Finite Element Simulation Research Based On Early Fatigue Cracks Of Laser Infrared

Based on COMSOL software,this paper studies the laser infrared nondestructive testing(IR NDT)of early fatigue cracks in the coated and uncoated substrates.It is difficult for the penetrating liquid to penetrate into the fine fatigue cracks in the early stage,especially the coated subsurface cracks,and the conventional non-destructive testing methods can’t work.In this paper,it is proposed to use a laser point source as the excitation heat source whose non-contact excitation can adapt to irregular surfaces.The heat formed by the point heat source inside the material conducts along the radius of the hemisphere,and a symmetrical temperature field is easily formed at the excitation interface.When heat conduction is blocked by cracks,even fine cracks,will cause asymmetry of the temperature field within a certain period of time.Reasonable selection of the width of the excitation pulse is beneficial to prolong the duration of this asymmetry,which is convenient for the thermal imager to capture and realize the effective detection.Based on the theory of heat conduction in isotropic materials,the COMSOL software was used to experimentally study the process elements of laser IR NDT affecting fatigue cracks,including: power of excitation laser,pulse width,spot radius,excitation scanning speed,scanning distance,sampling point position,sampling period,thermal imager sampling frequency,and sampling temperature resolution,etc.;Combined with defective cracks of commonly used aluminum alloy surface under1 mm,the specific value range of a set of process parameters was given;The laser IR NDT of fatigue crack was summarized on this basis.Using the researched testing method,the detection of fine cracks from 12 kinds of surface openings with a minimum size of 5μm*1mm*2mm for two kinds of stainless steel and aluminum alloy substrates was realized.There is a temperature difference of more than 1K between the two points on the defective side and non-defective side about symmetrical excitation points for the stainless steel substrate with a coated thickness of less than 0.3mm,12 kinds of defects with a minimum size of 10μm*3mm*5mm were set to simulate fatigue cracks,and the simulation experiments tested the temperature difference of more than 1K between the defective side and the non-defective side of the excitation point symmetry,which showed that the laser IR NDT for early fatigue cracks proposed in this paper is feasible and effective.In this paper,the feature extraction method of temperature gradient across defects is introduced in the processing of precise measurement data,which can further improve the positioning accuracy of defects.