Strain-controlled Low Cycle Fatigue Research Articles

Comprehensive Assessment of Low Cycle Fatigue Behavior of Nickel-Base Superalloy Superni 263 in the Range 298–1023 K

The effects of temperature on low cycle fatigue (LCF) behavior of Superni 263 superalloy in the solution annealed plus aged state were examined between 298 and 1023 K. The initial microstructure consisted of intragranular spherical γʹ (22 nm), intergranular M23C6 carbides and a few un-dissolved primary MC particles. Strain-controlled LCF tests were performed at a strain rate of 10−3 s−1. Correlations between the initial microstructure, macroscopic cyclic deformation, fatigue life, crack initiation and propagation modes, evolving deformation substructure and phase changes have been attempted. Cyclic stress response displayed a marked variation in its temperature dependence. In general, the alloy exhibited initial hardening followed by very slight softening up to 473 K, continuous hardening to a maximum stress in between 573 and 873 K, and initial hardening succeeded by rapid softening at and above 923 K. Dynamic Strain Ageing (DSA) was noticed during 573–923 K. In the DSA domain, the alloy exhibited pronounced cyclic hardening at 673 K, displayed planar slip, stacking faults, high dislocation density and shearing of γʹ. The macroscopic features revealed serrated flow in the plastic deformation portions of stress–strain hysteresis loops and inverse temperature dependence of maximum stress and inelastic strain generated at half-life. Fresh precipitation of γʹ occurred at 973 K and 1023 K during LCF testing while prior existing γʹ coarsened marginally above 923 K. The operating deformation mechanisms of γʹ were identified and correlated with microscopic and macroscopic aspects. LCF life displayed a maximum at ~ 473 K and the reduction in life above 473 K was attributed to the deleterious effects of DSA and oxidation.

Experiment and modelling on the effect of microstructural morphology on fatigue life of a Ni-based superalloy

A directionally solidified Ni-based superalloy was pre-treated under high temperature with/without the applied stress to obtain different microstructural morphology. Then, strain-controlled low cycle fatigue experiments were carried out under 850°C with a 0 strain ratio to measure the reduction in the fatigue resistance. The scanning electron microscopy (SEM) was employed to characterize the microstructures of the alloy with different treatments. The micro-deformation and dislocation movement were investigated via transmission electron microscopy (TEM). The results show that both coarsening and rafting morphology have negative effects on the fatigue properties, which is closely related to the deformation mechanism of the alloys with different γ' morphology. The coarsening morphology decreases the shear resistance of the γ' precipitates, resulting in shearing of γ' particles. More plastic deformation introduces during the cyclic process, leading in the reduction of fatigue life. On the other hand, the wide γ channel promoted the piling up and movement of dislocations in the matrix, also resulting in reduction of plastic deformation resistance and lifespan. Finally, a fatigue life prediction model was proposed based on the continuum damage mechanics (CDM) by introducing the variation of matrix channels’ width, showing a good competency compared with the experimental results.

Effect of hold time on high temperature creep-fatigue behavior of Fe–25Ni–20Cr (wt.%) austenitic stainless steel (Alloy 709)

To understand high temperature creep-fatigue interaction of the Alloy 709, strain-controlled low-cycle fatigue (LCF) tests were performed at strain ranges varying from 0.3% to 1.2% with fully reversible cycle of triangular waveform at 750 °C. In addition, different hold times of 60, 600, 1800 and 3600 s were introduced at the maximum tensile strain to investigate the effect of creep damage on the fatigue-life at strain range of 1% at 750 °C. The creep-fatigue life and the number of cycles to macro-crack initiation and failure are found to decrease with increasing hold time indicating higher crack initiation and growth rates. Creep-fatigue life is evaluated by a linear summation of fractions of cyclic and creep damages according to ASME code. The fractographs of the samples deformed at 1% strain range indicated that fatigue might have been the dominant mode of deformation whereas, for the samples deformed at the same strain range with different hold times, both fatigue and creep have contributed to the overall deformation and fracture of the alloy.

