Strain-controlled Low Cycle Fatigue Research Articles

Low Cycle Fatigue Properties of Alloy 617 base Metal and Weld Joint at Room Temperature

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 0.25Hz 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.

Life modeling of anisotropic fatigue behavior for a single crystal nickel-base superalloy

Strain-controlled low cycle fatigue (LCF) experiments of a second generation single crystal (SC) nickel-base superalloy are carried out to investigate the effect of crystallographic orientations on its LCF life. Further, the life modeling of the SC superalloy is considered by employing the modified Mücke’s anisotropic model. The results show that the LCF life of the SC superalloy has a strong crystallographic orientation dependence; the local strain approach fails to predict the fatigue life but the modified Mücke’s anisotropic model is able to predict the fatigue life for the SC superalloy at the three characteristic orientations simultaneously.

Strain-Controlled Low-Cycle Fatigue Behavior of Friction Stir-Welded AZ31 Magnesium Alloy

Strain-controlled low-cycle fatigue (LCF) behavior of friction stir-welded (FSW) AZ31 joints, produced at rotation rates of 800 and 3500 rpm, was studied. The joints exhibited symmetric hysteresis loops, whereas asymmetric loops were observed for the parent material (PM). The fatigue resistance of the FSW joints was slightly improved as the rotation rate increased, and both the FSW joints possessed a fatigue life similar to that of the PM at the low strain amplitude of 0.1 pct. The obtained fatigue data for the PM and FSW joints can be well described using the Coffin–Manson and Basquin’s relationships. For the FSW joints, during LCF deformation, the \( \left\{ {10\bar{1}2} \right\} \) twinning originated from the nugget zone (NZ)/thermomechanically affected zone (TMAZ) boundary and then propagated to the NZ interior. This was attributed to different textures in these regions: the center of the NZ exhibited a hard orientation, whereas a soft orientation was observed in the region around the NZ/TMAZ boundary. The fatigue cracks initiated at the bottom of the joints and propagated along the NZ/TMAZ boundary or the NZ adjacent to the NZ/TMAZ boundary.

Low cycle fatigue behavior of Cr–Mo–V low alloy steel used for railway brake discs

The cyclic stress–strain response and the low cycle fatigue (LCF) behavior of Cr–Mo–V low alloy steel which was used for forged railway brake discs was studied. Tensile strength and LCF properties were examined over a range from room temperature (RT) to 600°C using specimens cut from circumferential direction of a forged disk. The fully reversed strain-controlled LCF tests were conducted at a constant total strain rate with different axial strain amplitude levels. The cyclic strain–stress relationships and the strain–life relationships were obtained through the test results, and related LCF parameters of the steel were calculated. The studied steel exhibits cyclic softening behavior and behaves Masing type, especially at higher strain amplitudes. At higher than 600°C, carbide particles aggregated and a decarburized layer developed near the specimen surface. Micro voids distribute within the depth of 50μm from the specimen surface could coalesce with fatigue cracks. Multiple crack initiation sites were observed on the fracture surface. The oxide film that generated at 600°C covered the fatigue striations and accelerated the crack propagation. Final fracture area with bigger and deeper dimples showed better ductility at higher temperature. The investigated LCF behavior can provide reference for brake disc life assessment and fracture mechanisms analysis.

Cyclic deformation and fatigue behaviors of Hadfield manganese steel

The cyclic deformation characteristics and fatigue behaviors of Hadfield manganese steel have been investigated by means of its ability to memorize strain and stress history. Detailed studies were performed on the strain-controlled low cycle fatigue (LCF) and stress-controlled high cycle fatigue (HCF). Initial cyclic hardening to saturation or peak stress followed by softening to fracture occurred in LCF. Internal stress made the dominant contribution to the fatigue crack propagation until failure. Effective stress evolution revealed the existence of C–Mn clusters with short-range ordering in Hadfield manganese steel and demonstrated that the interaction between C atoms in the C–Mn cluster and dislocation was essential for its cyclic hardening. The developing/developed dislocation cells and stacking faults were the main cyclic deformation microstructures on the fractured sample surface in LCF and HCF, which manifested that fatigue failure behavior of Hadfield manganese steel was induced by plastic deformation during strain-controlled or stress-controlled testing.

