Study On Fracture Initiation And Propagation Behavior Under Mixed Mode Dynamic Loading

Fracture initiation and propagation for mixed mode cracks under dynamic loading is now become one of the frontier topics of fracture mechanics.In-depth research on this issue is helpful for the mechanical properties,damage and damage and safety assessment of engineering structures such as spacecraft,aircraft,ships,and bridge under dynamic loads,and provides an important basis for the design of these structures.At present,the research on the initiation and propagation criteria of mixed-mode cracks is not sufficient because of the lack of suitable dynamic fracture criteria.The fracture mode of specimens cannot be accurately controlled owing to the insufficiency of dynamic loading technique.In addition,the factors which influence the solution of dynamic stress intensity factors,especially T-stress,have not been studied in-depth.Therefore,it is of significance to conduct research on crack initiation and propagation under mixed mode dynamic loading.Based on the above points,the following work has been carried out in this paper:Fracture mode mixity is one of the basic parameters in the formula of fracture criteria.Aiming at the disadvantage of split Hopkinson bar system that the fracture mode mixity can not be controlled,a novel technique that achieves constant fracture mode mixity during dynamic loading is proposed in this paper.A modified CTS(MCTS)specimen has been proposed by trimming the edges of CTS specimen according to the loading angle,and the clamps and constraining structures for MCTS specimen are also designed.By applying lateral displacement constraints on the edges of the MCTS specimen,the pure mode II loading and controllable fracture mode mixity loading is achieved.Numerical simulation of all-fracture-mode(AFM)specimen,CTS specimen and MCTS specimen loaded by SHTB apparatus has been carried out using ABAQUS.The variation of the dynamic stress intensity factor and fracture mode mixity of AFM specimen,CTS specimen and MCTS specimen are studied.The finite element analysis results indicate that dynamic loading with constant fracture mode mixity can be achieved using MCTS specimen applying constraints of transverse displacement.The numerical results demonstrate that the average value of the dynamic stress intensity factor ratio of the MCTS specimen with lateral displacement constraints during loading is approximately equal to the tangent of the loading angle.It isproved that the pure mode II and the mixed-mode loading with controlled mode mixity can be achieved using MCTS specimen,but the AFM specimen and the CTS specimen cannot.A comprehensive discussion was carried out about the related problems of dynamic stress intensity factor calculation and solution,and the influence of various factors on the solution of dynamic stress intensity factors is revealed.The advantages and disadvantages of the empirical formula method,the relative displacement method,the J-integral method,and the strain gauge method in the calculation of the dynamic stress intensity factor are discussed and their applicability are compared,the path dependence of the J-integral is also discussed for the mixed-mode fracture problems;Comprehensive consideration of calculation accuracy,test cost and other factors,the dual strain gauge method based on asymptotic solution of strain field is suitable for the calculation of dynamic stress intensity factors.The principle of the dual strain gauge method is to solve a system of linear equations that contain strain components.This relationship between the condition number of the coefficient matrix and the pasting position of the strain gauge has been revealed and the effect of random error on the dynamic stress intensity factor solution relate to the order of the condition number of the coefficient matrix is also pointed out;The numerical simulation method was used to analyze the error of the asymptotic solution and numerical solution of the crack tip strain field,and the appropriate position for attaching the strain gauge was pointed out.The normalized Tstress of the MCTS specimens under different loading angles is calculated,and the asymptotic solution of the crack tip strain field is also modified considering the effect of T stress.The accuracy of dynamic stress intensity factor calculation is relatively improved after modification.Multi-angle mixed mode loading tests of MCTS specimen are carried out using SHTB apparatus.The fracture initiation and propagation behavior of brittle materials represented by PMMA under mixed-mode dynamic loading was studied.Four fracture criteria including T-terms(GMTS criterion,EMTSN criterion,GSED criterion and GMERR criterion)were applied to predict the fracture initiation angles of MCTS specimen.By comparing the prediction of various fracture criteria with the experimental results,the feasibility of these fracture criteria in prediction of dynamic crack initiation and propagation behavior is demonstrated and the validity of the dynamic mixed-mode loading technique based on SHTB apparatus and the dual strain gauge method for calculating the dynamic stress intensity factoris also verified.The experimental results indicated that the prediction errors of the four fracture criteria for the crack initiation angle are all within a reasonable range.The GSED criterion has the smallest average error,and the GMTS criterion has the lowest variance.