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Load-controlled cyclic three-point bending tests were conducted on a 316LN austenitic stainless steel at temperatures ranging from 20 to 450 °C. A fair method based on the concept of equivalent maximum nominal stress (MNS) was proposed to quantitatively characterize the temperature-dependent bending ratcheting behavior of the steel. The equivalent MNS was determined by ensuring consistent residual deflections after the first bending cycle at varied temperatures. Two loading paths, namely simple cycling and dwelling-cycling, were employed under the equivalent MNS. Under the simple cycling path, the ratcheting behavior of the steel tends to shakedown as temperature rises. Under the simple cycling path where a dwelling was introduced at the maximum loading, the ratcheting potential of the steel appears to be exhausted at relatively low temperatures. These facts are presumably attributed to the loss of viscosity induced by the dynamic strain aging effect. [ABSTRACT FROM AUTHOR]

Miniaturization of modern devices drives the exploration into how materials behave in small dimensions. However, the mechanical behaviors of micro-scale specimens under varied loading conditions are still challenging to characterize and predict. In this work, specimen size-dependent weakening effect was experimentally revealed in 316L stainless steel thin wires under tension with wire diameter decreasing from 200 μm down to 61 μm. The underlying mechanisms were primarily associated with surface grain softening and strain localization, which were largely dependent on the number of grains across wire diameter. Through an analog composite model with surface and interior grains that follows the Hall-Petch relation, the extent of surface grain softening was found to coincide with the flow stress reduction due to lack of boundary strengthening. Further using elasto-plastic self-consistent polycrystal modeling, the stress-strain behaviors of interior and surface grains were well described in analogy to those of bulk materials with similar-size grains and extremely large grains, respectively. Subsequently, the analog composite model well simulated the specimen size-dependent weakening effect under tension. This work provided more insights into the size effect exploration and proposed a practical model for describing size-dependent mechanical behaviors of small-scale materials to be applied to miniaturized devices. [ABSTRACT FROM AUTHOR]

Highlights • Oligo-crystalline 316L steel thin wires showed a "thinner is stronger" size effect. • Reduced wire diameter led to lower ratcheting strain under tension-torsion cycling. • Multiaxial ratcheting behavior of thin wires appeared loading path independent. Abstract 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. [ABSTRACT FROM AUTHOR]

This work reports the low cycle fatigue behavior of a wrought 316LN stainless steel under different control modes at room temperature. Under symmetrical strain and stress cycling, the steel exhibits consistent loading-amplitude-dependent cyclic hardening/softening and fatigue life characteristics. Under asymmetrical stress cycling, the steel is significantly hardened due to mean stress, and the fatigue life at the same strain amplitude is significantly reduced due to ratcheting strain. With the increase of mean stress, though the ratcheting strain level is increased, the fatigue life is prolonged. The effect of mean-stress hardening and ratcheting strain on fatigue life is discussed in terms of strain amplitude and micro-crack initiation and propagation. The Smith-Walker-Topper (SWT) model and a newly proposed fatigue life model based on the Coffin-Manson equation were used to predict the fatigue life under mean stress, and the proposed model yields more robust predictions. [ABSTRACT FROM AUTHOR]