FRACTURE OF 30CRMNSIA STEEL UNDER MIXED LOADING MODES

Abstract

Tests were conducted on notched specimens made of 30CrMnSiA steel in loading mode I with the fracture surface perpendicular to the loading direction (90°) and in mode II with shear angles of 45° and 15°. Shear loading was performed using Richard’s grips that allow for a transition from pure mode I to pure mode II by varying the angle of the notch in relation to the direction of the tensile load. Using digital image correlation, the acoustic emission (AE) signals and the strain field were measured during testing, and the damage and microhardness of the polished side face of the specimens were evaluated after failure. The macro- and microscopic research of fracture surfaces revealed that an increase in the shear component during tension leads to changes in the mechanical and acoustic properties and the ductile-to-brittle-transition temperature. This is evidenced by a change of the fracture surface morphology from ductile to brittle at 45° loading direction, accompanied by a change in all estimated acoustic parameters (the total number of events, the angular coefficient of the cumulative distribution of acoustic signals by amplitude-b-value, and the activity of AE signals). An increase in the shear component also leads to a nonlinear dependence of the damage parameters on the load application angle, such as the relative microcrack area S*, average microcrack length lav, and microcrack inclination angle with respect to the loading axis, which correlate with the change in the principal strains estimated by digital image correlation.