Effect of strain ratio on cyclic deformation behavior of a rare-earth containing extruded magnesium alloy

Cyclic deformation characteristics of an extruded Mg–10Gd–3Y–0.5Zr (GW103K) magnesium alloy were determined via the strain-controlled low cycle fatigue tests with varying strain ratios at a constant strain amplitude. Unlike the rare-earth (RE)-free extruded magnesium alloys, the present alloy exhibited symmetrical hysteresis loops in tension and compression in the fully reversed strain-control tests at a strain ratio of Rε=−1. This was due to the presence of relatively weak crystallographic textures and the suppression of twinning–detwinning activities arising from the fine grain sizes and RE-rich particles. At a strain ratio of Rε=0 and 0.5, a large amount of plastic deformation occurred in the tensile phase of the first cycle of hysteresis loops due to the high positive mean strain values. With decreasing strain ratio, the hysteresis loops became wider. Fatigue life of this alloy was observed to be the longest in the fully reversed strain control at Rε=−1, and it decreased as the strain ratio was deviated from Rε=−1. A certain degree of mean stress relaxation was also observed in the non-fully reversed strain control (i.e., Rε≠−1 tests).

Fatigue behavior of ultrafine-grained Ti–24Nb–4Zr–8Sn multifunctional biomedical titanium alloy

A multifunctional titanium alloy Ti–24Nb–4Zr–8Sn (wt%) was fabricated by warm swaging and rolling to get homogeneous microstructure with ultrafine-grained (UFG) β phase and nanostructured (NS) α phase, and their stress-controlled high cycle fatigue (HCF) and strain-controlled low cycle fatigue (LCF) behaviors were investigated in the study. The results showed that the UFG alloy exhibits much higher HCF strength than that of the hot-rolled alloy with coarse grains whereas its LCF endurance is worse slightly. This was explained by its stable microstructure originated from low homologous temperature being ∼0.15 of melting temperature and the improvement of phase stability by grain refinement and NS precipitations. The former leads to dynamic recovery instead of dynamic recrystallization occurring in other UFG materials while the latter results in comparable cyclic stress stability with the hot-rolled alloy. The study also found that the shear bands have an identical angle of ∼26° with the loading direction, which is independent of the strain ratios. Since the ductile UFG alloy has a nonlinear elastic deformation behavior with large recoverable strain, its LCF endurance is much better than those of other materials exhibiting linear elasticity and would be further improved by the depression of NS precipitation.

Low cycle fatigue behavior of a semi-solid processed AM60B magnesium alloy

Strain-controlled low cycle fatigue tests were conducted on a thixomolded (semi-solid processed) AM60B alloy, and its fatigue life, fatigue parameters and cyclic deformation characteristics were evaluated. The microstructure consisted of primary α-Mg and network-like eutectic structure containing β-Mg17Al12 particles. The alloy showed basically symmetrical hysteresis loops in tension and compression and superior fatigue resistance to its die cast counterpart at lower strain amplitudes. The hysteresis loops exhibited a characteristic clockwise rotation, and the elastic modulus decreased with increasing strain amplitude. This was attributed to the non-linear (pseudoelastic) behavior stemming from twinning–detwinning. In comparison with extruded magnesium alloys, while the thixomolded alloy had less extensive twinning during cyclic deformation due to the presence of a large number of β-Mg17Al12 particles along with relatively small grain sizes, two types of twins (namely wider lenticular extension twins and narrower banded contraction twins) still occurred in some favorably oriented and relatively large primary α-Mg grains in the vicinity of fracture surface. With increasing strain ratio fatigue life decreased and the extent of mean stress relaxation increased. While the monotonic and cyclic yield strengths of this alloy were similar, the cyclic strain hardening exponent was higher than the monotonic strain hardening exponent.

Effect of Strain Rate on the Low Cycle Fatigue Behavior of 316L(N) Stainless Steel Weld Joints

Abstract The aim of the present paper is to study the effect of strain rate on the low cycle fatigue (LCF) behavior of indigenously developed 316LN stainless steel weld joint (0.14 N wt. % base metal welded with 0.1 wt. % N electrodes). Fully reversed total axial strain-controlled LCF tests were conducted at 823 K using a servohydraulic machine, equipped with a resistance heating furnace. Tests were conducted in air at ±0.6% strain amplitude employing strain rates 3×10−3s−1, 3×10−4s−1 and 3×10−5s−1 to investigate the effect of strain rate in the dynamic strain aging regime (DSA). Initial brief hardening, negative strain rate stress response and lowering of fatigue life with decreasing strain rate have been correlated to DSA operating at 823 K. The LCF behaviour of weld joints are correlated to the microstructural changes during fatigue cycling in order to explain the underlying deformation mechanism and fracture behaviour.

