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To investigate the fatigue behavior of cyclically softening and hardening steels under multiaxial elastic-plastic strains, axial strain, and shear strain controlled fatigue tests under constant amplitude loading were carried out. S-N curves under axial strain and torsional pure shear as well as under combined axial strain and shear, in and out of phase, were obtained for the cyclically softening high-strength steel 30Cr-Ni-Mo 8 (similar to AISI Type 4340) and the cyclically hardening stainless steel X 10Cr-Ni-Ti 18 9 (AISI Type 321) in the region of low-cycle fatigue.
Annotation Examines the factors that contribute to overall steel deformation problems. The 27 articles address the effect of materials and processing, the measurement and prediction of residual stress and distortion, and residual stress formation in the shaping of materials, during hardening processes, and during manufacturing processes. Some of the topics are the stability and relaxation behavior of macro and micro residual stresses, stress determination in coatings, the effects of process equipment design, the application of metallo- thermo-mechanic to quenching, inducing compressive stresses through controlled shot peening, and the origin and assessment of residual stresses during welding and brazing. Annotation c. Book News, Inc., Portland, OR (booknews.com)
This study investigates fatigue damage and deformation behavior under multiaxial loading conditions, with the aim of evaluating reliable predictive models for life predictions. Life prediction for multiaxial variable amplitude loading involves a variety of issues to be considered. These include cyclic plasticity, material properties and variations with hardness and microstructure, fatigue damage evolution, fatigue damage quantification parameters, cycle counting procedure, damage accumulation rule, and effects of load non-proportionality. To evaluate the effect of hardness and microstructure on additional non-proportional hardening and fatigue behaviors, 1050 steel in normalized, quenched and tempered (QT), and induction hardened conditions as well as 304L stainless steel were utilized. Reduction in the non-proportional cyclic hardening was observed as the 1050 steel changed from low hardness to higher hardness. Significant non-proportional cyclic hardening was observed for 304L stainless steel. Multiaxial data generated in this study as well as from literature suggest non-proportional cyclic hardening can be related to uniaxial cyclic hardening. Non-proportional hardening coefficients predicted from a proposed equation based on this observation were found to be in very good agreement with the experimental values in this study and from literature. Multiaxial fatigue data for all hardness levels were satisfactorily correlated with the Fatemi-Socie parameter. In order to predict multiaxial fatigue life of steels in the absence of any fatigue data, the Roessle-Fatemi hardness method was extended to multiaxial loading. The applicability of the prediction method based on hardness was examined for several steels under a wide range of loading conditions. The great majority of the observed fatigue lives were found to be in good agreement with predicted lives. Several discriminating multiaxial cyclic strain paths with incremental and random sequences were used to investigate fatigue and cyclic deformation behaviors of materials with low and high additional hardening resulting from non-proportional loadings. Tubular specimens made of 1050 QT steel and 304L stainless steel were utilized for this purpose. The 1050 QT steel was found to exhibit very similar stress response under various multiaxial loading paths, whereas significant effects of loading sequence were observed on stress response of 304L stainless steel. In-phase cycles with a random sequence of axial-torsional cycles on an equivalent strain circle caused cyclic hardening levels similar to 90° out-of-phase loading of 304L stainless steel. In contrast, straining with a gradual sequence resulted in much lower stress than for 90° out-of-phase loading. Tanaka's non-proportionality parameter coupled with a Fredrick-Armstrong incremental plasticity model resulted in accurate prediction of the stabilized stress response. Kanazawa et al.'s empirical formulation as a representative of such empirical models could not distinguish between strain paths with random and incremental sequences of straining, resulting in significant over-prediction of stress for 304L stainless steel. Contrary to common expectations, fatigue lives for 1050 QT steel with no non-proportional hardening were found to be more sensitive to non-proportionality of loadings as compared to 304L stainless steel with significant non-proportional hardening. In-phase loading cycles with random sequences of axial-torsion strain ratio within an equivalent strain circle did not significantly affect fatigue life for either material. Experimentally observed failure planes were in good agreements with predicted failure planes based on the Fatemi-Socie critical plane parameter. Bannantine-Socie and Wang-Brown cycle counting methods were utilized to identify loading cycles for variable amplitude strain paths. Fatigue damage for each counted cycle was evaluated using Fatemi-Socie damage parameter and linear cumulative fatigue damage was then employed to account for accumulation of damage. Fatigue lives for both materials under these discriminating strain paths were predicted satisfactorily employing this approach and either Bannantine-Socie or Wang-Brown cycle counting method. Cracking behavior was analyzed for different materials investigated and under various loading conditions. Micro-cracks were observed to be around the maximum shear or critical plane. The ratio of crack initiation life to total fatigue life as well as the crack growth rate depended on a variety of factors including strain amplitude, load non-proportionality, material ductility, and specimen geometry. Crack growth rates for in-phase and 90° out-of-phase loading were correlated well by Reddy-Fatemi effective strain-based intensity factor.
Cyclic deformation behavior and fracture mechanisms of Ferrovac E iron under low cycle fatigue conditions are studied at temperatures ranging from 23 to 540 C, cyclic deformation rates from 4 x 10-4 to 2 x 10-1 s-1, and strain ranges from 0.010 to 0.040. The stress response during strain cycling shows three stages, primary hardening, steady state behavior, and secondary hardening. Both the onset and duration of secondary hardening are dependent on deformation rate and temperature through the process of dynamic strain aging. Stress response sensitivity to deformation rate increases under conditions which permit changes in both internal and effective stress components. There is a variation in the internal stress component at the blue brittleness temperature (370 C) caused by an increase in total dislocation density due to dynamic strain aging. However, at higher temperatures (485 to 540 C) a variation in the internal stress component is caused by accelerated thermal recovery. Steady state cyclic deformation in this temperature range is observed to be consistent with models for steady state creep and hot-working. The fracture mechanism and fatigue life are mainly influenced by the characteristics of stress and strain redistribution and the inhomogeneity of plastic-strains. This inhomogeneity of deformation results from dynamic strain aging effects during fatigue. A parameter which controls stress response, fatigue behavior, and fracture mode under dynamic strain aging conditions is presented.
This book provides practicing engineers, researchers, and students with a working knowledge of the fatigue design process and models under multiaxial states of stress and strain. Readers are introduced to the important considerations of multiaxial fatigue that differentiate it from uniaxial fatigue.
Room temperature fatigue behavior was obtained for eight different steels, including cast, hot-rolled, quenched and tempered, and case-hardened steels, under both constant and variable amplitude load conditions. Constant amplitude fatigue crack growth behavior was obtained with compact-type specimens with load ratios R ? 0 and -1. Variable amplitude fatigue behavior was obtained from keyhole-notched compact-type specimens by using the Society of Automotive Engineers' transmission history and three modifications of this history including single tensile overloads. The keyhole specimen and the variable amplitude spectra were chosen to simulate room temperature real-life situations including both crack initiation and crack propagation. Fatigue behavior of the eight steels is compared.