644.

Further classification
According to the method used to remove the excess penetrant from the part, the penetrants can be classified into: Method A - Water Washable Method B - Post Emulsifiable, Lipophilic Method C - Solvent Removable Method D - Post Emulsifiable, Hydrophilic Based on the strength or detectability of the indication that is produced for a number of very small and tight fatigue cracks, penetrants can be classified into five sensitivity levels are shown below: Level ½ - Ultra Low Sensitivity Level 1 - Low Sensitivity Level 2 - Medium Sensitivity Level 3 - High Sensitivity Level 4 - Ultra-High Sensitivity

Emulsifiers When removal of the penetrant from the defect due to over-washing of the part is a concern, a post emulsifiable penetrant system can be used. Post emulsifiable penetrants require a separate emulsifier to break the penetrant down and make it water washable. Method B - Lipophilic Emulsifier, Method D - Hydrophilic Emulsifier Lipophilic emulsification systems are oil-based materials that are supplied in ready-to-use form. Hydrophilic systems are water-based and supplied as a concentrate that must be diluted with water prior to use .

Developer The role of the developer is to pull the trapped penetrant material out of defects and to spread the developer out on the surface of the part so it can be seen by an inspector. The fine developer particles both reflect and refract the incident ultraviolet light, allowing more of it to interact with the penetrant, causing more efficient fluorescence. The developer also allows more light to be emitted through the same mechanism. This is why indications are brighter than the penetrant itself under UV light. Another function that some developers performs is to create a white background so there is a greater degree of contrast between the indication and the surrounding background.

Developer Types Dry powder developer –the least sensitive but inexpensive Water soluble – consist of a group of chemicals that are dissolved in water and form a developer layer when the water is evaporated away. Water suspendible – consist of insoluble developer particles suspended in water. Nonaqueous – suspend the developer in a volatile solvent and are typically applied with a spray gun. Using dye and developer from different manufacturers should be avoided.

3.3 Finding Leaks with Dye Penetrant

3.4 Primary Advantages The method has high sensitive to small surface discontinuities. The method has few material limitations, i.e. metallic and nonmetallic, magnetic and nonmagnetic, and conductive and nonconductive materials may be inspected. Large areas and large volumes of parts/materials can be inspected rapidly and at low cost. Parts with complex geometric shapes are routinely inspected. Indications are produced directly on the surface of the part and constitute a visual representation of the flaw. Aerosol spray cans make penetrant materials very portable. Penetrant materials and associated equipment are relatively inexpensive.

3.5 Primary Disadvantages
Only surface breaking defects can be detected. Only materials with a relative nonporous surface can be inspected. Precleaning is critical as contaminants can mask defects. Metal smearing from machining, grinding, and grit or vapor blasting must be removed prior to LPI. The inspector must have direct access to the surface being inspected. Surface finish and roughness can affect inspection sensitivity. Multiple process operations must be performed and controlled. Post cleaning of acceptable parts or materials is required. Chemical handling and proper disposal is required.

4. Radiography Radiography involves the use of penetrating gamma- or X-radiation to examine material's and product's defects and internal features. An X-ray machine or radioactive isotope is used as a source of radiation. Radiation is directed through a part and onto film or other media. The resulting shadowgraph shows the internal features and soundness of the part. Material thickness and density changes are indicated as lighter or darker areas on the film. The darker areas in the radiograph below represent internal voids in the component. High Electrical Potential Electrons - + X-ray Generator or Radioactive Source Creates Radiation Exposure Recording Device Radiation Penetrate the Sample

Properties and Generation of X-ray
4.1 Radiation sources X-rays or gamma radiation is used 4.1.1 x-ray source Properties and Generation of X-ray X-rays are electromagnetic radiation with very short wavelength ( m) The energy of the x-ray can be calculated with the equation E = h = hc/ e.g. the x-ray photon with wavelength 1Å has energy 12.5 keV

Production of X-rays X-rays are produced whenever high-speed electrons
collide with a metal target. A source of electrons – hot W filament, a high accelerating voltage (30-50kV) between the cathode (W) and the anode and a metal target. The anode is a water-cooled block of Cu containing desired target metal. target X-rays W Vacuum

X-ray Spectrum A spectrum of x-ray is produced as a result of the interaction between the incoming electrons and the inner shell electrons of the target element. Two components of the spectrum can be identified, namely, the continuous spectrum and the characteristic spectrum. I k characteristic radiation continuous radiation k  SWL - short-wavelength limit

Fast moving e- will then be deflected or decelerated and EM radiation will be emitted.
The energy of the radiation depends on the severity of the deceleration, which is more or less random, and thus has a continuous distribution. These radiation is called white radiation or bremsstrahlung (German word for ‘braking radiation’). If an incoming electron has sufficient kinetic energy for knocking out an electron of the K shell (the inner-most shell), it may excite the atom to an high-energy state (K state). One of the outer electron falls into the K-shell vacancy, emitting the excess energy as a x-ray photon -- K-shell emission Radiation.