Low cycle fatigue and creep-fatigue behavior of Ni-based alloy 230 at 850 °C

Strain-controlled low cycle fatigue (LCF) and creep-fatigue testing of Ni-based alloy 230 were carried out at 850°C. The material creep-fatigue life decreased compared with its low cycle fatigue life at the same total strain range. Longer hold time at peak tensile strain further reduced the material creep-fatigue life. Based on the electron backscatter diffraction, a novel material deformation characterization method was applied, which revealed that in low cycle fatigue testing as the total strain range increased, the deformation was segregated to grain boundaries since the test temperature was higher than the material equicohesive temperature and grain boundaries became weaker regions compared with grains. Creep-fatigue tests enhanced the localized deformation, resulting in material interior intergranular cracking, and accelerated material damage. Precipitation in alloy 230 helped slip dispersion, favorable for fatigue property, but grain boundary cellular precipitates formed after material exposure to the elevated temperature had a deleterious effect on the material low cycle fatigue and creep-fatigue property.

Strain-Controlled Low-Cycle Fatigue Properties of Extruded 6061-T6 Aluminum Alloy

Strain-Controlled Low-Cycle Fatigue Properties of Extruded 6061-T6 Aluminum Alloy

Cyclic deformation characteristics and fatigue crack growth behaviour of a newly developed aerospace superalloy Haynes 282

Strain-controlled low cycle fatigue (LCF) and fatigue crack growth (FCG) tests were performed on a newly developed Aerospace superalloy, Haynes 282, in its standard heat treatment condition which consists of a solution heat treatment of 1130°C/2h/water-quench followed by ageing at 1010°C/2h/air-cool and 788°C/8h/air-cool. Cyclic deformation characteristics of the Haynes 282 at room temperature (25°C) were obtained from the LCF test. The LCF test results showed that Haynes 282 experienced a relatively short period of initial cyclic hardening followed by a regime of cyclic softening to specimen failure at all the strain amplitudes employed in the work. Additionally, cyclic deformation parameters determined in the work indicate strong fatigue deformation resistance of the newly developed superalloy. On the other hand, room temperature FCG test results showed that an increase in the stress ratio leads to a rise in the crack growth rate. However, in contrast to common assumption, the loading frequency was observed to have an effect on the crack growth behaviour at room temperature. Furthermore, an increase in temperature from 25°C to 600°C at a testing frequency of 15Hz resulted in an increase in the fatigue crack growth at all stress intensities. However, at a frequency of 0.05Hz, over a relatively large stress intensity range (ΔK), the fatigue crack growth rate decreased with an increase in temperature. The effect of temperature on the fatigue crack growth rate of Haynes 282 are discussed in terms of homogeneity of slip, roughness induced crack closure and dynamic strain ageing.

Cyclic deformation behavior of a super-vacuum die cast magnesium alloy

Magnesium alloy as a lightweight structural material has recently kindled considerable interest in the automotive and aerospace industries, since lightweighting is considered as one of the salient strategies in reducing fuel consumption and anthropogenic greenhouse gas emissions. The structural applications of magnesium alloys inevitably involve fatigue resistance. This study was aimed at evaluating cyclic deformation behavior and fatigue life of a super-vacuum die cast (SVDC) AM60B alloy using strain-controlled low cycle fatigue tests at two strain ratios of Rs = −1 and Rs = 0.1. The SVDC AM60B alloy exhibited a superior fatigue resistance to the conventional die cast AM60 alloy especially in the high-cycle fatigue region. Fatigue life was longer at Rs = −1 than at Rs = 0.1 at lower strain amplitudes. With increasing total strain amplitude, cyclic stress amplitudes increased, hysteresis loops exhibited a clockwise rotation despite the symmetry in tension and compression at Rs = −1, and fatigue life and psuedoelastic modulus decreased at both strain ratios. Cyclic hardening increased with increasing strain amplitude and strain ratio due to the formation of more twins and their interaction with dislocations. Two types of twins (wider lenticular extension twins and narrower banded contraction twins) were observed in some favorably oriented large α-Mg cells near the fracture surface. Mean stress relaxation occurred mainly within the initial 10–30% of fatigue life. Cyclic hardening exponent was higher than monotonic hardening exponent. Fatigue crack initiation occurred from the specimen surface or near-surface defects, and crack propagation was mainly characterized by fatigue striations coupled with some secondary cracks and tear ridges.

