1. Introduction of Non Destructive Test
Date : July 2008
Peter Yang

2. NDT Introduction 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 reactors, boiler and pressure vessel to fail, pipelines to burst, ship to crack and a variety of less visible, but equally troubling events.
These 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. 

3. NDT Introduction 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. . 

4. NDT Introduction 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 common 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.

5. NDT Introduction 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.

6. NDT Introduction Penetrant Testing (PT)
Test objects are coated with visible or fluorescent dye solution.
Excess dye is then removed from the surface, and a developer is applied.
The developer acts as blotter, drawing trapped penetrant out of imperfections open to the surface.
With visible dyes, vivid color contrasts between the penetrant and developer make "bleedout" easy to see.
With fluorescent dyes, ultraviolet light is used to make the bleedout fluoresce brightly, thus allowing imperfections to be readily seen. .

7. IACS Rec. No. 20 - Non-destructive Testing of Ship Hull Steel Welds
Liquid Penetrant Testing (PT)
The procedure should detail as a minimum the calibration equipment, surface preparation, cleaning and drying prior to testing, temperature range, type of penetrant, cleaner and developer used, penetrant application and removal, penetration time, developer application and development time and lighting conditions during testing.
The surface to be examined should be clean and free from scale, oil, grease, dirt or paint and should include the weld bead and base metal for at least 10mm on each side of the weld, or the width of the heat affected zone, whichever is greater.
The temperature of parts examined should be typically between 5ºC and 50ºC, outside this temperature range special low/high temperature penetrant and reference comparator blocks should be used.
The penetration time should not be less than 10 minutes and in accordance with the manufacturer’s specification. The development time should not be less than 10 minutes and in accordance with the manufacturer’s specification, normally between minutes.

8. NDT Introduction Magnetic Particle Testing (MT)
This NDE method is accomplished by inducing a magnetic field in a ferromagnetic material and then dusting the surface with iron particles (either dry or suspended in liquid).
Surface and near-surface imperfections distort the magnetic field and concentrate iron particles near imperfections, previewing a visual indication of the flaw. . .

9. IACS Rec. No. 20 - Non-destructive Testing of Ship Hull Steel Welds
Magnetic particle testing (MT)
The procedure should detail as a minimum the surface preparation, magnetizing equipment, calibration methods, detection media and application, viewing conditions and post demagnetization.
The surface to be examined should be free from scale, weld spatter, oil, grease, dirt or paint and should be clean and dry.
When using current flow equipment with prods, care shall be taken to avoid local damage to the material. Copper prod tips must not be used. The prod tips should be lead, steel, aluminium or aluminium- copper braid.
To ensure detection of discontinuities of any orientation, the welds are magnetized in two directions approximately perpendicular to each other with a maximum deviation of 30º. Adequate overlapping shall ensure testing of the whole zone.
Continuous wet particle method should be used as far as practicable.

10. NDT Introduction Eddy Current Testing (ET)
Electrical currents are generated in a conductive material by an induced alternating magnetic field. The electrical currents are called eddy currents because they 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. .

11. NDT Introduction Radiography Testing (RT)
Radiography involves the use of penetrating gamma or X-radiation to examine parts and products for imperfections.
An X-ray generator or radioactive isotope is used as a source of radiation.
Radiation is directed through a part and onto film or other imaging media.
The resulting shadowgraph shows the dimensional features of the part.
Possible imperfections are indicated as density changes on the film in the same manner as a medical X-ray shows broken bones. .

12. IACS Rec. No. 20 - Non-destructive Testing of Ship Hull Steel Welds
Radiographic testing (RT1)
The procedure should detail as a minimum the type of radiation source, considering the thickness to be radiographed, test arrangement and films overlapping, type and position of image quality indicators (IQI), image quality, film system and intensifying screens used if any, exposure conditions, scattered radiation control, film processing, film density and viewing conditions.
Processed films should display hull no., frame no., weld boundary indicators, Port/Starboard, location (or film serial number) and date as radiographic image.
The Classification Society may require to duplicate some radiographs in order that some processed films are handed over to the Society together with testing reports. Alternative method to duplicate the processed film can be agreed with the Society.
The type of source is selected by the shipbuilder in accordance with item 7.2 of ISO ..

