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.

Effects of stress ratio on high-cycle and very-high-cycle fatigue behavior of a Ti-6Al-4V alloy

Abstract: The effects of stress ratio on high-cycle fatigue (HCF) and very-high-cycle fatigue (VHCF) behavior of a Ti-6Al-4V alloy were systematically investigated in this paper. Fatigue tests with ultrasonic frequency (20 kHz) were performed on specimens of a bimodal Ti-6Al-4V alloy with stress ratios of -1, -0.5, -0.1, 0.1 and 0.5. Three types of crack initiation mode were observed on the fracture surfaces of the specimens that failed in the HCF and the VHCF regimes, which were explicitly classified as surface-without-facets, surface-with-facets and interior-with-facets. With the increase of stress ratio from 1 to 0.5, the number of specimens for surface-without-facets decreased, that for surface-with-facets increased, and that for interior-with-facets increased first and then decreased. For the failure types of surface-with-facets and interior-with-facets, the fatigue strength decreased sharply in the VHCF regime, and the S-N curve switched from an asymptote shape to a duplex shape. Then, a new model based on Poisson defect distribution was proposed to describe the effects of stress ratio on the occurrence of different failure types, i.e., the competition of alternative failure types. The observations also showed that there is a rough area at the crack initiation region for interior initiation cases, and the values of the stress intensity factor range for the rough area are within a small range, with the mean value being close to the threshold for the crack starting to grow in vacuum environment of the alloy. The estimated value of plastic zone size at the periphery of rough area is close to the average diameter of the primary cc grains of the alloy. (C) 2014 Elsevier B.V. All rights reserved.

Fatigue Response of As-Built DMLS Maraging Steel and Effects of Aging, Machining, and Peening Treatments

Abstract: The main motivations for this study arise from the need for an assessment of the fatigue performance of DMLS-produced Maraging Steel MS1, when it is used in the “as fabricated” state. The literature indicates a lack of knowledge from this point of view; moreover, the great potentials of the additive process may be more and more incremented, if an easier and cheaper procedure could be used after the building stage. The topic has been tackled experimentally, investigating the impact of heat treatment, machining, and micro-shot-peening on the fatigue strength with respect to the “as built state”. The results indicate that heat treatment may improve the fatigue response, as an effect of the relaxation of the process-induced tensile residual stresses. Machining can also be effective, but it must be followed (not preceded) by shot-peening, to benefit from the compressive residual stress state generated by the latter. Moreover, heat treatment and machining are related by a strong positive interaction, meaning their effects are synergistically magnified when they are applied together. The experimental study has been completed by fractographic as well as micrographic analyses, investigating the impact of the heat treatment on the actual microstructure induced by the stacking process.

Fatigue of as-extruded AZ61A magnesium alloy

Abstract: Samples prepared from as-extruded AZ61A bars (18 mm in diameter) were used in a rotating bending test. The relation between stress amplitude and cycles to failure has been constructed, as well as the cycles to failure at two specific stress amplitudes. The probability of failure at these two specific stress levels was also analyzed. This study finally provided the predicted fatigue strength at 10 7 cycles with different probabilities (10 to 90%). A fractography study indicated that fatigue cracking initiated from subsurface or surface inclusions. These inclusions near the surface served as stress raisers and induced clusters of slip bands during the rotating bending test. After initiation, the cracks grew under the dominant shear stress and resulted in a cleavage fracture over a large area. Microscopic cracks occurred, resulting from the induced deformation twins that developed from the blunting process. Consequently, the propagation of cracks followed the existence of microscopic cracks and resulted in a transgranular fracture.