Chi Feng Lin

High strain rate shear deformation and fracture behaviour of biomedical titanium alloy

Abstract The rate and temperature dependent shear deformation behaviour of biomedical titanium alloy (Ti–15Mo–5Zr–3Al) is investigated using a torsional split Hopkinson pressure bar at strain rates ranging from 1·2 × 103 to 2·8 × 103 s–1 and temperatures of –150, 25 and 300°C. It is found that both the strain rate and the temperature have a significant effect on the high strain rate shear properties and fracture characteristics of the alloy. As the strain rate is increased or the temperature is reduced, the flow stress, workhardening rate, strain rate and temperature sensitivity increase, but the activation volume and activation energy decrease. The fracture strain increases with both increasing strain rate and increasing temperature. The shear flow behaviour of the Ti–15Mo–5Zr–3Al specimens is accurately described by the Kobayashi–Dodd constitutive equation. Metallographic observations reveal that the failure of the present Ti–15Mo–5Zr–3Al alloy is dominated by intensive localised shearing. Moreover, the SEM fractographs show that the fracture surfaces are characterised by a dimple-like structure. The dimple density increases with an increasing strain rate or temperature. By contrast, the dimple size reduces at higher strain rates and temperatures and gives rise to a significant improvement in the fracture resistance.

Dynamic Mechanical Response of Biomedical 316L Stainless Steel as Function of Strain Rate and Temperature

A split Hopkinson pressure bar is used to investigate the dynamic mechanical properties of biomedical 316L stainless steel under strain rates ranging from 1 × 103 s−1 to 5 × 103 s−1 and temperatures between 25°C and 800°C. The results indicate that the flow stress, work-hardening rate, strain rate sensitivity, and thermal activation energy are all significantly dependent on the strain, strain rate, and temperature. For a constant temperature, the flow stress, work-hardening rate, and strain rate sensitivity increase with increasing strain rate, while the thermal activation energy decreases. Catastrophic failure occurs only for the specimens deformed at a strain rate of 5 × 103 s−1 and temperatures of 25°C or 200°C. Scanning electron microscopy observations show that the specimens fracture in a ductile shear mode. Optical microscopy analyses reveal that the number of slip bands within the grains increases with an increasing strain rate. Moreover, a dynamic recrystallisation of the deformed microstructure is observed in the specimens tested at the highest temperature of 800°C.