Shiguo Long

Indentation depth dependence of the mechanical strength of Ni films

The indentation depth effect has been systematically examined on the mechanical properties of electrodeposited nickel films under 0% and 10% tensile strains. It is found that the indentation depth is proportional to the square root of the loads applied and the depth profiles of hardness and elastic modulus follow the similar trend of change with maximal values at the surface skins. The hardness and modulus then attenuate to a value of about half of the maximum, which follows the model proposed by Graca et al., Surf. Coat. Technol. (in press) with the mechanism of geometrically necessary dislocations and surface free energy. We suggest that the effect of surface oxidation and surface bond contraction [C. Q. Sun, Prog. Solid State Chem. 35, 1 (2007)] contributes intrinsically to the anomalous skin strengthening because of the local strain and energy trapping caused by surface bonds breaking.

Thermal failure mechanism and failure threshold of SiC particle reinforced metal matrix composites induced by laser beam

Abstract The failure of particulate-reinforced metal matrix composites (MMCs) induced by laser beam thermal shock is experimentally and theoretically studied. It is found that the initial crack occurs in the notched-tip region, wherein the initial crack is induced by void nucleation, growth and subsequent coalescence in the matrix materials or interface separation. However, crack propagation occurs by fracture of the SiC particle and it is much different from the crack initiation mechanism. The damage threshold and complete failure threshold can be described by a plane of applied mechanical load σ max , and laser beam energy density E J . A theoretical model is proposed to explain the damage/failure mechanism and to calculate the damage threshold and complete failure threshold. This model is based on the idea of stress transfer between a reinforced-particle and the matrix, as well as the calculation of the applied mechanical stress intensity factor and local thermal stress intensity factor. In order to check the validity of the theoretical model, the finite element simulations are carried out for the temperature field induced by laser heating, the stress fields induced by the combined laser heating and applied mechanical tensile load. Both the theoretical model and the finite element simulation can explain the experimental phenomenon. The theoretical model can predict the damage threshold and failure threshold. The failure of MMCs induced by laser thermal shock and applied mechanical load is non-linear.