Absorption of x-ray The absorption follows the equation
All x-rays are absorbed to some extent in passing through matter due to electron ejection or scattering. The absorption follows the equation where I is the transmitted intensity; x is the thickness of the matter;  is the linear absorption coefficient (element dependent);  is the density of the matter; (/) is the mass absorption coefficient (cm2/gm). I0 I ,  x

4.1.2 Radio Isotope (Gamma) Sources
Emitted gamma radiation is one of the three types of natural radioactivity. It is the most energetic form of electromagnetic radiation, with a very short wavelength of less than one-tenth of a nano-meter. Gamma rays are essentially very energetic x-rays emitted by excited nuclei. They often accompany alpha or beta particles, because a nucleus emitting those particles may be left in an excited (higher-energy) state. Man made sources are produced by introducing an extra neutron to atoms of the source material. As the material rids itself of the neutron, energy is released in the form of gamma rays. Two of the more common industrial Gamma-ray sources are Iridium-192 and Colbalt-60. These isotopes emit radiation in two or three discreet wavelengths. Cobalt 60 will emit a 1.33 and a 1.17 MeV gamma ray, and iridium-192 will emit 0.31, 0.47, and 0.60 MeV gamma rays. Advantages of gamma ray sources include portability and the ability to penetrate thick materials in a relativity short time. Disadvantages include shielding requirements and safety considerations. Advantages of gamma ray sources include portability and the ability to penetrate thick materials in a relativity short time. As can be noted above cobalt will produce energies comparable to a 1.25 MeV x-ray system. Iridium will produce energies comparable to a 460 kV x-ray system. Not requiring electrical sources the gamma radiography is well adapted for use in remote locations. Disadvantages include shielding requirements and safety considerations. Depleted uranium is used as a shielding material for sources. The storage container (camera) for iridium sources will contain 45 pounds of shielding materials. Cobalt will require 500 pounds of shielding. Cobalt cameras are often fixed to a trailer and transported to and from inspection sites. Iridium is used whenever possible, and not all companies using source material will have a cobalt source. Source materials are constantly generating very penetrating radiation and in a short time considerable damage can be done to living tissue. Technicians must be trained in potential hazards to themselves and the public associated with use of gamma radiography.

4.2 Film Radiography The part is placed between the radiation source and a piece of film. The part will stop some of the radiation. Thicker and more dense area will stop more of the radiation. The film darkness (density) will vary with the amount of radiation reaching the film through the test object. Defects, such as voids, cracks, inclusions, etc., can be detected. X-ray film The most common detector used in industrial radiography is film. The high sensitivity to ionizing radiation provides excellent detail and sensitivity to density changes when producing images of industrial materials. = less exposure = more exposure Top view of developed film

Contrast and Definition
The first subjective criteria for determining radiographic quality is radiographic contrast. Essentially, radiographic contrast is the degree of density difference between adjacent areas on a radiograph. It is essential that sufficient contrast exist between the defect of interest and the surrounding area. There is no viewing technique that can extract information that does not already exist in the original radiograph low kilovoltage high kilovoltage

Definition Radiographic definition is the abruptness of change in going from one density to another. In the example to the left, a two-step step tablet with the transition from step to step represented by Line BC is quite sharp or abrupt. Translated into a radiograph, we see that the transition from the high density to the low density is abrupt. The Edge Line BC is still a vertical line quite similar to the step tablet itself. We can say that the detail portrayed in the radiograph is equivalent to physical change present in the step tablet. Hence, we can say that the imaging system produced a faithful visual reproduction of the step table. It produced essentially all of the information present in the step tablet on the radiograph. In the example on the right, the same two-step step tablet has been radiographed. However, here we note that, for some reason, the imaging system did not produce a faithful visual reproduction. The Edge Line BC on the step tablet is not vertical. This is evidenced by the gradual transition between the high and low density areas on the radiograph. The edge definition or detail is not present because of certain factors or conditions which exist in the imaging system. good poor High definition: the detail portrayed in the radiograph is equivalent to physical change present in the part. Hence, the imaging system produced a faithful visual reproduction.

4.3 Areas of Application Can be used in any situation when one wishes to view the interior of an object To check for internal faults and construction defects, e.g. faulty welding To ‘see’ through what is inside an object To perform measurements of size, e.g. thickness measurements of pipes Standard: ASTM ASTM E94-84a Radiographic Testing ASTM E Radiographic Examination of Weldments ASTM E Radiographic Testing of Metallic Castings

Radiographic Images

4.4 Limitations of Radiography
There is an upper limit of thickness through which the radiation can penetrate, e.g. -ray from Co-60 can penetrate up to 150mm of steel The operator must have access to both sides of an object Highly skilled operator is required because of the potential health hazard of the energetic radiations Relative expensive equipment

4.5 Examples of radiographs
Cracking can be detected in a radiograph only the crack is propagating in a direction that produced a change in thickness that is parallel to the x-ray beam. Cracks will appear as jagged and often very faint irregular lines. Cracks can sometimes appearing as "tails" on inclusions or porosity.

Burn through (icicles) results when too much heat causes excessive weld metal to penetrate the weld zone. Lumps of metal sag through the weld creating a thick globular condition on the back of the weld. On a radiograph, burn through appears as dark spots surrounded by light globular areas.

Gas porosity or blow holes are caused by accumulated gas or air which is trapped by the metal. These discontinuities are usually smooth-walled rounded cavities of a spherical, elongated or flattened shape. Sand inclusions and dross are nonmetallic oxides, appearing on the radiograph as irregular, dark blotches. Gas porosity or blow holes are caused by accumulated gas or air which is trapped by the metal. These discontinuities are usually smooth-walled rounded cavities of a spherical, elongated or flattened shape. If the sprue is not high enough to provide the necessary heat transfer needed to force the gas or air out of the mold, the gas or air will be trapped as the molten metal begins to solidify. Blows can also be caused by sand that is too fine, too wet, or by sand that has a low permeability so that gas can't escape. Too high a moisture content in the sand makes it difficult to carry the excessive volumes of water vapor away from the casting. Another cause of blows can be attributed to using green ladles, rusty or damp chills and chaplets.

