Research On Fracture Brhavior Of Materials Based On Small Specimen And Cohesive Model

Fracture toughness is an important index for remaining life prediction and safety assessment of in-service equipment and irradiated material.In the case of limited sampling conditions,the fracture toughness cannot be measured by the mechanical tests of conventional specimens,so extrapolation of small specimen is required.There are two types of small specimen: conventional small-scale specimen and small punch specimen.In this paper,the cohesive model was used to simulate the fracture process of these two kinds of specimens,and the fracture toughness parameters of material were obtained by the inverse finite element method based on the experimental results of the conventional small-scale specimens.The optimal design of the small punch specimen was implemented to verify the applicability of the inverse finite element method to the fracture analysis of small punch test.The constitutive behavior and the process of ductile fracture and crack propagation of conventional small-scale specimen can be described by the extended plastic constitutive model and the cohesive model,respectively.The parameters of the extended constitutive model were calibrated by the standard round bar test and the ring-notched round bar test,and two parameters of the cohesive model were calibrated by the ring-notched round bar test and the quasi-static crack growth test of small-scale CT specimen,respectively.The method of calibration adopts the inverse finite element method based on the Nelder-Mead algorithm.For small punch specimen,the stress concentration should be introduced through the ring notch,and the parameters of cohesive model should be extracted by the inverse finite element method.The notched specimen needs to be geometrically optimized to improve the calculation convergence of the cohesive model and the sensitivity of the load-displacement curve at the stage of fracture damage to the parameters of the cohesive model.On this basis,the simulated load-displacement curve with the selected parameters was taken as the approached target,and the genetic algorithm and random walk algorithm,which were easy to obtain the global optimal solution,were used for iterative fitting by the inverse finite element method to extract the parameters of cohesive model.The calculated results show that the error between the obtained parameters and the pre-selected parameters is less than 1%,which verifies the sensitivity of the specimen design and the accuracy of inverse finite element method.