Tsuchin Philip Chu

Non-destructive evaluation of composite repairs by using infrared thermography:

Composite structures are found in modern aircraft designs ranging from air transport to general aviation. Maintenance repair technology varies for each original equipment manufacturer and aircraft type. This research reports on two different composite repair methods commonly used within the composite aviation industry and how they compare when inspected with transient infrared thermography non-destructive evaluation technique. Composite sandwich test coupons made with carbon fiber laminate, nomex honeycomb, and glass fiber laminate were used for this work. Impact damages were generated in the sandwich test coupons and repairs were conducted by following repair procedures of two leading general aviation composite aircraft manufactures. During the repair process, controlled flaws were also induced to simulate bad repairs with weak bond areas, disbonds, and excessive porosity. During transient infrared thermography, several patches were identified that contained wrinkles, porosity, and disbond defects in the repaired panels. The indications were evaluated utilizing the time vs. temperature plot curves and profile data. The porosity indications displayed significant variations compared to the surrounding areas and were subsequently deemed defects as a result of the data.

Determination of optimal experimental parameters for transient thermography imaging using finite-element models

Abstract A study was conducted to determine the optimal inspection parameters such as range and time for finding defects in carbon/epoxy composite panels using IR thermography imaging. The present paper presents an innovative method for automatically selecting these parameters for evaluating composites based on a series of finite-element models. Such finite-element models of composite panels with flaws at different depth locations were constructed a priori and analysed to estimate the optimal operating parameters. The optimal inspection range and time were identified in the contour plots obtained from the appropriate finite-element analysis results. A graphite–epoxy composite panel with phantom defects at various depth locations was built, and experiments were performed using a thermographic system to verify and validate the proposed method.

Defect characterization in commercial carbon- carbon composites

A relatively inexpensive and easy to operate custom built infrared thermography (IRT) system was developed and utilized for the detection and characterization of defects in Carbon-Carbon (C/C) composite aircraft brake disks. This method uses an active infrared thermography (IRT) approach, i.e. Flash Heating method, for a non-destructive evaluation (NDE) of C/C brake disk materials. The experimental results obtained from the developed system were then compared with commercial IRT turn-key system. In addition, Finite Element Analysis (FEA) was also carried out to determine the detectible defect depth and diameter of the defects in C/C composites to validate the experimental results. The experimental results were compared to the FEA results and it was found that they were in good agreement with one another.

C/C composite brake disk nondestructive evaluation by IR thermography

This paper discusses the non-destructive evaluation of thick Carbon/Carbon (C/C) composite aircraft brake disks by using transient infrared thermography (IRT) approach. Thermal diffusivity measurement technique was applied to identify the subsurface anomalies in thick C/C brake disks. In addition, finite element analysis (FEA) modeling tool was used to determine the transient thermal response of the C/C disks that were subjected to flash heating. For this, series of finite element models were built and thermal responses with various thermal diffusivities subjected to different heating conditions were investigated. Experiments were conducted to verify the models by using custom built in-house IRT system and commercial turnkey system. The analysis and experimental results showed good correlation between thermal diffusivity value and anomalies within the disk. It was demonstrated that the step-heating transient thermal approach could be effectively applied to obtain the whole field thermal diffusivity value of C/C composites.

Ultrasonic Defect Mapping Using Signal Correlation for Nondestructive Evaluation (NDE)

This article presents the application of a signal correlation technique to automatically classify ultrasonic A-scan signals for defect and defect-free regions in isotropic and anisotropic materials. First, feature extraction was implemented by generating a reference A-scan signal of a defect-free area using an autocorrelation function and statistics. Then, a cross-correlation function, utilized as a feature detector, was applied to the reference signal and a signal of interest (SOI) to detect defect-free features in an SOI. The correlation result was considered as a pattern containing both defect and defect-free features. Next, the pattern was classified by measuring the similarity between features of the reference signal and an SOI based on their Euclidean distance. Each A-scan signal classification result was then plotted on a 2D map based on its position on the specimen. The present work uses multiple correlation functions and statistics to classify defect signals rather than relying on an inspector’s prior knowledge to interpret C-scan data, and has particular value in automated ultrasonic signal classification and characterization.