A comparative assessment of cyclic deformation behavior of SA333 Gr-6 steel at ambient and elevated temperatures

Abstract The aim of this investigation is to study the comparative cyclic deformation behavior of SA333 Gr-6 steel at two different temperatures. Strain-controlled low cycle fatigue experiments were carried out at 1 × 10–3 s–1 strain rate at room temperature and 300 °C. Cyclic hardening of the steel was observed at both the temperatures. However, greater cyclic hardening was observed at 300 °C for all the strain amplitudes. Non-Masing behavior was observed at both the temperatures. Greater deviation from Masing behavior was observed at 300 °C due to generation of more dislocation activities. Higher dislocation density and sub-cell structure formation is associated with the hardening characteristic of the material. Lower fatigue lives were observed for all strain amplitudes at 300 °C in comparison to room temperature. This is due to the deleterious effect of dynamic strain aging, which is activated at 300 °C. The fracture surface reveals that with an increase in strain amplitude the number of crack initiation sites increases and the fatigue region decreases, which leads to a reduction in fatigue life.

Hysteresis energy based low cycle fatigue properties analysis in extruded Al-7Zn-2Mg-1.5Cu-0.2Sc-0.1Zr alloy at low temperature

The influences of low-temperature on low cycle fatigue (LCF) behaviors and fracture mechanisms of extruded Al-7Zn-2Mg-1.5Cu-0.2Sc-0.1Zr alloy with T6 state were investigated by strain-controlled LCF tests. The results show that the tensile strength increases, elongation and LCF life decreases with decreasing the temperature. The cyclic stress response behaviors of alloy are stable at various temperatures of room temperature (RT), 0 °C, −20 °C and −40 °C. And cyclic hysteresis energy densities are also stable after a short period of fluctuations at different conditions. In consideration of the different temperatures, the fatigue life evaluation model based on hysteresis energy is adopted. And the relationship between temperature and LCF life is derived from the parameters (Wo and β) of hysteresis energy model. The increase of hysteretic energy will lead to greater plastic damage, which is the main reason for the decrease of fatigue life, and the corresponding value of Wo significantly decreases. In addition, the fatigue crack initiates from the free surface of specimens and there exist a mass of fatigue striations in crack propagation region. The secondary cracks appear at low temperature, and the corresponding value of β slightly increases.

Ductility and fatigue properties of low nickel content type 316L austenitic stainless steel after gaseous thermal pre-charging with hydrogen

The susceptibility of low nickel content type 316L austenitic stainless steel to hydrogen was quantified using low strain rate tensile tests and strain-controlled low-cycle fatigue life measurements. Both tests were performed under air condition after charging with high-pressure 10-MPa hydrogen gas at 300 °C for eight days. No significant influence of hydrogen was recognized in 0.2% proof stress, but the strain at fracture and reduction area was decreased significantly in both hydrogen pre-charged and in gaseous hydrogen conditions compared to companion tests conducted in air. The decrease of fatigue life in the high strain amplitude region was related to a significant decrease in the plastic component while the effect of hydrogen on the elastic component was negligible. Highly localized deformation and a pronounced martensite transformation occurred near the site of the fracture surface in the high strain amplitude regime, resulting in the early formation of abundant micro-surface cracks in this regime of the hydrogen pre-charged samples.

Effect of HIPing and degassing on the low cycle fatigue behavior of A319 cast alloy

Microstructure and low-cycle fatigue (LCF) behavior of A319 cast alloy were investigated in this work. Microstructure analysis indicated that both degassing and hot isostatic pressing (HIPing) process could help to reduce and eliminate porosity. The strain-controlled low-cycle fatigue tests were conducted at total strain amplitude ranging from 0.2% to 0.4%. The stress-strain hysteresis loops were acquired and the effect of HIPing and degassing on cyclic deformation behavior was analyzed combining the cyclic stress response curves. Moreover, the effect of HIPing and degassing on LCF life was analyzed, and a fatigue life model based on the area fraction of defects was proposed which was found to be suitable to predict life. Finally, the fatigue fractography observation showed that the presence of porosity could accelerate the initiation and propagation of cracks and the fatigue failure of materials.

