New Experimental Procedure for the Analysis of Micro-Scale Surface Damage at High Temperature

A new experimental procedure has been developed to study micro-crack formation at the surface of compression specimens deformed at high temperature under thermo-mechanical conditions representative of hot forming operations for steels. Damage formation in a Fe-30wt%Ni alloy has been studied in relation to strain distributions measured at the scale of the microstructure using a microgrid technique. 20 μm-pitch microgrids have been deposited at the surface of a tapered specimen designed to generate critical levels of stress triaxiality representative to those leading to edge cracking in hot rolled steel products. The specimen was deformed using a revised plane strain compression test carried out under vacuum in a Gleeble machine at 1000 °C. The grid distortion after the test has then been analysed to produce strain distribution maps which show local strain values ranging from 0 to 75 % for an applied compression of 25 %. Strain localisation has been commonly observed along grain boundaries where damage is initiated at high temperature. Results therefore provide new insight into the early stages of damage formation at the surface of hot rolled products.