Multiscale Mechanical Analysis Of Fracture Properties Of Cracked Specimens Repaired By Laser

Fracture is one of the most common failure modes in engineering structures,which occurs suddenly and may cause catastrophic failure.Cracks repaired at early stage of propagation provides substantial savings both for resources and economy.Cracks can be repaired by utilizing high-powered laser beam.In this paper,the effect of micro/nano materials addition and laser process parameters on the laser repair effect of 304 stainless steel compact tension specimen was studied.Variable-node finite element based trans-scale calculation method for polycrystalline materials is established,and multiscale analysis of microstructural effects on the fracture properties of laser repaired cracks is proceed from macro-scale and meso-scale.The main research contents and results are as follows:(1)Digital image correlation method combined with J-integral theory is used to measure and calculate J-integrals of the repaired specimens.Fracture properties of the specimens repaired with different weight fractions of nano WC are compared and studied.The residual stress of specimens after being repaired by laser was also studied to analyze the feasibility of J-integral to evaluate the fracture properties of the repaired specimens.It is found that the influence of residual stress can be neglected when calculating J-integral in a certain region in case of when the residual stress is small.The J-integral obtained by the digital image correlation measurement is accurate and can effectively evaluate laser repairing effects.(2)Variable-node finite element based trans-scale calculation method for polycrystalline materials is proposed to investigate the interaction between mechanical properties at macroand meso-scale.The model analyzes the interaction between macro-scale factors such as structure and loads,and meso-scale factors such as polycrystalline geometry,size and distribution.Variable-node finite element was created by user subroutines of the ABAQUS.Also Voronoi diagram based polycrystalline finite element was realized by MATLAB.The crystal plasticity finite element and the conventional finite element were connected using the variable-node finite element.The feasibility of the model were verified with a laser repaired compact tension specimen.The number of degrees of freedom of variable-node finite element based trans-scale calculation model for polycrystalline materials is much smaller than that of the model with the fine-scale elements,but the reduction in calculation accuracy is negligible.The model can be used to study the mechanical behavior of heterogenous materials simultaneously by combining meso-scale and macro-scale with a smaller amount of computation.(3)The change of mechanical properties of laser repaired layer was studied by indentation,and crystal plasticity constants were reversed by nanoindentation.In the process of extracting the crystal plastic material constants of the laser repaired layer,the conventional finite element model of nanoindentation was established to obtain the macroscopic elastic-plastic parameters of the material,and the experimental load-displacement curves were modified by the pileup/sink-in parameters.Then,crystal plasticity material parameters were reversed by crystal plastic finite element model of tensile specimen.This method can be used to extract crystal plasticity material parameters at small computational cost with high accuracy.(4)Variable-node finite element based trans-scale calculation method for polycrystalline materials was used to study the influence of different microstructure characteristics on the fracture properties of the laser repaired specimens.J-integral of different repaired specimens were calculated by the crystal plasticity material constants of the repaired layer calculated by nanoindentation test and also DIC-based J-integral calculation software.Based on the experimental and computational results,the effect of microstructure on the fracture properties of the repaired layer is studied.The results show that,in the process of crack being repaired by laser,the grain size of the repaired layer is greatly reduced compared with the substrate,so the grain size has a greater impact on the fracture properties.Within a certain range,reducing the grain size and increasing the volume fraction of the intragranular second phase can improve the fracture properties of the repaired specimens,while the existence of pores will greatly reduce the fracture properties of the repaired specimens.These results clarify the effect of microstructure on the fracture properties of laser repaired specimens,and provide a theoretical supports for laser repair of crack with micro/nano materials addition.