Papers by Keyword: Cyclic Creep

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Authors: P.L. Liu, Zhong Guang Wang, Hiroyuki Toda, Takayuki Kobayashi
Authors: Tzu Yin Jean Hsu, Zhi Rui Wang
Abstract: Systematic experimental investigation was carried out to examine the cyclic deformation response and study the crack nucleation mechanisms under pure compression fatigue condition using OFHC Cu. Results show that the cyclic stress strain response and micro-structural evolution of copper under pure compression fatigue exhibits rather dissimilar responses compared to those under general push-pull fatigue conditions. Both rapid hardening and the compressive cyclic creep were observed in all tested conditions. Like under all fatigue conditions, surface micro-structural changes were detected by optical, SEM, and specifically AFM. It was revealed that cyclic plastic strain accommodated by the sample was not in any major way through dislocation activities, as there was only moderate slip activities observed on the surface and no PSB features were detected from TEM observations. Instead, cyclic creep was observed to be the major form of plastic strain accommodation. In addition, the observed surface phenomenon was found to have led to the eventual crack nucleation when the applied stress amplitude was high.
Authors: Jaroslav Polák, Martin Petrenec, Jiří Man, Tomáš Kruml
Abstract: Smooth specimens made from austenitic-ferritic duplex steel were subjected to constant stress amplitude loading with positive mean stresses. Hysteresis loops were recorded during the fatigue life and plastic strain amplitude and cyclic creep rate were determined. Fatigue hardening/softening curves, cyclic creep curves and cyclic stress-strain curves for different positive mean stresses were evaluated. Typical dislocation structures developed in both phases of the duplex steel were identified using TEM, compared with the saturated plastic strain amplitude and correlated with the decrease of the cyclic creep rate during cycling and the slope of the cyclic stress-strain curve.
Authors: Tick Hon Yip, Zhi Rui Wang
Authors: Xiang Guo Zeng, Zhan Hua Gao, Jing Hong Fan, Hua Yan Chen
Abstract: The cast magnesium alloys as AM50 offer a good strength, ductility and surface finish for automotive industry. But the poor creep resistance limited its application to power components such as engine and transmission cases at temperatures in excess of 100°C. In order to investigate the cyclic creep behavior of Magnesium Alloy at high temperature, creep tests of plate specimens AM50 were conducted in this work. Based on the analysis about the microstructure and defects of AM50 under the condition of cyclic creep, a cyclic creep constitutive model with isotropic and scalar damage parameter was developed. Furthermore, the proposed model was experimentally verified by analyzing the cyclic creep and recovery response of Cast Magnesium alloy under cyclic loading with dwell time. Comparisons between calculated results and experimental data showed good agreement.
Authors: Jaroslav Polák, Martin Petrenec
Abstract: The fatigue properties of ferritic-pearlitic-bainitic steel using specimens produced from massive forging were measured in stress controlled regime with positive mean stress. The cyclic creep curves and cyclic hardening/softening curves were evaluated. The fatigue life was plotted in dependence on the mean stress and on the plastic strain amplitude. The principal contribution to the drop of the fatigue life with the mean stress is due to the increase of the plastic strain amplitude in cycling with mean stress.
Authors: Rui Feng Wang, You Tang Li, Hu Ping An
Abstract: The low cycle fatigue behaviors of TI-6AL-4V alloy controlled by strain were investigated by experiment. The fatigue tests were performed at room temperature, and cyclic strain and stress ratio are 0.1 with triangle load wave. The results show that TI-6AL-4V alloy is soften rapidly under the cyclic tensile stresses and it is harden rapidly under the cyclic compressive stresses during the initial-stage of strain controlled fatigue, and the rates of cyclic soften and cyclic harden are decreased with the fatigue progress. The soften rate is related to the cyclic strain but little to the cyclic stress during the overall fatigue progress. The change of cyclic stress is related to the macro friction stresses. The results of experiment show that obvious cyclic creep occurs under the stress controlled low cycle fatigue conditions, and the magnitude of cyclic creep strain is related to the maximum cyclic stress. The softening of tensile friction stresses is the main factor of cyclic creep.
Authors: Tilmann Beck, Stephan A. Kovacs, Fabian Ritz
Abstract: Low-pressure steam turbine blades undergo VHCF-loadings induced by inhomogenous flow behind the vanes resulting in excitation frequencies of ≈ 2 kHz for rotational speeds of 50 Hz and a typical number of stator vanes of ≈ 60. The VHCF loading is superimposed by considerable mean stresses caused by centrifugal forces. In the present study, the VHCF-behavior of the ferritic-martensitic turbine blade steel X10CrNiMoV12-2-2 is investigated using an ultrasonic fatigue testing system up to cycle numbers of 5∙109 at stress ratios from R = -1 up to 0.7, i.e. up to very high mean stresses. Generally, crack initiation changes from the surface to internal inclusions at fatigue lives around 4∙107. The transition between fatigue failure and run-outs is shifted to higher lifetime with increasing R, and fine grained areas (FGAs) at the crack initiation sites only occur at R < -0.1. However, the fracture mechanics approach proposed by Murakami consistently describes the lifetime behavior for all load ratios over 4 decades of lifetime. At R up from 0.5 considerable cyclic creep occurs, even for lifetimes above 108 cycles, resulting in cyclic hardening which was proved by microhardness measurements at longitudinal sections. This effect at least partially explains the high maximum stresses close to the tensile strength of the material occurring in the VHCF regime at load ratios ≥ 0.5.
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