Experimental And Numerical Studies On Effect Of Meso-scale Characterization On Mechanical Behaviors Of SiCp/Al Composites

There is mounting experimental evidence that materials whose characteristic length scale associated non-nuniform plastic deformation is on the order of microns or submicrons are very different from conventional ones. In this thesis, SiCp/Al composites were chose as the study objects. Both the experimental and numerical approaches were applied to investigate the effects of meso-scale characterization on mechanical properties of SiCp/Al composites.The thesis begins with the experimental studies of the damage mechanism and dynamic mechanical properties of SiCp/Al composites. The SiCp/Al composites are fabricated by using Hot-Pressing sitering (HP), their dynamic mechanical properties are tested by using split Hopkinson pressure bar and the microstructure characterization before and after the experiment was observed by scanning electron microscope(SEM).The experimental research has three major objectives as belows:(1)The effects of volume fraction of SiC particles on the damage mechanism and dynamic mechanical properties of SiCp/Al composites. (2) The effects of particle size of SiC on the damage mechanism and dynamic mechanical properties of SiCp/Al composites. (3)Under the repeated low-energy impacts, the effects of work hardening, dislocation aggregation, particle fracture and interfacial debonding on the damage mechanism and dynamic mechanical properties of SiCp/Al composites. The experimental results were as follows:(1)SiCp/Al composites with higher content particles have better dynamic mechanical properties, but worse impact resistance toughness. (2)Under impact loading, SiCp/Al composites show high dependence of particle size and SiCp/Al composites with smaller particles have higher flow stress and yield stress. (3)Under repeated impact loading, with the increase of the impact times, more cracked particles can be observed in the composite samples and the debonding between the matrix and the SiC particles was more evident. The impact resistance toughness of SiCp/Al composites became lower. But the dynamic mechanical properties first increased remarkably, then changed relatively gentlely, didn't show a sharp decline.The strain hardening of the matrix may be one key reason. In addition, the accumulation of geometrically neccesary dislocation and the residual compressive strength of cracked partivles may also play a part.Subsequently, a numerical simulation is implemented to study the effects of meso-scale characterization on quasi-static mechanical properties. The materials display significant size effects when its characteristic length scale associated non-nuniform plastic deformation is on the order of microns or submicrons.The classical theories of plasticity cannot explain the observed size dependence of metallic materials since their constitutive models possess no internal material lengths.The Taylor-based nonlocal flow theory of plasticity(TNT) was used to accout for the size dependence of plastic deformation at micron and submicron length scales. This theory links Taylor's model of dislocation harding to a nonlocal theory of plasticity in which the density of geometrically neccesary dislocations is calculated as nonlocal variables express in terms of a weighted averge of plastic strain. In this thesis, a farther development of the common code ABAQUS was implemented to build a new user defining element,which is an eight-node isoparametric element with 9 integral points,so as to study size effects in SiCp/Al composites. The thesis is intended to illustrate the validity of TNT in SiCp/Al composites by contrast with the related foreign documents. Finally, the effects of meso-scale characterization (particle's volume fraction, shape,size and orientation) on the quasi-static mechanical properties were analyzed by a cell model with the user defining element. Given the TNT flow theory's complexity, the numerical simulation on dynamical mechanical properties has not been done and particle fracture and interfacial debonding have not been taken into account in this thesis. The results were as follows:(1)By contrast with experiment, simplified model and MSG theory, TNT can explain the observed size dependence in SiCp/Al composites and accurately predict the mechanical properties of SiCp/Al composites without any material failure. (2)The mean equivalent strain gradient and Von Mises stress show high dependence of particle size in TNT. They increase with the decrease of the particle's size and vary drastically at the particles's size of aboutl1?m. With the increasing of particles'size, the variance is starting to flatten. (3)As predicted by TNT, the effects of meso-scale characterization (particle volume fraction,size, shape, aspect ratio and orientation) on mechanical properties of SiCp/Al composites are regular. The yield strength, flow stress and mean equivalent strain gradient of SiCp/Al composites increase,with the increasing of particles'volume fraction. As the reinforced phase, the angular particles have better enhancement effects. In addition, the greater the aspect ratio is and the closer the orientation of the particles is to the load axis or transverse axis, the higher the strength of SiCp/Al composites.