Chin Liu strain rate sensitivity index–strain equation of superplasticity and influence of specimen geometry

Abstract

The influence of specimen geometry on the strain rate sensitivity index–strain (m–δ) relationship has been determined for a Zn–5Al (wt-%) eutectic alloy at a strain rate ɛ of 0·025 min−1 and a temperature of 250°C. It was found that all the m–δ curves were of the type mL=mmax>m0 and > mF for δ0 < δL < δF, where mL is the strain rate sensitivity corresponding to the ‘limit strain’ δL; δ0 (=0) is the initial strain; and δF is the total elongation at fracture. Tests were performed for five values of specimen width/length w0/l0 between 0·15 and 0·39. With increase in w0/l0, it was observed that the m–σ curves shifted to higher positions; the values of total elongation at fracture δF and strain rate sensitivity index at zero strain (δ0) m0 and at the limit strain (δL) mL increased; and the strain rate sensitivity index at fracture strain (δF) mF varied little. Consistent with these findings, increasing the thickness of the specimen from 1 to 2 mm at constant w0/l0 caused a shift in the m–σ curve to the right and increased the value of δF and also the limit strain at maximum value of m. From the m–δ data obtained, a modification to the Chin Liu equation is proposed to take into account specimen geometry δF=(C3,F(m0′− mL′)/(w0/l0)ɛmF−1[ (m0′(w0/l0)]−1)100%, where C is the slope of the m-δ curve; (m0′(w0/l0)=m0, m0′>m0 and (mL′(w0/l0)=mL, mL′>m0 for w0/l0 between 0·15 and 0·39. When w0/l0=1(or w0=l0), the notch effect is eliminated and the total elongation at fracture will take its maximum value.MST/1034