Diffusion and Flow in Impression Creep Testing

Some steady state vacancy diffusion problems caused by impression loading were analyzed. It was found that the impression velocity is proportional to the punching stress and the self-diffusivity of atoms for impression creep of a thin film by a flat end straight or cylindrical punch. For the same punching stress it is proportional to the thickness of the film and inversely to the square of the punch width or diameter for the film deposited on a rigid and impermeable substrate. Without the substrate, the impression velocity is inversely proportional to the thickness of the film but independent of the punch dimension. This analysis gives a simple way of detecting film decohesion from its substrate. For the situation of low Reynolds numbers, the viscous flow induced by a flat-end straight/cylindrical punch impressing into a half space was analyzed. It is shown that the impression velocity is proportional to the punching stress and the punch width/diameter, and inversely proportional to the viscosity. This result suggests a simple way of measuring the viscosity. Impression creep and stress relaxation experiments on a Sn-Pb eutectic alloy were carried out in the RSA II modified to use a 0.5 mm diameter cylindrical punch under 1.5 to 47 MPa punching stress and within a temperature range between 25^circC and 110^circC. Using the hyperbolic sine function between the impression velocity and the punching stress, a single activation energy, 55 kJ/mole was obtained. Based on the experimental data, a single mechanism of interfacial viscous shearing between the two eutectic phases is proposed for both creep and stress relaxation. The impression creep of a half-space with the Eyring hyperbolic sine constitutive equation was simulated by finite element analysis using the ABAQUS program. It turns out that the impression velocity is also a hyperbolic sine function of the punching stress. Application of this result to the Sn-Pb experiment shows that the elementary process of interface shearing also obeys the hyperbolic sine law.