Mechanical Behaviour and Failure Mode of High Interstitially Alloyed Austenite under Combined Compression and Cyclic Torsion

Abstract

multiaxial stress states frequently occur in technical components and, due to the multitude of possible load situations and variations in behaviour of different materials, are to date not fully predictable. This is particularly the case when loads lie in the plastic range, when strain accumulation, hardening and softening play a decisive role for the material reaction. This study therefore aims at adding to the understanding of material behaviour under complex load conditions. Fatigue tests conducted under cyclic torsional angles (5°, 7.5°, 10° and 15°), with superimposed axial static compression loads (250 MPa and 350 MPa), were carried out using smooth specimens at room temperature. A high nitrogen alloyed austenitic stainless steel (nickel free), was employed to determine not only the number of cycles to failure but particularly to aid in the understanding of the mechanical material reaction to the multiaxial stresses as well as modes of crack formation and growth. Experimental test results indicate that strain hardening occurs under the compressive strain, while at the same time cyclic softening is observable in the torsional shear stresses. Furthermore, the cracks’ nature is unusual with multiple branching and presence of cracks perpendicular in direction to the surface cracks, indicative of the varying multiaxial stress states across the samples’ cross section as cross slip is activated in different directions. In addition, it is believed that the static compressive stress facilitated the Stage I (mode II) crack to change direction from the axial direction to a plane perpendicular to the specimen’s axis.

Mechanical Behavior of Austenitic Steel under Multi-Axial Cyclic Loading

“…This compressive creep occurs as soon as a critical angle for the cyclic torsion is exceeded, such that it remains unclear whether an equivalent stress of the applied stress tensor can be defined that describes this deformation mode with sufficient accuracy. Ngeru et al [ 11 ] reported similar behavior in a high nitrogen stainless steel subjected to similar loading conditions. It is noted here, that this kind of creep deformation is purely mediated by dislocation motion, whereas diffusive or other thermally activated mechanisms do not play a role.…”

Crack bifurcation behavior of coarse-grained copper under cyclic torsion combined with axial static loading

“…Some researchers have investigated and discussed the growth behavior of torsional fatigue cracks under axial tension or compression superposition (Zhang and Akid, 1997a;Makabe and Socie, 2001;Yang and Kuang, 2005;Macek, 2021;Ngeru et al, 2022). Zhang and Akid (1997b) found that in pure cyclic torsion, cracks on the surface of high strength spring steel and 316L stainless steel both extended longitudinally in mode II.…”

“…An axial tensile mean stress promoted a change in the direction of the mode II crack from the longitudinal direction to a plane normal to the specimen axis in the high strength steel but not in the stainless steel. Nevertheless, Ngeru (2022) believed that the static compressive stress facilitated the Stage I (mode II) crack to change direction from the axial direction to a plane perpendicular to the specimen's axis. Yang and Kuang (2005) studied the crack growth of carbon steel S45 under cyclic torsion with axial static tension/compression, using specimens with a straight-fronted surface flaw in the cross section.…”