Micro-indentation tests at scales on the order of sub-micron have shown that the measured hardness increases strongly with the indent depth or indent size decreasing, which is frequently referred to as the size effect. However, the trend is at odds with the size-independence implied by conventional elastic-plastic theory. In this paper, strain gradient plasticity theory is used to model the size effect for materials undergoing the micro-indenting. Meanwhile, the micro-indentation experiments for single crystal copper and single crystal aluminum are carried out. By the comparison of the theoretical predictions with experimental measurements, the micro-scale parameter of strain gradient plasticity theory is predicted, which is fallen into the region of 0.8—1.5 micron for the conventional metals such as copper (Cu), aluminum (Al) and silver (Ag). Moreover, the phenomena of the pile-up and sink-in near micro-indent boundary are investigated and analyzed in detail.
Based on the microscopic observations and measurements, the mechanical behavior of the surface-nanocrystallized Al-alloy material at microscale is investigated experimentally and theoretically. In the experimental research, the compressive stress-strain curves and the hardness depth curves are measured. In the theoretical simulation, based on the material microstructure characteristics and the experimental features of the compression and indentation, the microstructure cell models are developed and the strain gradient plasticity theory is adopted. The material compressive stress-strain curves and the hardness depth curves are predicted and simulated. Through comparison of the experimental results with the simulation results, the material and model parameters are determined.
The influences of particle size on the mechanical properties of theparticulate metal matrix composite are obviously displayed in the experimental ob-servations. However, the phenomenon can not be predicted directly using the conven-tional elastic-plastic theory. It is because that no length scale parameters are involvedin the conventional theory. In the present research, using the strain gradient plas-ticity theory, a systematic research of the particle size effect in the particulate metalmatrix composite is carried out. The roles of many composite factors, such as: theparticle size, the Young’s modulus of the particle, the particle aspect ratio and vol-ume fraction, as well as the plastic strain hardening exponent of the matrix material,are studied in detail. In order to obtain a general understanding tor the compos-ite behavior, two kinds of particle shapes, ellipsoid and cylinder, are considered tocheck the strength dependence of the smooth or non-smooth particle surface. Finally,the prediction results will be applied to the several experiments about the ceramicparticle-reinforced metal-matrix composites. The material length scale parameter ispredicted.
The interface adhesion strength(or interface toughness)of a thinfilm/substrate system is often assessed by the micro-scratch test.For a brittle filmmaterial,the interface adhesion strength is easily obtained through measuring thescratch driving forces.However,to measure the interface adhesion strength(or in-terface toughness)for a metal thin film material(the ductile material)by the micro-scratch test is very difficult,because intense plastic deformation is involved and theproblem is a three-dimensional elastic-plastic one.In the present research,usinga double-cohesive zone model,the failure characteristics of the thin film/substratesystem can be described and further simulated.For a steady-state scratching pro-cess,a three-dimensional elastic-plastic finite element method based on the doublecohesive zone model is developed and adopted,and the steady-state fracture workof the total system is calculated.The parameter relations between the horizontaldriving forces(or energy release rate of the scratching process)and the separationstrength of thin film/substrate interface,and the material shear strength,as well asthe material parameters are developed.Furthermore,a scratch experiment for theAl/Si film/substrate system is carried out and the failure mechanisms are explored.Finally,the prediction results are applied to a scratch experiment for the Pt/NiOmaterial system given in the literature.
