Damage analysis during hot deformation of a resulfurised stainless steel

Ductile damage evolution and fracture of a resulfurised stainless steel (AISI 303) were analysed by high temperature tension tests. Void nucleation, growth and coalescence were investigated. The existence of two distinct void coalescence mechanisms has been demonstrated. High temperature damage models assume the existence of a critical value of the void volume fraction at the onset of void coalescence. In the present work, experimental evidence of the existence of two threshold fractions: (i) a critical value of the void volume fraction f crit at the onset of void coalescence and of (ii) a critical value of the void volume fraction at final failure f fail is given. Values of the threshold f crit were measured by high temperature tension tests on axisymmetric smooth and notched specimens and values of the threshold f fail on deeply notched specimens. It has been shown that f crit and f fail are physically meaningful concepts. Two simple criteria for the determination of f crit and f fail are given. f crit corresponds to the sudden increase of the void volume fraction in the necking area of tensile specimen at the transition from non-catastrophic to catastrophic void coalescence. f fail corresponds to the maximum void volume fraction in front of a moving crack.