Cyclic Stress Response Behavior Research Articles

Influence of temperature on cyclic stress response and fracture behavior of aluminum alloy 6061

The cyclic stress response and fracture characteristics of aluminum alloy 6061 was studied at different temperatures. The specimens were cyclically deformed using tension-compression loading under total strain-amplitude control. The alloy showed evidence of softening at all test temperatures. The degree of cyclic softening was observed to increase with an increase in test temperature. The presence of shearable matrix precipitates in the alloy results in a local decrease in resistance to dislocation movement, thereby causing a progressive loss of strengthening contribution. At the elevated temperatures, localized oxidation and embrittlement at the grain boundaries are promoted by the applied cyclic stress and play an important role in accelerating crack initiation and subsequent crack propagation. The fracture behavior of the alloy is discussed in terms of competing influences of intrinsic microstructural effects, deformation characteristics arising from a combination of mechanical and microstructural contributions, plastic strain amplitude and concomitant response stress, and test temperature.

Cyclic stress response, strain resistance and fracture behavior of modified 1070 steel

A combined experimental and finite element study of fatigue crack closure in modified 1070 steel has been conducted. In this paper, the material property evaluations required for this study are presented. The monotonic and cyclic stress-strain properties, cyclic stress response, cyclic strain resistance, low cycle fatigue life and fracture behavior are examined. The low cycle fatigue tests were conducted using tension-compression cycling, under total strain amplitude control, over a wide range of strain levels. The material was found to possess medium strength and high ductility; while displaying a strain level dependent combination of cyclic strain softening and hardening behavior. The observed softening behavior is attributed to the rearrangement of dislocations produced by processing, formation of slip bands on the specimen surface and the formation of microcracks. The observed hardening behavior is ascribed to contributions from synergistic influences of dislocation multiplication, dislocation-dislocation interactions and dislocation-microstructural feature interactions. The material followed the strain-life relationships attributed to Basquin and Coffin-Manson. The fracture surfaces of the fatigue specimens showed distinct regions of crack initiation, microscopic-macroscopic crack growth and sudden fracture. The low-cycle fatigue characteristics and fracture behavior are discussed in the light of competing and mutually interactive influences of cyclic strain amplitude, concomitant response stress, intrinsic microstructural effects and dislocation-microstructure interactions during cyclic straining.

Temperature and strain-rate effects on low-cycle fatigue behavior of alloy 800H

The effects of strain rate (4 × 10-6 to 4 × 10-3 s-1) and temperature on the low-cycle fatigue (LCF) behavior of alloy 800H have been evaluated in the range 750 ° to 950 °. Total axial strain controlled LCF tests were conducted in air at a strain amplitude of ± 0.30 pct. Low-cycle fatigue life decreased with decreasing strain rate and increasing temperature. The cyclic stress response behavior showed a marked variation with temperature and strain rate. The time- and temperature-dependent processes which influence the cyclic stress response and life have been identified and their relative importance assessed. Dynamic strain aging, time-dependent deformation, precipitation of parallel platelets of M23C6on grain boundaries and incoherent ledges of twins, and oxidation were found to operate depending on the test conditions. The largest effect on life was shown by oxidation processes.

High temperature low cycle fatigue

Fatigue at high temperature is a complex phenomenon as it is influenced by a number of time-dependent processes which become important at elevated temperatures. These processes include creep, oxidation, phase instabilities and dynamic strain ageing (DSA), acting either independently or synergistically influence fatigue behaviour, often lowering the fatigue life. Current design approaches employ linear summation of fatigue and creep damage with suitable factors on permissible damage to take care of uncertainties in interaction between cyclic and time-dependent processes. It is, therefore, important to develop a deeper understanding of the processes that occur during high temperature fatigue so that realistic life predictions could be made. Results on the high temperature fatigue behaviour of austenitic stainless steels, ferritic steels and nickel base alloys are presented here. The important mechanisms of interaction of high temperature time-dependent processes with fatigue under various conditions are discussed in detail. Emphasis is placed on cyclic stress response, fatigue life, deformation substructure and fracture behaviour. This is followed by a review of important life prediction techniques under combined creep-fatigue loading conditions. Life prediction techniques considered here include linear damage summation, strain range partitioning, ductility exhaustion approach, frequency modified and frequency separation methods, techniques based on hysteresis energy and damage rate models, and methods based on crack-cavitation interation models.

The influence of Al2O3 particulate reinforcement on cyclic stress response and fracture behavior of 6061 aluminum alloy

The cyclic stress response characteristics and cyclic fracture behavior of aluminum alloy 6061 discontinuously reinforced with particulates of Al2O3 are presented and discussed. The 6061/Al2O3 composite specimens and the unreinforced 6061 aluminum alloy were cyclically deformed using tension-compression loading under constant total strain amplitude control. Both the composite and the unreinforced alloy exhibited softening to failure from the onset of cyclic deformation. The degree of softening was observed to increase at the elevated test temperature for both the composite and the unreinforced counterpart. The intrisic micromechanisms controlling the stress response characteristics during fully-reversed cyclic straining are highlighted and rationale for the observed behavior is discussed. The cyclic fracture behavior of the composite is discussed in terms of the competing influences of intrinsic microstructural effects, deformation characteristics arising from a combination of mechanical and microstructural contributions, cyclic stress response, and test temperature.