5. Ultrasonic Testing 5.1 Introduction
In ultrasonic testing, high-frequency sound waves are transmitted into a material to detect imperfections or to locate changes in material properties. The most commonly used ultrasonic testing technique is pulse echo, whereby sound is introduced into a test object and reflections (echoes) from internal imperfections or the part's geometrical surfaces are returned to a receiver. The time interval between the transmission and reception of pulses give clues to the internal structure of the material. Below is an example of shear wave weld inspection. Notice the indication extending to the upper limits of the screen. This indication is produced by sound reflected from a defect within the weld.

Ultrasonic Inspection (Pulse-Echo)
High frequency sound waves are introduced into a material and they are reflected back from surfaces or flaws. Reflected sound energy is displayed versus time, and inspector can visualize a cross section of the specimen showing the depth of features that reflect sound. f 2 4 6 8 10 initial pulse back surface echo crack echo crack plate Oscilloscope, or flaw detector screen

Generation of Ultrasonic Waves
Piezoelectric transducers are used for converting electrical pulses to mechanical vibrations and vice versa Commonly used piezoelectric materials are quartz, Li2SO4, and polarized ceramics such as BaTiO3 and PbZrO3. Usually the transducers generate ultrasonic waves with frequencies in the range 2.25 to 5.0 MHz

Ultrasonic Wave Propagation
Wave Propagation Direction Longitudinal or compression waves Shear or transverse waves Surface or Rayleigh waves Plate or Lamb waves Symmetrical Asymmetrical In solids, sound waves can propagate in four principle modes that are based on the way the particles oscillate. Sound can propagate as longitudinal waves, shear waves, surface waves, and in thin materials as plate waves. Longitudinal and shear waves are the two modes of propagation most widely used in ultrasonic testing. The particle movement responsible for the propagation of longitudinal and shear waves is illustrated below. Different types of waves As the reed vibrates back and forth, the sound waves produced move the same direction (left and right). Waves that move in the same direction, or are parallel to their source are called longitudinal waves. Longitudinal sound waves are the easiest to produce and have the highest speed, however, it is possible to produce other types. Waves which move perpendicular to the way their source does are called shear waves. Shear waves travel at slower speeds than longitudinal waves, and can only be made in solids. Another type of wave is the surface wave. Surface waves travel at the surface of a material and move in elliptical orbits. They are slightly slower than shear waves but difficult to make. A final type of sound wave is the plate wave. These waves also move in elliptical orbits but are much more complex. They can only be created in very thin pieces of material.

Longitudinal waves Shear waves Similar to audible sound waves
Longitudinal waves Similar to audible sound waves the only type of wave which can travel through liquid Shear waves generated by passing the ultrasonic beam through the material at an angle Usually a plastic wedge is used to couple the transducer to the material In longitudinal waves, the oscillations occur in the longitudinal direction or the direction of wave propagation. Since compressional and dilational forces are active in these waves, they are also called pressure or compressional waves. They are also sometimes called density waves because their particle density fluctuates as they move. Compression waves can be generated in liquids, as well as solids because the energy travels through the atomic structure by a series of comparison and expansion (rarefaction) movements. In the transverse or shear wave, the particles oscillate at a right angle or transverse to the direction of propagation. Shear waves require an acoustically solid material for effective propagation and, therefore, are not effectively propagated in materials such as liquids or gasses. Shear waves are relatively weak when compared to longitudinal waves In fact, shear waves are usually generated in materials using some of the energy from longitudinal waves.

Surface waves Lamb waves
travel with little attenuation in the direction of propagation but weaken rapidly as the wave penetrates below the material surface particle displacement follows an elliptical orbit Lamb waves observed in relatively thin plates only velocity depends on the thickness of the material and frequency Rayleigh waves travel the surface of a relative thick solid material penetrating to a depth of one wavelength. Rayleigh waves are useful because they are very sensitive to surface defects and since they will follow the surface around, curves can also be used to inspect areas that other waves might have difficulty reaching. Lamb waves, also known as plate waves, can be propagated only in very thin metals. Lamb waves are a complex vibrational wave that travels through the entire thickness of a material. Lamb waves provide a means for inspection of very thin materials. Propagation of Lamb waves depends on density, elastic, and material properties of a component, and they are influenced by a great deal by selected frequency and material thickness.

