Research On Ultrasound Infrared Lock-in Thermography For Non-destructive Testing

Cracks and delaminations at surface or interior can induce material and component failure. So this is important especially to monitor quality of structure and performance of equipment, that nondestructive testing and evaluating defects. Ultrasound infrared lock-in thermography (ULT), which uses modulated ultrasonic wave to excite surface of a sample and lock-in method to process thermal image sequence, is active infrared non-destructive testing. It can be used to detect surface's and interior contacting interface-type defects in metal and composite, as such cracks.It has advantages in detection efficiency and detection depth. So ultrasound infrared lock-in thermography has broad application prospects in aerospace, automotive industry, railroad transportation. In order to develop the detectability, theoretically analyzing detection's principle, selecting inspection's parameters, processing infrared image sequence and quantitatively evaluating defects are investigated.Firstly, detecting defect by Ultrasound infrared lock-in thermography is analyzed theoretically using wave theory, contact dynamics and heat transfer theory. Mechanical-heat coupling, that amplitude modulated ultrasound is injected into the sample to be converted into heat mainly in defects, and the heat travels to the surface by conduction and induces surface's temperature change, is calculated and simulated. Calculated surface's temperature is analyzed on frequency domain. Effects of parameters of processing infrared image sequence and detecting defect on detection are researched.Secondly, ultrasonic horn, included transducer and amplitude transformer, ultrasonic power amplifier are designed and developed. According to requirement of detection simple pneumatic control system and worktable with 5-D are designed and manufactured. Ultrasonic horn can reached the excited position at sample and excite sample effectively using developed ultrasonic exciting system.Thirdly, collected data by a camera with higher performance is analyzed and read to obtain infrared image sequence. Lock-in algorithms for processing infrared image sequence are investigated, algorithms included amplitude and phase extremum method based on short time Fourier transform with box window and enhancement method based on Wavelet transform are proposed. By compare proposed algorithms with conventional algorithms included correlation algorithm and fast Fourier transform, advantages of proposed algorithms is validated.Fourthly,some experiments are carried out in a natural ambient environment to detect prefabricated surface's cracks of metal plats and interior simulated defects using developed inspecting system of ULT. Performance of the inspecting system and detectability of ULT is verified. Effects of inspection's parameters and interfere between thermal waves produced by some defects or heat sources on detecting defects are researched. Detections defects at different depth are completed to analyze effect of inspection's parameter on detectable depth of ULT.Finally, in order to detect defect quantitatively using ULT, advanced image processing technology and fuzzy control theory are introduced into the ultrasound lock-in thermography technology. Based on distribution of amplitude and phase in amplitude image and phase image for detecting defect, optimization method using threshold based on operator'canny'is proposed to recognize geometric feature of defect and size of defect is calculated. Adaptive Neuro-Fuzzy Inference Stystem is obtained, when modulation frequency and phase as inputs and the defect depth as output are used for training fuzzy-neural network. The system can used for measuring defect depth.