Anish Poudel

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.

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.

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.