5.2 Equipment & Transducers
5.2 Equipment & Transducers 5.2.1 Piezoelectric Transducers The active element of most acoustic transducers is piezoelectric ceramic. This ceramic is the heart of the transducer which converts electrical to acoustic energy, and vice versa. A thin wafer vibrates with a wavelength that is twice its thickness, therefore, piezoelectric crystals are cut to a thickness that is 1/2 the desired radiated wavelength. Optimal impedance matching is achieved by a matching layer with thickness 1/4 wavelength. When piezoelectric ceramics were introduced they soon became the dominant material for transducers due to their good piezoelectric properties and their ease of manufacture into a variety of shapes and sizes. The first piezoceramic in general use was barium titanate, and that was followed during the 1960's by lead zirconate titanate compositions, which are now the most commonly employed ceramic for making transducers. In selecting a transducer the piezoelectric material is always a consideration as some materials are more efficient transmitters and some are more efficient receivers. Understanding the internal structure of the material to be inspected, as well as type, size, and probable location of defects is helpful when selecting a transducer. A transducer that performs well in one application will not always produce similar results when material properties change. For example, sensitivity to small defects is proportional to the product of the efficiency of the transducer as a transmitter and a receiver. Resolution, the ability to locate defects near surface or in close proximity in the material, requires a highly damped transducer. The backing material supporting the crystal has a great influence on damping characteristics of a transducer. Using a backing material with an impedance similar to that of the crystal will produce the most effective damping. Such a transducer will have a narrow bandwidth resulting in higher sensitivity. As the mismatch in impedance between crystal and backing material increases, transducer sensitivity is reduced and material penetration increased. It is of importance to understand the concept of bandwidth, or range of frequencies, associated with a transducer. The frequency noted on a transducer is the central or center frequency and depends primarily on the backing material. Highly damped transducers will respond to frequencies above and below the central frequency. The broad frequency range provides a transducer with high resolving power. Less damped transducers will exhibit a narrower frequency range, poorer resolving power, but greater penetration. The central frequency will also define capabilities of a transducers. Lower frequencies (0.5Mhz-2.25Mhz) provide greater energy and penetration in a material, while high frequency crystals (15.0Mhz-25.0Mhz) provide reduced penetration but greater sensitivity to small discontinuities. Direction of wave propagation

Characteristics of Piezoelectric Transducers
Transducers are classified into groups according to the application. Contact: are used for direct contact inspections. Coupling materials of water, grease, oils, or commercial materials are used to smooth rough surfaces and prevent an air gap between the transducer and the component inspected. Contact type Immersion: do not contact the component. These transducers are designed to operate in a liquid environment and all connections are watertight. Wheel and squirter transducers are examples of such immersion applications. Many factors, including material, mechanical and electrical construction, and the external mechanical and electrical load conditions, influence the behavior a transducer. Mechanical construction is the factor that influences performance, with important parameters such as radiation surface area, mechanical damping, housing, and other variables of physical construction. As of this writing, transducer manufactures are hard pressed when constructing two transducers that have identical performance characteristics. Transducer manufacture still has something of a "black art" component. Contact Transducers have elements protected in a rugged casing to withstand direct contact with a variety of materials. These transducers have an ergonomic design so that they are easy to grip and move along a surface. They also often have replaceable wear plates to lengthen their useful life. Immersion Transducers are designed to transmit ultrasound in situations where the test part is immersed in water. Immersion transducers are typically used inside a water tank or as part of a squirter or bubbler system in scanning applications. Immersion transducers usually have a impedance matching layer that helps to get more sound energy into the water and, in turn, into the component being inspected. Immersion transducers can be purchased with in a planner, cylindrically focused or spherically focused lens. A focused transducer can improve sensitivity and axial resolution by concentrating the sound energy to a smaller area. immersion

Dual element Angle beam
Dual Element: contain two independently operating elements in a single housing. One of the elements transmits and the other receives. Dual element transducers are very useful when making thickness measurements of thin materials and when inspecting for near surface defects. Dual element Angle Beam: and wedges are typically used to introduce a refracted shear wave into the test material. Transducers can be purchased in a variety of fixed angles or in adjustable versions where the user determines the angles of incident and refraction. They are used to generate surface waves for use in detecting defects on the surface of a component. Dual Element Transducers contain two independently operating elements in a single housing. One of the elements transmits and the other receives. Dual element transducers are especially well suited for making measurements in applications where reflectors are very near the transducer since this design eliminates the ring down effect that single-element transducers experience. (When single-element transducers are operating in pulse echo mode, the element can not start receiving reflected signals until the element has stopped ringing from it transmit function.) Dual element transducers are very useful when making thickness measurements of thin materials and when inspecting for near surface defects. The two elements are angled towards each other to create a crossed-beam sound path in the test material. Angle Beam Transducers and wedges are typically used to introduce a refracted shear wave into the test material. Transducers can be purchased in a variety of fixed angles or in adjustable versions where the user determines the angles of incident and refraction. In the fixed angle versions, the angle of refraction that is marked on the transducer is only accurate for a particular material, which is usually steel. The angled sound path allows the sound beam to be reflected from the back wall to improve detectability of flaws in and around welded areas. They are also used to generate surface waves for use in detecting defects on the surface of a component. Normal Incidence Shear Wave Transducers are unique because they allow introduction of shear waves directly into a test piece without the use of an angle beam wedge. Careful design has enabled manufacturing of transducers with minimal longitudinal wave contamination. The ratio of the longitudinal to shear wave components is generally below -30dB. Delay Line Transducers provide versatility with a variety of replaceable options. Removable delay line, surface conforming membrane, and protective wear cap options can make a single transducer effective for a wide range of applications. As the name implies, the primary function of a delay line transducer is to introduce a time delay between the generation of the sound wave and the arrival of any reflected waves. This allows the transducer to complete its "sending" function before it starts it "listening" function. Delay line transducers are recommended for applications that require a contact transducer with good near surface resolution. They are designed for use in applications such as high precision thickness gauging of thin materials and delamination checks in composite materials. They are also useful in high-temperature measurement applications since the delay line provides some insulation to the piezoelectric element from the heat. High Frequency Transducers, when used with the proper instrumentation, can improve flaw resolution and thickness measurement capabilities dramatically. Broadband transducers with frequencies between 20 MHz and 150 MHz are commercially available. Angle beam

5.2.2 Electromagnetic Acoustic Transducers (EMATs)
When a wire is placed near the surface of an electrically conducting object and is driven by a current at the desired ultrasonic frequency, eddy currents will be induced in a near surface region of the object. If a static magnetic field is also present, these eddy currents will experience Lorentz forces of the form F = J x B F is a body force per unit volume, J is the induced dynamic current density, and B is the static magnetic induction. EMAT: Couplant free transduction allows operation without contact at elevated temperatures and in remote locations. The coil and magnet structure can also be designed to excite complex wave patterns and polarization's that would be difficult to realize with fluid coupled piezoelectric probes (Lamb and Shear waves). In the inference of material properties from precise velocity or attenuation measurements, use of EMATs can eliminate errors associated with couplant variation, particularly in contact measurements.

