The Research On Mechanical Properties Of Particle Reinforced Composites Considering Size Effect And Micro-damage Mechanism

Particle reinforced metal matrix composites are ductile metal matrix consisting of stiff but brittle particulate reinforcements which enhance their strength and modulus,so that better overall performance can be obtained.The strengthening effect of particlereinforced metal matrix composites stems from the combined effect of direct strengthening mechanisms and indirect strengthening mechanisms.Among them,the indirect one presents obvious size effect.Up to now,the understanding of how factors including microstructure,interface properties and damage mechanisms effect macro composite properties under indirect strengthening mechanism remains to be further studied.To this end,mechanical properties of particle-reinforced metal matrix composites with indirect strengthening mechanism are studied in this thesis,the effects of microstructure features and interface characteristics under interface damage mechanism are investigated at first,after that the effects of the particle size and it's strength under coupling damage mechanism are also analyzed.The main content of the thesis includes four aspects as below:Firstly,the dual indirect mechanisms including quenching hardening and strain gradient strengthening is introduced to an elastoplastic constitutive model to realize the size effect of macro composites performance.Next,the RVE based FE method is used to predict the macro composites behavior under uniaxial tensile loading.The RVE model is three-dimensional and containing multiple particles.In order to study the mechanisms of interface damage and the mixed-mode damage mechanism(both at interfaces and inside particles),a bilinear cohesive model is applied to interface layers to simulate interface damage,and a continuum damage model is used to characterize brittle damage related to particle fracture.The predicted equivalent responses agree well with experimental results,which verifies the model's predictive ability for the size-related macro composites behavior;besides,the simulation results present that the interfacial damage develops from both ends of the particles parallel to the tensile direction to the middle part.Then the result of RVE models with no internal damage shows that strain gradient strengthening effect is stronger in RVE with smaller particles.Then,based on the validated model with interface damage mechanism,a comprehensive analysis is conducted from the two aspects of microstructure features(particle size and particle volume fraction)and interface characteristics(interface strength and interface fracture energy).The strength and ductility are improved simultaneously with decreasing particle size,while increasing particle volume fraction results in higher strength and worse ductility.The interface strength casts an evident influence on the macroscopic composites behavior at plastic stage,whereas the effect of the interface fracture energy is only reflected at the end of composites response.With increasing interfacial strength,the initiation and development of interfacial damage get slower;the interfacial damage develops faster with lower interfacial fracture energy,while the damage initiation is not affected.Finally,comparison between the prediction of coupling damage model and that of interface damage model presents that results of model with the former one agree better with the experiment results.Under the coupling damage mechanism,the two damage modes are competing and affecting the evolution of each other.Under this mechanism,the particle strength and the interface strength exert joint effort to determine the dominant damage mode.The innovations of the thesis are concluded below: indirect strengthening mechanism are introduced in matrix's constitutive model to realize the size effect of macro composites performance by writing UMAT subroutine;the subroutine is highly practical,as it adapts to hexahedral mesh as well as tetrahedral mesh;introducing particle fracture to interface damage mechanism bring about better predictability.This study explored the design space of particle-reinforced composite materials comprehensively,and provides reference value for the following opt imal design research.