Evaluation of Friction Stir Welds

An Image correlation technique to evaluate the structural properties and behavior of the Friction Stir Welds (FSW) is presented. The technique used is Sub-Pixel Digital Image Correlation (SPDIC). Four weld samples made of Al-2219 and Al-2195 alloys but different surface treatments are tested under tensile loads and the digital images of these samples are correlated using SPDIC. Stress-strain graphs and strain contours are obtained from the results. From the stress-strain graphs the structural properties and the behavior of the weld material are determined. The results indicate that SPDIC is a steadfast tool to determine the structural behavior and material properties of the weld samples.Copyright

Influence of Diamond-like Carbon Coatings on the Fatigue Behavior of Spinal Implant Rod

The purpose of this paper is to discuss the influence of Diamond–Like Carbon (DLC) coatings on the fatigue behavior of Spinal implant rod. DLC exists in seven different forms of amorphous carbon materials that display some of the unique properties of diamond. They are usually applied as coatings to other materials that could benefit from some of unique diamond properties. In this work, we compared bending fatigue behavior between Stainless Ti6Al4V and DLC Coating Spinal implant rod. The positive effect has been observed by fatigue tests carried out at ASTM Standard D790. Fatigue life and endurance limit of spinal implant rod have been remarkably increased by means of thin DLC coatings. The improvement of fatigue characteristics of spinal implant rod by DLC coatings has been ascribed to the sealing of external defects which retards crack initiation.

Polynomial Fitting Techniques for IRT Inspection

This paper discusses the use of polynomial surface fitting techniques for the infrared thermography (IRT) evaluation of the graphite epoxy composite laminate. The composite laminate had 12 inserted Teflon films in different layers so as to simulate delamination defects. Based on the initial IRT inspection, it was determined that the raw IR images were not able to detect all 12 defects. In order to improve the detection capability and remove the heating pattern, a fitted heating pattern was generated by using the polynomial surface fitting method. Then, this pattern was deleted by applying the subtract function. It was demonstrated that this applied method not only removed the non-uniform heating effect, but also preserved all defect information and enhanced the thermal contrast of the raw IR images.

Super-Resolution in Ultrasonic NDE

This paper discusses the use of an iterative back projection (IBP) super-resolution (SR) image reconstruction technique on the carbon epoxy laminates with simulated porosity defects. In order to first validate and evaluate the application of the proposed method, three artificially simulated delamination defects in carbon epoxy laminates were considered. Based on the preliminary results, it was verified that the contrast signal-to-noise ratio (CNR) of the SR image was higher than the bi-cubic interpolation image. Further, the peak signal-to-noise ratio (PSNR) value of SR result had an average increase of 5.7088 dB compared to the bi-cubic interpolation method. This validates the proposed approach used to generate the reconstructed SR images with image quality similar to the original simulated UT images. After the validation, the UT image reconstruction algorithm was applied to the ultrasonic C-scan amplitude images of a porosity sample. Based on the results, it was demonstrated that the SR image achieved better visual quality with an improved image resolution. It was also demonstrated that this method was capable of detecting the defects with more confidence by recovering the defect outline compared to the LR C-scan image. The defect outline in SR images is more distinct to recognize, allowing post-processing work such as measurement of defect size, shape, and location to be much easier.

Investigating the thermo-mechanical behavior of clutch facing materials using image processing techniques

The purpose of this paper is to investigate the thermo-mechanical behavior of clutch facing materials using image processing techniques. Infrared thermography and digital image correlation were used to analyze the mechanical behavior of clutch facing material under tensile loads. For specimens under tension loads, Infrared thermography clearly showed the thermo-elastic coupling as well as thermal dissipations due to the microcracking state of damaged clutch facing specimens. During an up to failure tensile test, localized thermal effects were observed at the macro-crack location before it appeared. Strain fields at the surface of specimens under tensile loadings were determined using the digital image correlation (DIC) technique. These experiments showed that the strain fields early became inhomogeneous. Strains concentrated in multiple localization zones which highlighted the role of the fibers in transmitting the internal forces.