5.3 Ultrasonic Test Methods
Fluid couplant or a fluid bath is needed for effective transmission of ultrasonic from the transducer to the material Straight beam contact search unit project a beam of ultrasonic vibrations perpendicular to the surface Angle beam contact units send ultrasonic beam into the test material at a predetermined angle to the surface

5.3.1Normal Beam Inspection
Pulse-echo ultrasonic measurements can determine the location of a discontinuity in a part or structure by accurately measuring the time required for a short ultrasonic pulse generated by a transducer to travel through a thickness of material, reflect from the back or the surface of a discontinuity, and be returned to the transducer. In most applications, this time interval is a few microseconds or less. d = vt/2 or v = 2d/t where d is the distance from the surface to the discontinuity in the test piece, v is the velocity of sound waves in the material, and t is the measured round-trip transit time.

5.3.2 Angles beam inspection
Angle Beam Transducers and wedges are typically used to introduce a refracted shear wave into the test material. An angled sound path allows the sound beam to come in from the side, thereby improving detectability of flaws in and around welded areas. Can be used for testing flat sheet and plate or pipe and tubing Angle beam units are designed to induce vibrations in Lamb, longitudinal, and shear wave modes

The geometry of the sample below allows the sound beam to be reflected from the back wall to improve detectability of flaws in and around welded areas.

Crack Tip Diffraction When the geometry of the part is relatively uncomplicated and the orientation of a flaw is well known, the length (a) of a crack can be determined by a technique known as tip diffraction. One common application of the tip diffraction technique is to determine the length of a crack originating from on the backside of a flat plate. When an angle beam transducer is scanned over the area of the flaw, the principle echo comes from the base of the crack to locate the position of the flaw (Image 1). A second, much weaker echo comes from the tip of the crack and since the distance traveled by the ultrasound is less, the second signal appears earlier in time on the scope (Image 2).

Crack height (a) is a function of the ultrasound velocity (v) in the material, the incident angle (2) and the difference in arrival times between the two signal (dt). The variable dt is really the difference in time but can easily be converted to a distance by dividing the time in half (to get the one-way travel time) and multiplying this value by the velocity of the sound in the material. Using trigonometry an equation for estimating crack height from these variables can be derived.

Surface Wave Contact Units
With increased incident angle so that the refracted angle is 90° Surface waves are influenced most by defects close to the surface Will travel along gradual curves with little or no reflection from the curve

5.4 Data Presentation Ultrasonic data can be collected and displayed in a number of different formats. The three most common formats are know in the NDT world as A-scan, B-scan and C-scan presentations. Each presentation mode provides a different way of looking at and evaluating the region of material being inspected. Modern computerized ultrasonic scanning systems can display data in all three presentation forms simultaneously

5.4.1 A-Scan The A-scan presentation displays the amount of received ultrasonic energy as a function of time. The relative amount of received energy is plotted along the vertical axis and elapsed time (which may be related to the sound energy travel time within the material) is display along the horizontal axis. Relative discontinuity size can be estimated by comparing the signal amplitude obtained from an unknown reflector to that from a known reflector. Reflector depth can be determined by the position of the signal on the horizontal sweep. In the illustration of the A-scan presentation to the right, the initial pulse generated by the transducer is represented by the signal IP, which is near time zero. As the transducer is scanned along the surface of the part, four other signals are likely to appear at different times on the screen. When the transducer is in its far left position, only the IP signal and signal A, the sound energy reflecting from surface A, will be seen on the trace. As the transducer is scanned to the right, a signal from the backwall BW will appear latter in time showing that the sound has traveled farther to reach this surface. When the transducer is over flaw B, signal B, will appear at a point on the time scale that is approximately halfway between the IP signal and the BW signal. Since the IP signal corresponds to the front surface of the material, this indicates that flaw B is about halfway between the front and back surfaces of the sample. When the transducer is moved over flaw C, signal C will appear earlier in time since the sound travel path is shorter and signal B will disappear since sound will no longer be reflecting from it.

5.4.2 B-Scan The B-scan presentations is a profile (cross-sectional) view of the a test specimen. In the B-scan, the time-of-flight (travel time) of the sound energy is displayed along the vertical and the linear position of the transducer is displayed along the horizontal axis. From the B-scan, the depth of the reflector and its approximate linear dimensions in the scan direction can be determined. The B-scan is typically produced by establishing a trigger gate on the A-scan. Whenever the signal intensity is great enough to trigger the gate, a point is produced on the B-scan. The gate is triggered by the sound reflecting from the backwall of the specimen and by smaller reflectors within the material. The B-scan presentations is a profile (cross-sectional) view of the a test specimen. In the B-scan, the time-of-flight (travel time) of the sound energy is displayed along the vertical and the linear position of the transducer is displayed along the horizontal axis. From the B-scan, the depth of the reflector and its approximate linear dimensions in the scan direction can be determined. The B-scan is typically produced by establishing a trigger gate on the A-scan. Whenever the signal intensity is great enough to trigger the gate, a point is produced on the B-scan. The gate is triggered by the sound reflecting from the backwall of the specimen and by smaller reflectors within the material. In the B-scan image above, line A is produced as the transducer is scanned over the reduced thickness portion of the specimen. When the transducer moves to the right of this section, the backwall line BW is produced. When the transducer is over flaws B and C lines that are similar to the length of the flaws and at similar depths within the material are drawn on the B-scan. It should be noted that a limitation to this display technique is that reflectors may be masked by larger reflectors near the surface.

