Size effects on uniaxial tension and multiaxial ratcheting of oligo-crystalline stainless steel thin wires

The uniaxial tensile and multiaxial ratcheting behaviors of oligo-crystalline 316L stainless steel thin wires of two diameters (190 and 90 μm) with less than 3.5 grains across diameter were experimentally investigated. The size effect of “thinner is stronger” was observed on the yield strength, though the ultimate tensile strength and elongation were lower in the thinner wire. Under six differently combined axial stress and shear strain cycling conditions, the ratcheting strains were all found to be relatively suppressed in the thinner wire, accompanied by enhanced shear stress flow. Moreover, unlike in bulk materials, the multiaxial ratcheting strain in both thin wires tended to be loading path independent in the cases with the same shear strain amplitude, axial mean stress and stress amplitude. The mechanisms of these size effects were analyzed to be related to the surface dislocation source limitation and strain gradient hardening.