Effect of strain rate on the high-temperature low-cycle fatigue properties of a nimonic PE-16 superalloy

Strain-rate effects on the low-cycle fatigue (LCF) behavior of a NIMONIC PE-16 superalloy have been evaluated in the temperature range of 523 to 923 K. Total-strain-controlled fatigue tests were per-formed at a strain amplitude of +/-0.6 pct on samples possessing two different prior microstructures: microstructure A, in the solution-annealed condition (free of gamma' and carbides); and microstructure B, in a double-aged condition with gamma' of 18-nm diameter and M23C6 carbides. The cyclic stress response behavior of the alloy was found to depend on the prior microstructure, testing temperature, and strain rate. A softening regime was found to be associated with shearing of ordered gamma' that were either formed during testing or present in the prior microstructure. Various manifestations of dynamic strain aging (DSA) included negative strain rate-stress response, serrations on the stress-strain hysteresis loops, and increased work-hardening rate. The calculated activation energy matched well with that for self-diffusion of Al and Ti in the matrix. Fatigue life increased with an increase in strain rate from 3 x 10(-5) to 3 x 10(-3) s-1, but decreased with further increases in strain rate. At 723 and 823 K and low strain rates, DSA influenced the deformation and fracture behavior of the alloy. Dynamic strain aging increased the strain localization in planar slip bands, and impingement of these bands caused internal grain-boundary cracks and reduced fatigue life. However, at 923 K and low strain rates, fatigue crack initiation and propagation were accelerated by high-temperature oxidation, and the reduced fatigue life was attributed to oxidation-fatigue interaction. Fatigue life was maximum at the intermediate strain rates, where strain localization was lower. Strain localization as a function of strain rate and temperature was quantified by optical and scanning electron microscopy and correlated with fatigue life.

Rolling contact fatigue of case-hardened parts: Stangner, H. and Zoch, H.-W. Harterei-Techn. Mitteil. July–Aug. 1990 5(4), 223–229 (in German)

Alloy 617 is the one of the leading candidate materials for intermediate heat exchangers (IHX) of a Very High Temperature Reactor (VHTR). System start-ups and shut-downs as well as power transients will produce low cycle fatigue (LCF) loadings of components. As a series of the work to better understand the LCF properties of Alloy 617 weld joints at high temperature, firstly, in this work, strain-controlled LCF testing of Alloy 617 base metal (BM) and weld joint (WJ) by a gas tungsten arc weld process (GTAW) were carried out. Fully reserved total-strain controlled LCF tests have been conducted at room temperature with four total strain ranges of 1.5, 1.2, 0.9, and 0.6%. For the LCF tests triangular test waveforms with a frequency of 0.25 Hz were applied. The present paper is to characterize the LCF properties for Alloy 617 base metal (BM) and weld joint (WJ) from the cyclic stress response behavior and fatigue fracture behavior, with the comparative method. The cyclic stress response behavior was influenced by the level of total strain ranges and the material properties. Though base metal (BM) had shown higher plastic strain accumulation, the observed fatigue life of the weld joint (WJ) is lower than the base metal (BM). Coffin-Manson relationship and energy-life models can be used to determine the fatigue life.

Cyclic stress response and cyclic fracture behavior of silicon carbide particulate reinforced aluminum metal-matrix composite

The cyclic stress response characteristics of a powder metallurgy high purity aluminum alloy 2124 discontinuously reinforced with a varying volume fraction of silicon carbide particulates was studied over a range of plastic strains. The specimens were cycled using tension-compression loading under total strain control. The material, in the as-extruded condition, displayed cyclic hardening at all plastic strain amplitudes and for different volume fractions of ceramic reinforcement. The degree of cyclic hardening was observed to be greater at the higher cyclic strain amplitudes than at corresponding lower cyclic strain amplitudes. Overall, hardening was also found to increase with an increase in volume fraction of carbide particle reinforcement in the aluminum matrix. The observed hardening behavior is attributed to concurrent and competing influences of an increase in dislocation-dislocation interaction, dislocation multiplication and dislocation-particle interactions, and is a mechanical effect. The cyclic fracture behavior of the alloy is discussed in light of composite microstructural effects, plastic strain amplitude and concomitant response stress.

Cyclic stress response and deformation behaviour of precipitation-hardened aluminium-lithium alloys

The cyclic stress response of two lithium-containing aluminium alloys aged to contain ordered precipitates was studied in different environments over a range of plastic strains. The specimens were cycled using tension-compression loading under total strain control. The peak-aged AlLiMn alloy cyclically hardened to failure, whereas the peak-aged AlLiCu alloy displayed softening for most of the fatigue life. The presence of shearable softening for most of the fatigue life. The presence of shearable precipitates in the two alloys results in a local decrease in resistance to dislocation movement, leading to a progressive loss of ordering contributions to hardening and slip concentration. This, coupled with the presence of precipitate free zones, promotes strain localization in intense slip bands and results in early crack nucleation. Transmission electron microscopy observations revealed homogeneous deformation in specimens cycled at high plastic strain amplitudes. However, at lower plastic strain amplitudes, deformation was inhomogeneous in the two alloy systems with the formation of intense planar slip bands. Results of this study reveal that the initial hardening observed is due to dislocation-dislocation and dislocation-precipitate interaction and that the softening observed in the AlLiCu alloy is a mechanical and not an environmental effect.