Study on cyclic deformation behavior of extruded Mg–3Al–1Zn alloy

Abstract Fatigue properties and cyclic deformation behavior of an extruded Mg–3Al–1Zn (AZ31) alloy were investigated by performing the strain-controlled low-cycle fatigue tests at room temperature. We believed that the total strain amplitude should be closely related to fatigue properties and cyclic damage process. This study aimed to investigate the deformation mechanism from the viewpoint of strain amplitude. We found that when the total strain amplitude increased from 0.3% to 0.4%, there was a noticeable change in compressive peak stress amplitude, which might correspond to the competition between twinning–detwinning process and dislocation slips. Microstructure and fracture morphology were investigated using optical microscopy and scanning electron micrograph. At the total strain of great amplitude, fracture surfaces were characterized by the striation-like features and dimple-like structures.

Low-cycle fatigue properties of a titanium alloy exhibiting nonlinear elastic deformation behavior

Strain-controlled low-cycle fatigue (LCF) behavior of a metastable β-type titanium alloy Ti–24Nb–4Zr–8Sn (wt.%, termed Ti2448) was investigated at room temperature under different cycle strains. The results showed that the alloy possesses good LCF resistance because applied total cycle strains up to 4.5% are endured mainly by the recoverable elastic strain. The study also revealed a dependence of the elastic deformation behavior on the loading condition: nonlinear elasticity with large recoverable strains in uniaxial tension but almost linear elasticity with higher elastic modulus and lower recoverable strain in uniaxial compression. The above elastic asymmetry explains the facts that both fatigue life in LCF and fatigue strength in high-cycle fatigue are worse in tension–compression cycles than in tension–tension cycles. Microstructure analyses of these fatigue-tested specimens showed that the β phase is elastically stable at cycle stresses up to ∼400 MPa whereas further increasing the applied cycle stress and strain results in small amounts of the ω phase and the α′′ martensite. These two stress-induced phase transformations are competing and exclude each other across a wide range of cycle stress.

Energy-based approach to predict the fatigue life behavior of pre-strained Fe–18Mn TWIP steel

The influence of pre-strain on the fatigue life behavior of Fe–18Mn twinning-induced plasticity (TWIP) steel was investigated by performing strain-controlled low-cycle fatigue (LCF) and stress-controlled high-cycle fatigue (HCF) tests with variation of the amount of pre-strain. The pre-strain to the as-received plate was achieved through a cold rolling process; thickness reductions of 10 and 30% were applied. The results revealed that both the LCF and HCF resistances were strongly dependent on the amount of pre-strain but showed vastly different effects; with increasing pre-strain, LCF resistance was degraded but HCF resistance was improved. An energy-based concept was adopted and modified to capture the pre-strain effect on the fatigue life behavior by considering the energy consumed by pre-straining. The developed model was further extended to establish a correlation between the LCF and HCF life data. The results showed that the HCF life behavior could be successfully predicted from the LCF life data.

STRAIN RATE CHANGE FROM 0.04 TO 0.004%/S IN AN ENVIRONMENTAL FATIGUE TEST OF CF8M CAST STAINLESS STEEL

To define the effect of strain rate variation from 0.04% to 0.004%/s on environmental fatigue of CF8M cast stainless steel, which is used as a primary piping material in nuclear power plants, low-cycle fatigue tests were conducted at operating pressure and temperature condition of a pressurized water reactor, 15 MPa and <TEX>$315^{\circ}C$</TEX>, respectively. A high-pressure and high-temperature autoclave and cylindrical solid fatigue specimens were used for the strain-controlled low-cycle environmental fatigue tests. It was observed that the fatigue life of CF8M stainless steel is shortened as the strain rate decreases. Due to the effect of test temperature, the fatigue data of NUREG-6909 appears a slightly shorter than that obtained by KEPRI at the same stress amplitude of <TEX>$1{\times}10^3$</TEX> MPa. The environmental fatigue correction factor <TEX>$F_{en}$</TEX>'s calculated with inputs of the test data increases with high strain amplitude, while the <TEX>$F_{en}$</TEX>'s of NUREG-6909 remain constant regardless of strain amplitude.