On the low-cycle fatigue behavior of friction stir welded Al–Si12 parts produced by selective laser melting

The solid-state friction stir welding (FSW) process was used to join Al–Si12 parts fabricated via the selective laser melting (SLM) technique. The effect of the welding process on microstructural evolution and mechanical properties of the samples is investigated in present work. Microstructural studies demonstrate that FSW is capable of changing Si phase morphologies (i.e. shape and size) resulting in various mechanical properties. The stir zone of the welded joint shows significantly lower micro-hardness in comparison to the as-built SLM samples. Correspondingly, the friction stir welding process results in significant reduction of tensile strength, while ductility is strongly improved. The fully-reversed strain-controlled low-cycle fatigue (LCF) tests imply that at low strain amplitudes the FSW and SLM samples show almost the same fatigue life, while at the high strain amplitudes the SLM samples show superior LCF performance. Fracture analysis of fatigued samples reveals that the near-surface pores lead to the crack initiation in both SLM and FSW cases.

Cyclic deformation behavior of a high zinc-containing cast magnesium alloy

The objective of the present study was to evaluate strain-controlled low cycle fatigue behavior of a high zinc-containing Mg-10Zn-5Al (wt.%) cast magnesium alloy in relation to the microstructure characterized via EBSD and XRD. The addition of Zn with a Zn/Al ratio of ∼2 suppressed the presence of β-Mg17Al12 phase, and the as-cast alloy consisted mainly of primary α-Mg and eutectic-like τ-Mg32(Al,Zn)49 phase in a characteristic network form. The alloy exhibited superior tensile properties compared with several commercial cast alloys. Unlike extruded magnesium alloys, the alloy exhibited almost symmetrical stress-strain hysteresis loops due to the presence of τ-Mg32(Al,Zn)49 phase and nearly random textures. Although slight cyclic softening occurred at higher strain amplitudes, cyclic stabilization basically remained. This was also reflected by the nearly overlapped cyclic and monotonic stress-strain curves. The fatigue life predicted via strain energy based approach was in good agreement with the experimental value. Fatigue crack initiated from the near-surface imperfections, and crack propagation was characterized by fatigue striation-like features along with tear ridges.

단결정 니켈기 초내열합금 CMSX-4의 고온 저주기 피로특성

Isothermal low cycle fatigue (LCF) behavior of a crystal nickel-based superalloy CMSX-4, a material for high-pressure turbine first stage rotor blade, was investigated at elevated temperatures. Strain-controlled LCF tests were performed under various test conditions, such as mechanical strain amplitude. Stress response and cyclic deformation were investigated, and equations of LCF life prediction were derived through the Coffin-Manson method. In addition, fatigue-induced fracture mechanism and microstructural evolution were investigated, using scanning electron microscopy (SEM). Results revealed that cyclic behavior of the CMSX-4 superalloy, was characterized by cyclic softening with increasing number of cycles at 800oC and 900oC. LCF of the CMSX-4 superalloy at 800oC and 900oC could be affected mainly by elastic damage in fatigue processing. Fatigue cracks were initiated in the surface oxide layer of the specimen. The plane of fracture surface was tilted toward 001 direction. The fatigue fracture mechanism was quasi-cleavage fracture at 800oC and 900oC. In all broken specimens, the γˊ phase morphology maintained cuboidal shape.

Effect of Precompression Deformation on the Strain-Controlled Low-Cycle Fatigue Behavior of Extruded AZ31 Magnesium Alloy

In this work, the effect of precompression along the extrusion direction on the strain-controlled low-cycle fatigue behavior of an extruded AZ31 magnesium alloy was investigated. After precompression deformation occurred, the tensile yield strength decreased, whereas the tensile yield strength and ultimate tensile strength increased gradually as precompression deformation increased. The shape of the stress–strain hysteresis of a specimen with 5% precompression deformation changed from asymmetrical to symmetrical compared with that of the as-extruded specimen. Hence, the whole process was likely dominated by twinning–detwinning behavior. Fractographic analysis revealed that the crack propagation region of the specimen with 5% precompression deformation was flatter than that of the as-extruded specimen because of the compressive mean stress and finer grains in the specimen with 5% precompression deformation. The fatigue lives of the as-extruded and precompression specimens were predicted by the Coffin–Manson model, the Ellyin energy model, the Smith–Watson–Topper model, the Fatemi–Socie model, and the Jiang model, respectively. The life predicted by the Ellyin energy model and the Jiang model was consistent with the experimental data.