5.4.3 C-Scan: The C-scan presentation provides a plan-type view of the location and size of test specimen features. The plane of the image is parallel to the scan pattern of the transducer. C-scan presentations are produced with an automated data acquisition system, such as a computer controlled immersion scanning system. Typically, a data collection gate is established on the A-scan and the amplitude or the time-of-flight of the signal is recorded at regular intervals as the transducer is scanned over the test piece. The relative signal amplitude or the time-of-flight is displayed as a shade of gray or a color for each of the positions where data was recorded. The C-scan presentation provides an image of the features that reflect and scatter the sound within and on the surfaces of the test piece.

High resolution scan can produce very detailed images
High resolution scan can produce very detailed images. Both images were produced using a pulse-echo techniques with the transducer scanned over the head side in an immersion scanning system. For the C-scan image on the left, the gate was setup to capture the amplitude of the sound reflecting from the front surface of the quarter. Light areas in the image indicate area that reflected a greater amount of energy back to the transducer. In the C-scan image on the right, the gate was moved to record the intensity of the sound reflecting from the back surface of the coin. The details on the back surface are clearly visible but front surface features are also still visible since the sound energy is affected by these features as it travels through the front surface of the coin. Gray scale image produced using the sound reflected from the front surface of the coin Gray scale image produced using the sound reflected from the back surface of the coin (inspected from “heads” side)

6. Eddy Current Testing Electrical currents are generated in a conductive material by an induced alternating magnetic field. The electrical currents are called eddy currents because the flow in circles at and just below the surface of the material. Interruptions in the flow of eddy currents, caused by imperfections, dimensional changes, or changes in the material's conductive and permeability properties, can be detected with the proper equipment. Eddy current testing can be used on all electrically conducting materials with a reasonably smooth surface. The test equipment consists of a generator (AC power supply), a test coil and recording equipment, e.g. a galvanometer or an oscilloscope Used for crack detection, material thickness measurement (corrosion detection), sorting materials, coating thickness measurement, metal detection, etc.

6.1 Principle of Eddy Current Testing (I)
When a AC passes through a test coil, a primary magnetic field is set up around the coil The AC primary field induces eddy current in the test object held below the test coil A secondary magnetic field arises due to the eddy current

Mutual Inductance (The Basis for Eddy Current Inspection)
The magnetic field produced by circuit 1 will intersect the wire in circuit 2 and create current flow. The induced current flow in circuit 2 will have its own magnetic field which will interact with the magnetic field of circuit 1. At some point P on the magnetic field consists of a part due to i1 and a part due to i2. These fields are proportional to the currents producing them. The flux B through circuits as the sum of two parts. B1 = L1i1 + i2M B2 = L2i2 + i1M L1 and L2 represent the self inductance of each of the coils. The constant M, called the mutual inductance of the two circuits and it is dependent on the geometrical arrangement of both circuits.

Principle of Eddy Current Testing (II)
The strength of the secondary field depends on electrical and magnetic properties, structural integrity, etc., of the test object If cracks or other inhomogeneities are present, the eddy current, and hence the secondary field is affected.

Principle of Eddy Current Testing (III)
The changes in the secondary field will be a ‘feedback’ to the primary coil and affect the primary current. The variations of the primary current can be easily detected by a simple circuit which is zeroed properly beforehand The bridge circuit here is known as the Maxwell-Wien bridge (often called the Maxwell bridge), and is used to measure unknown inductances in terms of calibrated resistance and capacitance. Calibration-grade inductors are more difficult to manufacture than capacitors of similar precision, and so the use of a simple "symmetrical" inductance bridge is not always practical. Because the phase shifts of inductors and capacitors are exactly opposite each other, a capacitive impedance can balance out an inductive impedance if they are located in opposite legs of a bridge, as they are here. Unlike this straight Wien bridge, the balance of the Maxwell-Wien bridge is independent of source frequency, and in some cases this bridge can be made to balance in the presence of mixed frequencies from the AC voltage source, the limiting factor being the inductor's stability over a wide frequency range. In the simplest implementation, the standard capacitor (Cs) and the resistor in parallel with it are made variable, and both must be adjusted to achieve balance. However, the bridge can be made to work if the capacitor is fixed (non-variable) and more than one resistor is made variable (at least the resistor in parallel with the capacitor, and one of the other two). However, in the latter configuration it takes more trial-and-error adjustment to achieve balance as the different variable resistors interact in balancing magnitude and phase. Another advantage of using a Maxwell bridge to measure inductance rather than a symmetrical inductance bridge is the elimination of measurement error due to mutual inductance between two inductors. Magnetic fields can be difficult to shield, and even a small amount of coupling between coils in a bridge can introduce substantial errors in certain conditions. With no second inductor to react within the Maxwell bridge, this problem is eliminated.

