Nondestructive Testing Of Subsurface Defects By Laser Ultrasonic Surface Wave

With the advent of Industry 4.0 era,the manufacturing industry has once again developed rapidly.Additive manufacturing(AM)technology has been widely used as a new manufacturing method.However,due to the complex combination of control parameters and the influence of factors such as the environment,the AM parts inevitably have defects such as voids and cracks,which can directly affect the mechanical performance.Therefore,nondestructive testing(NDT)of subsurface defects in AM parts is a prerequisite for ensuring the safety.Laser ultrasonic(LU)technology has been gradually promoted and applied in the NDT field of AM,because it has non-contact,wide frequency domain,and high detection accuracy advantages.In order to realize the NDT of subsurface defects in AM,a LU surface wave(SW)method is proposed in this thesis,and the specific research includes the following aspects:Firstly,the wave equations of SW in bounded solids were calculated.And this thesis introduced two generation mechanisms of LU: the thermoelastic and ablation mechanism.The theoretical equation and finite element(FE)form of ultrasound excitation using a pulsed laser under thermoelastic mechanism were described,which lays a theoretical foundation for FE simulation.Secondly,the effects of subsurface defects with different sizes and depths on the propagation of SW were investigated using the COMSOL simulation software;the wave equations of were calculated based on the coupled results of the simulated temperature and solid stress fields,which provided the comparative results for subsequent LU experiments.Then,316 L stainless steel specimens with defects of different sizes and depths on the subsurface were fabricated using a selective laser melting(SLM);nickel alloy coated-45 steel substrate specimens with blind holes of different diameters were fabricated using a laser cladding;and the defect areas were inspected by the LU detection system,and the maximum amplitude,minimum amplitude,and the peak time distribution of SW were obtained.Finally,the detection accuracy was increased using the wavelet method to process the ultrasonic signals,and the LU C-scan image was smoothed using the image mean filtering,and then the Ostu method was used to determine the threshold and identify the defect area image.In summary,the subsurface defects can be fast and non-contact measured using the LU technology.It provides a potential online inspection method for AM with a certain engineering application value.