Presentation on theme: "Assessment of Advanced Ultrasonic and Infrared Inspection Methods to Detect Delaminations and Water Ingress in Composite Honeycomb Materials David G. Moore."— Presentation transcript:
Assessment of Advanced Ultrasonic and Infrared Inspection Methods to Detect Delaminations and Water Ingress in Composite Honeycomb Materials David G. Moore and Ciji L. Nelson Sandia National Laboratories Nondestructive Evaluation and Experimental Mechanics Department Post Office Box 5800 MS-0557 Albuquerque, New Mexico 87185, USA NDCM Conference May , 2013 Le Mans, France Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energys National Nuclear Security Administration under contract DE-AC04-94AL This presentation is declared a work of the U.S. Government and is not subject to copyright protection in the United States.
Sandia National Laboratories is studying composites due to their unique structure and potential for aerospace applications. New composite manufacturing techniques are requiring us to implement advanced inspection methods. Reference standard creation is key to the development of material characterization establishing inspection criteria limits and assessing equipment capabilities. Introduction
Overview of Composites Generally four types of flaws in composite materials: –Disbond –Core Damage –Delamination –Porosity These flaws can occur due to the following: –Impact damage –Lighting Strike (heat damage) –Manufactures Defect Defects found in Composite Structures Core Damage
Engineering Sample The specimen is 20.3 by 20.3 cm composed of a Nomex honeycomb structure sandwiched between two carbon graphite laminate weave skins (3 plies thick). Five flaws were created in the specimen: 1)2.54 cm diameter epoxy potted honeycomb core (full thickness); 2)2.54 cm square Teflon shim (located between plies 2 and 3); 3)12.7 mm diameter Teflon shim (located between plies 2 and 3); 4)1.27 square disbond (located at adhesive bondline); 5)2.54 cm diameter disbond (located at adhesive bondline).
Computed Tomography Baseline Inspection The Computed Tomography (CT) technique collects penetrating radiation measurements from the composite samples x-ray opacity using an amorphous silicon digital detector array (flat panel). The source and detector remain constant while the part is rotated. These slices are then collected and mapped together to create a three dimensional CT-density map. The fraction of the x-ray beam that is attenuated will directly relate to the density and thickness of the material through which the beam has traveled.
Shallower Deeper CT Inspection Results A Perkin Elmer (2048 by 2048 pixels) amorphous silicon flat panel detector with a 0.20 by 0.20 mm pixel pitch was used. The panel was scanned at a 1-to-1 ratio giving it a geometric resolution of 20 mils. The x-ray source operated at 160 Kilovolts and 4.4 mA Top of the laminate showing the weave pattern at the surface of the part. Interface between the laminate and Nomex honeycomb structure.
Shallower Deeper CT Inspection Results (continued) Midway through the honeycomb core crushing Honeycomb material (disbonds detected) Side view of the panel showing an area of core damage
Ultrasonic Properties Material Sound Velocity v (m/sec) Density ρ (kg/m 3 ) Acoustic Impedance Z = ρv (kg/m 2 sec) x 10 6 Aluminum Water (20 °C) Air (20 °C) Hysol Teflon Composite- graphite- Epoxy
Ultrasonic Inspection Ultrasonic transducers transmit sonic waves into a sample and measure the reflected responses. Near surface resolution can be improved with a delay line tip, to provide a time delay between sound generation and reception of reflected energy. Low-frequency Resonant, Ultrasonic Transducers transmit ultrasonic waves that penetrate through the laminate and enter the honeycomb cell wall at the node bond adhesive interface. Front Surface Echo Back Surface Echo & Bondline 0.22 mm
Ultrasonic (UT) Setup and Results The Composite Sample was UT inspected using the below equipment and setup, -MAUS V, (Mobile automated UT scanner) was used to acquire the images. -A 5 MHz probe, 6.35 mm in diameter. -Scanner resolution- 0.5 mm. -The gate was set to monitor the backwall signal from the laminate. Amplitude Gate Image Depth Gate Image
Resonance inspection requires a narrowband transducer that can be excited at its natural resonance frequency. A continuous standing wave is coupled into the material. Resonant inspection can be used to inspect honeycomb materials and compliment conventional ultrasonic inspection. The cursor tracks the shift in the signal phase (X) and amplitude (Y). Any changes in structural resonance (disbonds, or delaminations) is represented by changes in the Resonant Frequency at that point in the sample. Resonance Basics Shift in Resonant Frequency Amplitude Phase