6.2 Eddy Current Instruments
Voltmeter Coil's magnetic field Coil Eddy current's magnetic field Eddy currents The most basic eddy current testing instrument consists of an alternating current source, a coil of wire connected to this source, and a voltmeter to measure the voltage change across the coil. An ammeter could also be used to measure the current change in the circuit instead of using the voltmeter. While it might actually be possible to detect some types of defects with this type of an equipment, most eddy current instruments are a bit more sophisticated. In the following pages, a few of the more important aspects of eddy current instrumentation will be discussed. Conductive material

Depth of Penetration Eddy currents are closed loops of induced current circulating in planes perpendicular to the magnetic flux. They normally travel parallel to the coil's winding and flow is limited to the area of the inducing magnetic field. Eddy currents concentrate near the surface adjacent to an excitation coil and their strength decreases with distance from the coil as shown in the image. Eddy current density decreases exponentially with depth. This phenomenon is known as the skin effect. Skin effect arises when the eddy currents flowing in the test object at any depth produce magnetic fields which oppose the primary field, thus reducing net magnetic flux and causing a decrease in current flow as depth increases. Alternatively, eddy currents near the surface can be viewed as shielding the coil's magnetic field, thereby weakening the magnetic field at greater depths and reducing induced currents. The depth that eddy currents penetrate into a material is affected by the frequency of the excitation current and the electrical conductivity and magnetic permeability of the specimen. The depth of penetration decreases with increasing frequency and increasing conductivity and magnetic permeability. The depth at which eddy current density has decreased to 1/e, or about 37% of the surface density, is called the standard depth of penetration (d). The word 'standard' denotes plane wave electromagnetic field excitation within the test sample (conditions which are rarely achieved in practice). Although eddy currents penetrate deeper than one standard depth of penetration they decrease rapidly with depth. At two standard depths of penetration (2d), eddy current density has decreased to 1/e squared or 13.5% of the surface density. At three depths (3d) the eddy current density is down to only 5% of the surface density. The depth at which eddy current density has decreased to 1/e, or about 37% of the surface density, is called the standard depth of penetration ().

Three Major Types of Probes
The test coils are commonly used in three configurations Surface probe Internal bobbin probe Encircling probe

6.3 Result presentation The impedance plane diagram is a very useful way of displaying eddy current data. The strength of the eddy currents and the magnetic permeability of the test material cause the eddy current signal on the impedance plane to react in a variety of different ways. If the eddy current circuit is balanced in air and then placed on a piece of aluminum, the resistance component will increase (eddy currents are being generated in the aluminum and this takes energy away from the coil and this energy loss shows up as resistance) and the inductive reactance of the coil decreases (the magnetic field created by the eddy currents opposes the coil's magnetic field and the net effect is a weaker magnetic field to produce inductance). If a crack is present in the material, fewer eddy currents will be able to form and the resistance will go back down and the inductive reactance will go back up. Changes in conductivity will cause the eddy current signal to change in a different way. When a probe is placed on a magnetic material such as steel, something different happens. Just like with aluminum (conductive but not magnetic) eddy currents form which takes energy away from the coil and this shows up as an increase in the coils resistance. And, just like with the aluminum, the eddy currents generate their own magnetic field that opposes the coils magnetic field. However, you will note for the diagram that the reactance increase. This is because the magnetic permeability of the steel concentrates the coil's magnetic field this increase in the magnetic field strength completely overshadows the magnetic field of the eddy currents. The presence of a crack or a change in the conductive will produce a change in the eddy current signal similar to that seen with aluminum.

6.4 Applications Crack Detection Material Thickness Measurements
Coating Thickness Measurements Conductivity Measurements For: Material Identification Heat Damage Detection Case Depth Determination Heat Treatment Monitoring

Surface Breaking Cracks
Eddy current inspection is an excellent method for detecting surface and near surface defects when the probable defect location and orientation is well known. Successful detection requires: A knowledge of probable defect type, position, and orientation. Selection of the proper probe. The probe should fit the geometry of the part and the coil must produce eddy currents that will be disrupted by the flaw. Selection of a reasonable probe drive frequency. For surface flaws, the frequency should be as high as possible for maximum resolution and high sensitivity. For subsurface flaws, lower frequencies are necessary to get the required depth of penetration. Successful detection of surface breaking and near surface cracks requires: A knowledge of probable defect type, position, and orientation. Selection of the proper probe. The probe should fit the geometry of the part and the coil must produce eddy currents that will be disrupted by the flaw. Selection of a reasonable probe drive frequency. For surface flaws, the frequency should be as high as possible for maximum resolution and high sensitivity. For subsurface flaws, lower frequencies are necessary to get the required depth of penetration and this results in less sensitivity. Ferromagnetic or highly conductive materials require the use of an even lower frequency to arrive at some level of penetration. Setup or reference specimens of similar material to the component being inspected and with features that are representative of the defect or condition being inspected for. In the lower image, there is a flaw under the right side of the coil and it can be see that the eddy currents are weaker in this area.

