Fatigue limit

About: Fatigue limit is a research topic. Over the lifetime, 20489 publications have been published within this topic receiving 305744 citations. The topic is also known as: endurance limit & fatigue strength.

Ultra-long cycle fatigue of high-strength carbon steels part II: estimation of fatigue limit for failure from internal inclusions

Abstract: Ultra-long cycle fatigue fracture origins in high strength steels are mostly at non-metallic inclusions due to the influence of the trapped hydrogen. In the vicinity of a non-metallic inclusion at the fracture origin, an optically dark area (ODA) is often observed inside a fish-eye mark, which represents the particular morphology associated with mechanism of failure at an early stage: hydrogen assisted fatigue. From an analysis of several features involved in this mechanism of failure, as the development of the ODA, the threshold for pure fatigue propagation, as a function of crack length and inclusion size, an expression was deduced to estimate the internal fatigue limit, that is, the stress level below which fracture produced by cracks initiated from an internal inclusion is not found after 10 10 cycles: σ e Int =256 Δ K th R i max where Δ K th is the pure fatigue crack propagation threshold as a function of crack length estimated with the following expression: Δ K th =4×10 −3 (H V +120)(3R i max ) 1/3 If Δ K th ≤10 MPa m 1/2 Otherwise Δ K th =10 MPa m 1/2 R i max is the maximum radius of the non-metallic inclusions from which the crack initiates, in μm, and H V is the Vickers hardness, in kgf mm −2 . The internal fatigue limit was estimated for several steels and compared with experimental results obtained from the literature for internal fatigue lives in the range of 10 8 –10 10 cycles. Differences between the estimated internal fatigue limit and the experimental one ranged from 4 to −29%.

A Consideration of Allowable Equivalent Stresses for Fatigue Design of Welded Joints According to the Notch Stress Concept with the Reference Radii r ref = 1.00 and 0.05 mm

Abstract: In the literature, allowable stresses (FAT-values) for the fatigue design of welded joints are established according to the notch stress concept with the reference radii rref = 1.00 mm for thick connections (t ≥ 5 mm) and 0.05 mm for thin connections (t < 5 mm). However, it is not clear for which strength hypothesis they are valid. As local equivalent stresses may be calculated by the principal stress or von Mises hypotheses, it is necessary to distinguish between the applied hypotheses. The FAT-values according to the principal stress and von Mises hypotheses are compiled for steel, aluminium and magnesium for the reference radii rref = 1.00 and 0.05 mm. The allowable stresses are derived from normal as well as from shear stresses. However, the values derived from pure normal loading (axial or bending) and from pure torsion are not compatible when the principle stress or the von Mises hypotheses are applied. Therefore, in case of biaxial loading, the stated incompatibility between the values obtained from different loading modes should be overcome by the Gough-Pollard relationship.

Overview of fatigue properties of fine grain 5056 Al-Mg alloy processed by equal-channel angular pressing

Abstract: The fatigue performance of the fine-grain 5056 Al-Mg alloy processed by severe plastic deformation through equal-channel angular pressing (ECAP) is assessed in both stress- and plastic strain-controlled experiments. Compared to its conventional counterpart, the ECAP material exhibits a high tensile and low-cyclic fatigue strength under constant stress amplitude. However, its fatigue life under strain-controlled conditions is notably shorter than that of the O-temper specimens. Despite severe pre-straining of the specimen during ECA-pressing, cyclic softening was found to be rather small. It is shown that the mechanical characteristics obtained after ECAP can be significantly improved during short time annealing at moderate temperatures (150°C, 15 min) after fabrication. Such heat treatment is supposed to recover partially the grain boundary region, which has been most heavily distorted during processing. Mechanisms of fatigue in ECA-processed materials are discussed within a framework of a simple one-parametric model of dislocation kinetics.