Resonant inspection was moderately successful on the sample at 110 KHz. Resonance probe could not reliably detect the Teflon inserts but could detect the epoxy filled honeycomb core. The signal is attenuated around the perimeter of the Teflon inserts making the defect features hard to discern. The use of Teflon inserts in a resonant inspection reference standard should be carefully considered. Teflon inserts may not be usable for reliable instrument calibration or establishing a reject criterion. Amplitude (X) Image Phase (Y) Image Resonance Inspection Results
Thermal Material Properties Conduction: energy transfer from a more energetic particles to less energetic particles within a material. Interactions between particles are due to a thermal gradient. Fourier's law defines time rate of heat transfer through a material. The heat flux is proportional to the negative gradient in the temperature and to the area. The proportionality constant is the transport property thermal conductivity W/(m °C). Heat flux q" is the heat transfer rate in direction x per unit area perpendicular to the direction of transfer. Since heat transfer rate is a vector quantity it can be written in general of the conduction rate equation: T 1 > T 2 T2T2 x
Thermophysical Properties Thermophysical properties have two distinct categories: Transport and Thermodynamic. Transport properties include the diffusion rate coefficients thermal conductivity and kinematic viscosity. Thermodynamic properties are useful to define the state of equilibrium. Density – ρ (kg/ m 3 ) Heat capacity – c (J/ kg °C) and volumetric heat capacity ρc (J/ m 3 °C ). Solids can store large amounts of thermal energy when compared to gases. The ability of a material to conduct thermal energy relative to it ability to store thermal energy is termed thermal diffusivity α (how fast the material temperature adapts to the surrounding temperature). Thermal effusivity (ε) is a measure of a materials ability to exchange thermal energy with its surroundings
Material Conductivity, W/(m °C) Specific Heat, c p J/(kg °C) Density, ρ kg/(m 3 ) Diffusivity, α m 2 /sec 1 x Phenolic (resin pressed) Teflon Carbon Graphite CFRP Parallel Carbon Fibers CFRP Perpendicular Carbon Fibers Epoxy (hysol) Aluminum 2024 T Copper Stainless Steel GRP Parallel Glass Fibers GRP Perpendicular Glass Fibers Thermal Properties
Infrared Basics Active Thermography (AT) is a technique where a stimulus is applied to a surface and causes it to heat or cool in such a way to allow the surface characteristics to be observed by an infrared camera. Lamp IR camera defect PC heat conduction emitted IR light IR Image Source: Thermal Wave Imaging
EchoTherm Equipment The EchoTherm System used the following components and settings: -FLIR 6106 camera was ran at 60Hz at an image size of 640 X 512 pixels, with an InSb detector and 14 bit output. -The hood contained two xenon flash lamps partially surrounded by parabolic reflectors and 5000 Joules per lamp. -The collected images were processed with Thermographic Signal Reconstruction (TSR) technology in the Mosaiq software package, to provide a set of images representing the heat transfer. Source:
Bond separation Delaminations Dents/crushed core: No greater than 76.2 mm (3.0) diameter or deeper than 12.7 mm (0.050) Holes and punctures: No greater than 25.4 mm (1.0) diameter. Disbonds between core and skin: (core crush, split core) No greater than 30 cm (12.0) Core bond separations: No greater then 45.2 square cm (7.0 square inches). Liquid Intrusion in cells: No more than 40 cells Inspect to make sure water and moisture is within limits. No further water removal efforts are required provided entrapped water is within limits and leak path is found, repaired, and water drip test confirms repair. Damage Limits for Composites
Infrared Data Analysis CoolWarm First derivative of reconstructed intensity per unit time. First and second derivatives yield substantial contrast at earlier thermal decay times in the image sequence. The images will be sharper. (less lateral diffusion). Epoxy Teflon Source: High Diffusivity (α ) Low Diffusivity (α ) High Effusivity (ε) Low Effusivity (ε)
IR Results (Epoxy and Teflon) Intensity Time Plot 0.55 seconds after pulse Log of reconstructed intensity per time. First derivative of reconstructed intensity per time. Second derivative of reconstructed intensity per time.