Strain controlled low cycle fatigue properties of stepped aging heat treated AA5754

Abstract In present work strain controlled low cycle fatigue (SC-LCF) properties of stepped ageing heat treated(SAHT) AA5754 alloy is evaluated at three different strain amplitudes (0.4–0.6)% at fixed strain rate of 0.001 sec-1. SAHT process significantly improves the mechanical and fatigue life of the as-received alloy. The material shows cyclic hardening behavior at all the strain amplitudes. The hysteresis loop areas for SAHTed alloys are found to be significantly higher at corresponding elapsed cycles at higher strain amplitudes. Finite element simulation using Chaboche cyclic plasticity is utilized to predict the cyclic response of the alloy. Chaboche kinematic hardening material parameters are determined from stabilized hysteresis loop. Numerical results agree well with experimentally obtained results.

Influence of Notch Sensitivity and Crack Initiation Site on Low Cycle Fatigue Life of Notched Components under Multiaxial Non-proportional Loading

A series of strain-controlled multiaxial low cycle fatigue (LCF) tests under proportional and non-proportional loading conditions have been conducted on notched specimens. Cylindrical bars of Al 6061 aluminum alloy and AISI 316L stainless steel with four values of stress concentration factors referred to the net section Kt,n were employed. The experimental results evidenced a reduction of fatigue life due to non-proportional loading. Furthermore, the crack initiation site has been detected to be moved from the notch tip in the case of steel for high values of notch radius under non-proportional loading. Stress concentration factor evaluated in the elastic field Kt,n has been included in the Itoh-Sakane parameter to evaluate the fatigue life, returning a general underestimation of fatigue life especially for high values of Kt,n. Material notch sensitivity and crack initiation position have been taken into account to further modify the model, improving the original results and showing a better assessment.

Understanding strain controlled low cycle fatigue response of P91 steel through experiment and cyclic plasticity modeling

In this paper, constitutive models for cyclic plasticity, namely, Chaboche and the Ohno-Wang models, are used to simulate the low cycle fatigue response of tempered ferritic-martensitic P91steel. A series of uniaxial strain-controlled low cycle fatigue tests are conducted at room temperature to evaluate various fatigue parameters. P91 steel shows cyclic softening behavior at all strain amplitudes. The analysis of the shape of hysteresis loops reveals that P91 steel deviates from Masing behavior. Plastic strain energy is used as a parameter to characterize the fatigue damage. The Morrow energy model estimates experimental average plastic strain energy density accurately. Remnant tensile properties are evaluated to quantify the damage during low cycle fatigue loading. The evaluation of back stress components and material Jacobian are implemented through a user defined subroutine UMAT into the commercial finite element package ABAQUS. Ohno-Wang material model predicts the hysteresis loop shape, area, and cyclic softening nature better than the Chaboche model. The predicted fatigue lives by simulation correlates well with the experimental fatigue lives within ± 200 cycles. These results suggest that energy based parameters can provide a reliable alternative to strain based approach in evaluating the fatigue damage of P91 steel.

Microstructure analysis and low-cycle fatigue behavior of spray-formed Al–Li alloy 2195 extruded plate

