Experimental And Numerical Studies On The Dynamic Mechanical Properties Of Particle Reinforced Metal-Matrix Composites

Particle reinforced metal matrix composites (PRMMCs) are getting increasing attention in a great many fields because of their excellent mechanical properties, such as high specific stiffness, high specific strength, good wear resistance and good impact resistance. These properties make PRMMCs popular in fields like aerospace, automobile, electrical engineering and military etc. The deformation and damage mechanisms of PRMMCs under dynamic load conditions are very complicated, and the studies about them are important for the improvement of their dynamic mechanical properties and their applications.In this thesis, the dynamic properties of PRMMCs are studied by both experiments and numerical simulations. Different computational micro-mechanical models are built to investigate the effects of the particle's shape, distribution, orientation and the particle/matrix interface on the deformation, damage and dynamic mechanical properties of PRMMCs.The thesis begins with the experiment of the deformation mechanism and dynamic mechanical properties of SiC particle reinforced aluminum (SiC_p/Al) composites. The SiC_p/Al composites are fabricated by using spark plasma sintering (SPS), and their dynamic mechanical properties are tested by using split Hopkinson pressure bar (SHPB). The experiment focuses on the deformation mechanism and mechanical properties of the SiC_p/Al composites in relation to the process of powder mixing, SiC particle's size and volume fraction under the quasi-static and dynamic load conditions. The results show that the powder mixing processes significantly affect the micro-structures, interfacial strength and mechanical properties of the SiC_p/Al composites. The yield strength, dynamic flow stresses and strain rate sensitivity of the SiC_p/Al composites increase with the increase of the particle's volume fraction. Under the dynamic load conditions, due to the damage and fracture of SiC particles, debonding of the interfaces and the adiabatic temperature rising, strain softening occurs with major deformation. With the decrease of average particle's size, the yield strengths and dynamic flow stresses of the SiC_p/Al composites increase significantly, while their strain rate sensitivity decreases, and the strains at which the strain softening happens are reduced.Based on the experiments, two different computational micro-mechanical models are developed to predict the damage mechanisms and dynamic mechanical properties of PRMMCs. One is an artificial multi-particles model with the particles randomly and discontinuously distributed in the matrix, and the other is based on the representative actual micro-structure cut out of the SEM micrograph. In order to investigate the effect of the particle/matrix interface on the predicted results, the bonding between the matrix and the particles are simulated by three different algorithms, namely perfect interface, interlayer element interface and tied the matrix edge to the particle edge interface. In the models for the tensile simulations, the initialization and propagation of the cracks are simulated by two different algorithms, element erosion and failure of the tying of the coincident nodes.Subsequently, lots of numerical tests are carried out using the artificial multi-particles models. The predicted results by different interface models are compared with each other, and so are the predicted results by the models with different tensile crack modes. The effects of the mesh size, interface strength, particle's number and distribution on the predicted results are studied. The validity of different micro-structure models and material models is verified by comparing the predicted results by these models with those from the experiments. The findings indicate that under the dynamic compressive loadings, the micro-structure models with elastic ceramic model overestimate the flow stresses of PRMMCs at different strain rates, while the predicted stress-strain curves by Johnson-Holmquist ceramic model are quite consistent with experimental results. In the models based on SEM micrograph of PRMMCs, as the particles' shapes are highly irregular and their distribution is also very complicated, the constraint between the matrix and particles are very strong. As a result, the yield stresses and flow stresses predicted by these models are higher than those predicted by the micro-structure models with the regular particles. Under the dynamic tensile loadings, different micro-structure models can correctly predict the elastic properties and the initial yield stress of PRMMCs, and they also can better simulate the effect of the interface strength and particles' distribution on the fracture surface.In the last part of the thesis, the effects of the particle's volume fraction, shape and orientation on the dynamic compressive properties of PRMMCs are analyzed by the artificial multi-particles models with perfect interface between the matrix and the particles. The results show that the yield strength, flow stress and the strain rate sensitivity of PRMMCs increase with the increase of the particle's volume fraction, and the increase shows more clearly with the angular particles. The closer the orientation of the particles is to the load axis, the higher the strength of PRMMCs and the more serious the damage of the particles at large deformation. The particle's damage can reduce the strain hardening and strain rate sensitivity of PRMMCs. The strain rate sensitivity and strength of PRMMCs are also much higher in the model with the particle vertical to the load axis.