Microstructure and Low-Cycle Fatigue of a Friction-Stir-Welded 6061 Aluminum Alloy

Strain-controlled low-cycle fatigue (LCF) tests and microstructural evaluation were performed on a friction-stir-welded 6061Al-T651 alloy with varying welding parameters. Friction stir welding (FSW) resulted in fine recrystallized grains with uniformly distributed dispersoids and dissolution of primary strengthening precipitates β″ in the nugget zone (NZ). Two low-hardness zones (LHZs) appeared in the heat-affected zone (HAZ) adjacent to the border between the thermomechanically-affected zone (TMAZ) and HAZ, with the width decreasing with increasing welding speed. No obvious effect of the rotational rate on the LHZs was observed. Cyclic hardening of the friction-stir-welded joints was appreciably stronger than that of base metal (BM), and it also exhibited a two-stage character where cyclic hardening of the friction-stir-welded 6061Al-T651 alloy at higher strain amplitudes was initially stronger followed by an almost linear increase of cyclic stress amplitudes on the semilog scale. Fatigue life, cyclic yield strength, cyclic strain hardening exponent, and cyclic strength coefficient all increased with increasing welding speed, but were nearly independent of the rotational rate. Most friction-stir-welded joints failed along the LHZs and exhibited a shear fracture mode. Fatigue crack initiation was observed to occur from the specimen surface, and crack propagation was mainly characterized by the characteristic fatigue striations. Some distinctive tiremark patterns arising from the interaction between the hard dispersoids/inclusions and the relatively soft matrix in the LHZ under cyclic loading were observed to be present in-between the fatigue striations.

Effect of prior cyclic damage removal on high temperature low cycle fatigue endurance

In terms of fatigue life recovery, the beneficial effect of removing a surface layer from critical locations of components subject to high frequency elastic loading is well documented. Not so well known are the consequences of surface layer removal from parts previously subject to elasto-plastic low cycle fatigue (LCF) loading. The paper describes the results of a research project conducted to evaluate the effects of surface layer removal on subsequent LCF properties using uniaxial testpieces which have been prior cyclic deformed to different life fractions. Strain-controlled LCF tests performed on a 1%CrMoV steel at 565 °C with different strain amplitudes indicate that, following: (i) prior cyclic deformation (PCD) to nominal LCF life fractions of 0.5 and 0.8, (ii) subsequent surface damage layer removal, and (iii) test resumption; the consequent overall LCF endurances are at least as long as those accumulated without intermediate surface damage removal, and in some cases are significantly extended. Unlike the possibilities associated with elastic fatigue loading, LCF lifetimes are not fully recoverable following the removal of prior cyclic surface damage.

Dependence of the distribution of deformation twins on strain amplitudes in an extruded magnesium alloy after cyclic deformation

Cyclic deformation characteristics and the distribution of deformation twins in a recently developed magnesium alloy AM30 after strain-controlled low cycle fatigue tests were studied in the loading direction along the extrusion direction. The alloy was basically cyclically stable at a lower strain amplitude of 0.2% with low stress amplitude and small plastic strain amplitude, while it exhibited strong cyclic hardening at a higher strain amplitude of 0.6%. In addition to the initially high stress amplitude and large plastic strain amplitude, the stress amplitude increased and plastic strain amplitude decreased significantly as cyclic deformation proceeded at the higher strain amplitude. Twinning in compression and detwinning in tension occurred at both strain amplitudes, and the distribution of the residual twins observed near the fracture surface was a consequence of competition between twinning and detwinning, depending on the strain amplitude, cyclic hardening–softening characteristics, and the plastic zone size ahead of the crack tip. In the stable cyclic deformation at the lower strain amplitude, deformation twins appeared mostly in the area prior to the final rapid fracture, while at the higher strain amplitude with cyclic hardening the extensive and well-developed twins were mainly observed in the crack initiation area.