13. IACS Rec. No. 20 - Non-destructive Testing of Ship Hull Steel Welds
Radiographic testing (RT2)
Single-wall exposure technique should be used as far as practicable.
The image quality should be verified using an IQI (Image Quality Indicator) in accordance with ISO or equivalent. In general the IQI is to be placed on the source side of the weld examined. The minimum image quality should be in accordance with Class A of ISO or equivalent, as given in Table 2 for IQI’s of wire type placed on source side.
When using IQI’s of wire type, the image of a wire is considered visible on the film if a continuous length of at least 10mm is clearly visible in a section of uniform optical density.
The optical density of the radiographs should be selected by the shipbuilder in accordance with Table 5 of ISO
Traditional radiographic film may be replaced by digital radiographic techniques where it can be shown, to the satisfaction of the classification society, that the sensitivity of the digital image is better than or equal to the image obtained with traditional radiographic film.

14. IACS Rec. No. 20 - Non-destructive Testing of Ship Hull Steel Welds
Radiographic testing (RT3)

15. NDT Introduction Ultrasonic Testing (UT)
Ultrasonics use transmission of high-frequency sound waves into a material to detect imperfections or to locate changes in material properties.
The most commonly used ultrasonic testing technique is pulse echo, wherein sound is introduced into a test object and reflections (echoes) are returned to a receiver from internal imperfections or from the part's geometrical surfaces.

16. IACS Rec. No. 20 - Non-destructive Testing of Ship Hull Steel Welds
Ultrasonic testing (UT1)
The procedure should detail the equipment, type of probes (frequency, angle of incidence), coupling media, type of reference blocks, method for range and sensitivity setting, method for transfer corrections, scanning technique, sizing technique and intervals for calibration checks during testing.
The equipment (instrument and probes) should be verified by the use of appropriate standard calibration blocks at suitable time intervals.
The range and sensitivity should be set prior to each testing and checked at regular intervals as per the procedure and whenever needed.
The scanning surfaces should be sufficiently clean and free from irregularities like rust, loose scale, paint (excluding primer), weld spatter or grooves which may interfere with probe coupling.
The surface profile should be such to avoid loss of probe contact by rocking.
The scanning technique should be determined to allow the testing of the entire volume of the weld bead and base metal for at least 10mm on each side of the weld, or the width of the heat affected zone, whichever is greater.

17. IACS Rec. No. 20 - Non-destructive Testing of Ship Hull Steel Welds
Ultrasonic testing (UT1)
The probe frequency should be within the range 2 MHz to 5 MHz.
The reference level for testing should be set using a Distance-Amplitude- Corrected curve (DAC curve) for a series of 3mm diameter side-drilled holes in a reference block or other methods like the Distance-Gain-Size (DGS) system based on a disc shaped reflector provided the same sensitivity is achieved. The reference block used should be made in a material giving equivalent ultrasonic response to that of the material to be tested.
The indications with an echo height below 33% of DAC curve (DAC minus 10 dB) should be disregarded. The indications with an echo height equal to or exceeding 33% of DAC curve (DAC plus 10 dB) should be evaluated.
Base material in the scanning zone should be examined with a straight beam technique to check the absence of imperfections which would interfere with the angle beam technique, unless already demonstrated at a previous fabrication stage.
Angle beam technique should be used to search for longitudinal and transverse weld discontinuities. An angle probe with an incident angle of the sound wave equal to that of the weld preparation should be used as a minimum .

18. NDT Introduction Leakage Testing (LT)
Several techniques are used to detect and locate leaks in pressure containment parts, pressure vessels, and structures.
Leaks can be detected by using electronic listening devices, pressure gauge measurements, liquid and gas penetrant techniques, and/or a simple soap-bubble test.