Microstructure and low-cycle fatigue behavior of spray-formed Al–Li alloy 2195 extruded plate were investigated in this work. The spray-formed alloy after hot extrusion experiment was treated with solid solution treatment and artificial aging. Microstructure analysis indicated the aged plate was dominated by elongated unrecrystallized grains, and had a rolling-type texture along extrusion direction with the highest intensity at Brass component. The existence of T1 phase strengthened the alloy crucially, but δ′ phase was basically absent. Then, the fully-reversed strain-controlled low-cycle fatigue tests were conducted at total strain amplitudes ranging from 0.4% to 1.0% for samples along two orthogonal directions. The stress-strain hysteresis loops were acquired, and the cyclic stress response curves were derived. At low strain amplitudes (0.4–0.5%), the initial cyclic hardening was slight and followed by a cyclic stability, while at higher strain amplitudes (0.6–1.0%), the alloy merely presented a continuously increasing cycle hardening behavior. Moreover, the fatigue life model based on the total strain energy was built and found to be suitable to predict life. Finally, the fatigue fractography observation showed that the fatigue source is relatively concentrated and the fracture surface had typical fatigue striations at 0.5% strain amplitude, while multiple cracks originated on the sample surface and the final fracture zone showed a ductile characteristic at 1.0%. The deformed microstructure near fracture surfaces were observed, and it was found that the cyclic hardening and stability were closely associated with the interaction between moving dislocations and obstacles including (sub)grain boundaries and secondary phase particles against them.

Creep Fatigue Interaction under Different Test Variables: Mechanics and Mechanisms

Abstract We present detailed analyses of dwell characteristics of various waveforms of creep fatigue interaction tests performed on the nickel-based superalloy IN 718. We discuss the effects of different dwell modes (strain, stress, and mixed) on creep fatigue properties. Strain dwell tests cause relaxation/accumulation of the mean stress, and stress dwell tests cause accumulation/relaxation of mean strain. True interaction of strain-controlled low cycle fatigue and creep takes place when stress-controlled dwell has been introduced within a strain-controlled cycling. In this article, the effects of pure and mixed modes have been compared in terms of the creep strain accumulation rate at dwell positions. It has been found, irrespective of creep and fatigue, that there is also a second competitive process that goes on, like tensile strain accumulation and compressive strain accumulation. These two processes compete with each other and show a difference in evolution with cycles. Mean strain rates for different stress and mixed control tests have also been determined; by comparing the creep strain accumulation rate and plastic strain rate, the competition of creep and fatigue processes has also been demonstrated in this article. The mechanism of cracking and cavity formation has also been illustrated by computed microtomography and subsequent fractography.

Cyclic stress responses of a newly developed nickel-base superalloy at elevated temperatures

Total strain-controlled low cycle fatigue tests were conducted on the newly designed nickel-base superalloy M951G under different testing conditions; the relationship among cyclic stress responses, microstructural degradations, deformation mechanisms and testing conditions has been established. Results show that both the cyclic hardening and softening behaviors are dependent on the testing temperature and strain amplitude. As the strain amplitude increases both at 900 and 1000 °C, M951G alloy exhibits cyclic hardening under low strain amplitudes and cyclic softening under higher strain amplitudes. At 900 °C, the initial cyclic hardening is related to the coherent γ/γ′ interface, parallel dislocation arrays and dislocation bypassing the tiny γ′ particles. At higher strain amplitudes, the initial cyclic softening is due to the higher density of shearing dislocations in γ′ precipitates. At 1000 °C, plenty of parallel dislocation arrays present in γ channels, which reduces the possibility of dislocation interactions from different slip systems and results in initial cyclic hardening. Under higher strain amplitudes, apart from microstructural degradations, dislocations shearing into γ′ precipitates and formation of dislocation networks, the dislocation annihilation and partial loss of coherency of γ′ precipitates are also responsible for the initial cyclic softening at 1000 °C.

Influence of temperature on the low-cycle fatigue properties of compacted graphite iron

Strain-controlled low-cycle fatigue (LCF) tests at a fairly high strain rate were performed on as-cast compacted graphite iron (CGI) controlled with total strain amplitude at 25, 400 and 500 °C. The results show that CGI has the longest fatigue life, smallest stable hysteresis loop area at 400 °C. The reason is that the effective bearing size of the ferrite increases from 25 °C to 400 °C because slip deformation is retarded by carbon atom diffusion and carbide formation, and decreases from 400 °C to 500 °C because of the appearance of the ferrite grain boundary sliding. That effect of temperature is associated with the elastic strain amplitude instead of the plastic strain or stress amplitudes in macroscope. