Study On Fracture Simulation Method By Discretized Virtual Internal Bond Accounting For Elastoplastic And Thermal Effect

The fracturing behavior of engineering materials significantly impacts on the engineering stability and security.Hence,how to effectively simulate fracture has been a central issue in the practical engineering.The fracturing behaviors present multiscale features.The discretized virtual internal bond(DVIB)considers material to consist of bond cells on mesoscale.Each bond cell can take any geometry with any number of bonds,which makes DVIB enable to capture the meso-structural characteristics of material.Initial DVIB only considers the hyperelasticity of material.To make DVIB applicable to more cases,the present thesis extends the DVIB to the elastic-brittle,elastic-plastic and thermal-mechanical coupling cases.The strain energy density(SED)was used as the criterion to characterize stable and unstable crack growth in the fracture mechanics of linear and non-linear constitutive relations.It applies equally well for continuum mechanics problems in general.It is free from the restrictions of theories that implicate linear superposition.The energy density combines strain and stress and is not exclusive to strain alone nor stress alone.“Energy” and “density” for a point like element are the basic ingredients.The element can be microscopic and visualized as atoms or molecules.In this sense,the energy density criterion can be applied to the lattice structure and the results can also predict macro fracture.In this work,the limit strain of the micro bond is related to the lattice size.Bond rupture is associated with the strain and strain energy density factor as the failure criterion.The results show that the smaller the lattice is,the larger the limit bond strain.The simulation results show that the lattice structure can be modeled by the energy density and scaling of strain.And the simulation results are almost lattice size free.The method can be applied to develop scale shifting laws in general.A hyperelastic-bilinear potential(HBP)is proposed for lattice model to preserve the fracture energy in fracture simulation.This potential inherits the essence of hyperelastic interatomic potential in that it can capture the hyperelastic behaviors of material at the crack tip which plays a critically important role in dynamic fracture.Moreover,the potential retains the essence of the bilinear cohesive law in that it can preserve the fracture energy by setting a limit on the bond strain and reflect the strain softening behavior of materials.With this HBP,the lattice size sensitivity is eliminated to a great extent and the dynamic fracture can be more accurately simulated.The plastic deformation is always localized prior to fracture for ductile materials.The plastic deformation and fracture process should be uniformly considered.To unify the two processes together on the micro bond level,a plasticity-fracture-embedded bond potential is proposed for lattice model.With this bond potential,the DVIB model can capture the plastic deformation and simulate the plastic fracture propagation of materials.The simulation results are almost independent of mesh size due to the conservation of fracture energy.The unified bond potential stabilizes the lattice model and makes the simulation results more reliable in plastic fracture simulation.It provides a more straightforward,simple and efficient method for plastic fracture simulation.The thermal-mechanical coupled DVIB model is developed to simulate the fracture problem subjected to the thermal-mechanical coupled field.To enable DVIB to simulate the thermal conduction process,each bond is taken as the thermal channel.The micro bond thermal conductivity and heat capacity coefficient are calibrated based on a volume equivalence approach,which is unnecessary to consider the geometry details of cell.The relationship between the macro and the micro thermal parameters are derived.The thermal effect is incorporated into the mechanical process by means of bond deformation decomposition,where the bond deformation is decomposed into the mechanical and the thermal part.Through the bond potential,which characterizes the interaction between particles,the thermal effect on the mechanical response of material is accounted.The simulation results demonstrate that the present method can simulate the fracture behaviors of material subjected to the thermal-mechanical coupled field with very high accuracy.Because both the thermal and mechanical field simulation are based on the common discrete lattice structure in DVIB,it is highly efficient and straightforward to deal with the thermal-mechanical induced fracture problem.In addition,the thermal parameter calibration method makes the DVIB highly flexible.The present thesis extends the application of discretized virtual internal bond model into elastic-brittle,elastic-plastic and thermal-mechanical coupling cases.The conclusions drawn in this thesis is of significant importance for practical engineering analysis.