Study on the Fracture Mechanisms of a Grade 91 Steel at 600°C via Interrupted Miniature Tensile Testing

The study of the deformation and fracture mechanisms of power plant materials at elevated service temperature is of utmost importance in structural integrity assessment. In this work, interrupted miniature uniaxial tensile tests (MUTT) have been conducted to investigate the fracture evolution mechanisms for a Grade 91 steel at 600°C. The engineering stress–strain curves of the MUTT can be divided into six different stages, including the elastic stage, strain hardening stage, homogeneous plastic deformation stage, necking across the transverse and thickness direction stage, and finally, the fracture propagation stage. In addition to full history tests, MUTT were interrupted in each of the different stages to study the entire failure process evolution from the uniform deformation, necking to specimen separation, using optical microscopy and Scanning Electron Microscope (SEM). It has been shown that the necking of the interrupted testing specimens appears at Stage IV across the width direction prior to the thickness direction at Stage V. The thickness direction necking would lead to fracture rapidly. The void coalescence directly caused by damage has been observed in Stage VI.