Measurement And Analysis Of The Fatigue Crack-tip Deformation Field Under Resonant Loading Based On DIP And DSCM

Fatigue failure is the most common form of mechanical parts failure,the study of fatigue crack initiation,propagation and fracture characteristics is of great significance for the safety and reliability of mechanical products.The fracture characteristics of materials are mainly determined by the deformation field of crack tip(crack propagation path,direction,displacement field,strain field and crack tip plastic zone).The electromagnetic resonant fatigue crack growth test system has been widely used in various material laboratories for material fatigue test to study the fracture characteristics of material,because of its high working frequency,low energy consumption,short test time and good test waveforms,etc.However,because the high-speed vibration of specimen causes the propagation effect of stress wave and the inertia effect,which makes the theoretical calculation of displacement and strain field of crack tip complicated.So no effective method has yet been found to solve it.Accordingly,this paper combines the macroscopic fatigue crack detection method based on digital image processing(DIP)and the full-field and non-interference digital speckle correlation method(DSCM)to solve the measurement problem of fatigue crack growth path,size,and deformation field of fatigue crack tip under high-frequency resonant loading.And on this basis the shape and size of crack tip plastic zone are estimated,and the deformation field of fatigue crack tip is analyzed.The main work of this paper is as follows:1.The deformation field measurement system of fatigue crack tip under high-frequency resonant loading is established,including electromagnetic resonant high-frequency fatigue test system,online detection system of specimen fatigue crack and online acquisition system of specimen speckle image,and a matching method for specimen speckle images of feature loading position is proposed.2.A detection algorithm of specimen fatigue crack based on DIP method is proposed.This method determines the crack initiation point by template matching and corner detection,locates the crack area according to the gray mean square deviation of the specimen crack image,realizes image segmentation using adaptive threshold segmentation method,extracts the exact crack contour based on connected domain and smoothing and denoising algorithm,and finally the crack length and crack tip position are obtained.3.The basic principle and deformation-field measurement method at crack tip of DSCM are introduced,and the deformation field of crack tip under different loading cycles is calculated and verified.Aiming at the discontinuity region at the crack tip,the computing strategy based on reliable path guidance is adopted.Taking into account the accuracy and speed of computation,in DSCM the inverse compositional Gauss-Newton method algorithm is applied to calculate sub-pixel displacement,and the subset least squares plane fitting method is used to calculate strain.4.Based on the theoretical analysis of crack tip plastic zone,a method for estimating the shape and size of crack tip plastic zone based on Von Mises yield criterion and strain field data obtained by DSCM is presented.And the results are verified by the finite element method.5.The resonant fatigue crack growth test is carried out.Then the macroscopical size of fatigue crack(crack growth path,crack length,crack tip position)is measured under high-frequency resonant loading and crack steady growth stage.The deformation field of crack tip within one loading cycle and the deformation amplitude field of crack tip with different crack lengths are measured.And finally their change law are analyzed and studiedThe results of this paper can lay a foundation for further study on the mechanism of fatigue crack growth and the measurement of extended parameters under high-frequency resonant loading,which has theoretical and practical value.