Applications with Encircling Probes
Mainly for automatic production control Round bars, pipes, wires and similar items are generally inspected with encircling probes Discontinuities and dimensional changes can be revealed In-situ monitoring of wires used on cranes, elevators, towing cables is also an useful application

Applications with Internal Bobbin Probes
Primarily for examination of tubes in heat exchangers and oil pipes Become increasingly popular due to the wide acceptance of the philosophy of preventive maintenance

Applications with Internal Bobbin Probes

6.5 Advantages of ET Sensitive to small cracks and other defects
Detects surface and near surface defects Inspection gives immediate results Equipment is very portable Method can be used for much more than flaw detection Minimum part preparation is required Test probe does not need to contact the part Inspects complex shapes and sizes of conductive materials

Limitations of ET Only conductive materials can be inspected
Surface must be accessible to the probe Skill and training required is more extensive than other techniques Surface finish and and roughness may interfere Reference standards needed for setup Depth of penetration is limited Flaws such as delaminations that lie parallel to the probe coil winding and probe scan direction are undetectable

7. Common Application of NDT
Inspection of Raw Products Inspection Following Secondary Processing In-Services Damage Inspection

Inspection of Raw Products
Forgings, Castings, Extrusions, etc.

Inspection Following Secondary Processing
Machining Welding Grinding Heat treating Plating etc.

Inspection For In-Service Damage
Cracking Corrosion Erosion/Wear Heat Damage etc.

Power Plant Inspection
Periodically, power plants are shutdown for inspection. Inspectors feed eddy current probes into heat exchanger tubes to check for corrosion damage. Probe Pipe with damage Signals produced by various amounts of corrosion thinning.

Wire Rope Inspection Electromagnetic devices and visual inspections are used to find broken wires and other damage to the wire rope that is used in chairlifts, cranes and other lifting devices.

Storage Tank Inspection
Robotic crawlers use ultrasound to inspect the walls of large above ground tanks for signs of thinning due to corrosion. Cameras on long articulating arms are used to inspect underground storage tanks for damage.

Aircraft Inspection Nondestructive testing is used extensively during the manufacturing of aircraft. NDT is also used to find cracks and corrosion damage during operation of the aircraft. A fatigue crack that started at the site of a lightning strike is shown below.

Jet Engine Inspection Aircraft engines are overhauled after being in service for a period of time. They are completely disassembled, cleaned, inspected and then reassembled. Fluorescent penetrant inspection is used to check many of the parts for cracking.

Crash of United Flight 232 Sioux City, Iowa, July 19, 1989
A defect that went undetected in an engine disk was responsible for the crash of United Flight 232.

Pressure Vessel Inspection
The failure of a pressure vessel can result in the rapid release of a large amount of energy. To protect against this dangerous event, the tanks are inspected using radiography and ultrasonic testing.

Rail Inspection Special cars are used to inspect thousands of miles of rail to find cracks that could lead to a derailment. The heavy loads that trains place on the railroad tracks can result in the formation of cracks in the rail. If these cracks are not detected, they can lead to a derailment. Special rail cars equipped with NDT equipment are used to detect rail defects before they are big enough to cause serious problems.

Bridge Inspection The US has 578,000 highway bridges.
Corrosion, cracking and other damage can all affect a bridge’s performance. The collapse of the Silver Bridge in 1967 resulted in loss of 47 lives. Bridges get a visual inspection about every 2 years. Some bridges are fitted with acoustic emission sensors that “listen” for sounds of cracks growing. The US has 578,000 highway bridges, which are the lifelines of US commerce. Corrosion, cracking and other damage can all affect the bridges load carrying capacity. Therefore, all of the elements that directly affect performance of the bridge including the footing, substructure, deck, and superstructure must be periodically inspected or monitored. Visual inspection is the primary NDE method used to evaluate the condition of the majority of the nation's highway bridges. Inspectors periodically (about every two years) pay each bridge a visit to assess its condition. However, it is not uncommon for a fisherman, canoeist and other passerby to alert officials to major damage that may have occurred between inspections. The potential penalties for ineffective inspection of bridges can be very severe. Instances of major bridge collapse are very rare, but the results are truly catastrophic. The collapse of the famous Silver Bridge at Point Pleasant, Ohio in 1967 resulted in loss of 47 lives. The cost of this disaster was 175 million dollars but some experts estimate the same occurrence today would cost between 2.1 and 5.6 billion dollars. Furthermore, these cost figures do not take into account factors such as loss of business resulting from loss of access or detours, the cost resulting from blockage of a major river shipping channel, and potential environmental damage due to hazardous materials being transported over the bridge at the time of collapse. Fatigue cracking and corrosion will become increasingly important considerations as we go beyond the 75 year life expectancy and current visual inspection techniques will not suffice. The life extension approach will require increased use of NDE in a coordinated effort to obtain reliability assurance for these structures. NDE techniques such as magnetic particle inspection and ultrasonic inspection are being used with greater frequency. One of the newer NDE technologies being used is acoustic emission (AE) monitoring. Some bridges are being fitted with AE instruments that listen to the sounds that a bridge makes. These sophisticated systems can detect the sound energy produced when a crack grows and alert the inspector to the cracks presence. Sensors can be permanently fixed to the bridge and the data transmitted back to the lab so that continuous bridge condition monitoring is possible. The image provided here shows field engineers installing an AE monitoring system on the lift cables of the Ben Franklin Bridge in Philadelphia, PA

Pipeline Inspection NDT is used to inspect pipelines to prevent leaks that could damage the environment. Visual inspection, radiography and electromagnetic testing are some of the NDT methods used. Remote visual inspection using a robotic crawler. Magnetic flux leakage inspection. This device, known as a pig, is placed in the pipeline and collects data on the condition of the pipe as it is pushed along by whatever is being transported. Radiography of weld joints.

Special Measurements Boeing employees in Philadelphia were given the privilege of evaluating the Liberty Bell for damage using NDT techniques. Eddy current methods were used to measure the electrical conductivity of the Bell's bronze casing at a various points to evaluate its uniformity.

MCE Nondestructive Testing Methods

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