IR Results (Epoxy and Disbond) Intensity Time Plot 1.1 seconds after pulse Log of reconstructed intensity per time. First derivative of reconstructed intensity per time. Second derivative of reconstructed intensity per time.
IR Inspection Results (continued) Intensity Time Plot 4.6 seconds after pulse Log of reconstructed intensity per time. First derivative of reconstructed intensity per time. Second derivative of reconstructed intensity per g time.
Polymer Properties Hydrophilic polymers characterized by different swelling factors were evaluated by: Bogomil YOCHEV, Svetoslav KUTZAROV, Damyan GANCHEV, Krasimir STAYKOV, Technical University – Sofia, Bulgaria for their ultrasonic properties. -Preliminary experiments show that polymers having water content above 85% can not be used as solid dry-couplants for practical purposes because of their low mechanical strength. Might be used as a reference standard for water ingress into composite materials. -When the water content increases the longitudinal ultrasonic velocity decreases and nears that of water – 1480 m/s. -Hydrophilic polymers have longitudinal ultrasonic velocity significantly lower than the velocity of PMMA (type-H) – 2750 m/s, polyizoprene rubber (type-F) – 1840m/s and is comparable to that of Aqualene (type-G) – 1580m/s Reference: Bogomil YOCHEV et.al
Cut from Aircraft Sample The aircraft sample was cut from a secondary load bearing surface of a general use commercial aircraft. It has naturally occurring skin to core separation and ply damage. To simulate water ingress an elastomer (Aqualene) was placed at the ply to honeycomb surface. This elastic polymer is designed specifically for ultrasonic probes. The acoustic impedance is nearly the same as water. Its attenuation coefficient is lower than most elastomers and plastics mm skin thickness Aqualene placement 15.2 cm 22.9 cm
Elastomer Experiment Log of reconstructed intensity per time (2.61 seconds) First derivative of reconstructed intensity per time (2.61 seconds).
Water Ingress Experiment Cells were saturated with water and inspected with active IR. This technique works well for development of the technique but cannot be used for a long term reference standard.
Water Ingress Experiment (continued) Log of reconstructed intensity per time (2.56 seconds). First derivative of reconstructed intensity per time (2.56 seconds).
Water Ingress Experiment (continued) First derivative of reconstructed intensity per time. These six images were taken at different soak times from left to right (0, 5, 18 minutes, top row and 53, 74, and 178 minutes, bottom row). This allows the water to migrate through the composite.
Summary Core potting is an excellent way to produce a reference standard that will represent splices or honeycomb repairs. All inspection techniques were able to detect this manufacturing process. Resonant inspection and computed tomography could not reliability detect the embedded inserts between the plies. Ultrasonic resonant inspection can detect near surface disbonds in the laminate bonded to honeycomb but may not be able to establish a reference standard threshold. High frequency ultrasonics can detect the embedded inserts in the plies with a high signal-to-noise ratio. Computed tomography was a valuable assessment tool for evaluating through the thickness of the honeycomb material as well as the ply to ply variations, but is limited on the size of part that can be inspected.
Conclusions Honeycomb removal at the bondline to create a core disbond was identifiable with computed tomography and infrared inspection. The thermal properties of the epoxy and air are significantly different from the honeycomb and carbon graphite plies, however thermal properties of Teflon are not. The use of signal processing algorithms greatly enhances the detectability of flaws that are hidden in the raw infrared image. The TSR helps with the evaluation and characterization of indications in the sample. Flashed thermography can identify flaws within the first three plies and the bondline interface. The detectability of a Teflon insert embedded into the plies will depend on its size, depth from the surface and the degree to which its thermal properties differ from the surroundings. The use of representative test samples allows the signal process software to characterize fluid ingress into honeycomb cells and track the migration path. The elastomer material has material properties that almost simulate the thermophysical behavior of water ingress into honeycomb cell walls. The key difference in the thermal behavior is found in the first and second derivatives.