Fracture initiation in ductile metals

A major research and development programme that began more than a decade ago and is still underway at the Defence Metallurgical Research Laboratory seeks to, on the one hand, investigate certain aspects of ductile fracture in iron and binary iron solid solution alloys and, on the other, develop a suitable low alloy ultra-high strength (yield strength better than 1500 Mpa) steel with relatively high fracture toughness (fracture toughness better than 80 Mpa root m). Fracture toughness measurements were carried out on Armco Fe with varying grain size and at different temperatures and on a whole series of iron alloys: Fe-Si, Fe-Mo, Fe-Ni, Pe-Co, Fe-Cr, Fe-C, Fe-C-Ni, Fe-C-Co, Fe-C-Ni-Si-Cr, Fe-C-Ni-Si-Cr-Co, Fe-C-Ni-Si-Cr-Mo and Fe-C-Ni-Si-Cr-Mo-Co. Through scanning electron metallography, measurements have been made of the characteristic distance of the void ahead of the blunted crack tip and the stretch zone,width to determine the crack tip opening displacement, Using an approach involving these measurements and the plastic flow related energy dissipation in the HRR (Hutchinson-Rice-Rosengren) zone beyond the process zone and the calculation of J(Ic) in terms of the critical strain model originally developed by Rice and Johnson and Ritchie and Thompson, it is shown that measured J(Ic) covering a wide range (100-300 kJ/m(2)) can be estimated, The NiSiCrMoCo steel has been now produced on tonnage scale at MIDHANI, Hyderabad, Following the development of welding consumables and evaluation of the weld joint efficiencies, the use of this structural steel for high technology applications is currently under progress.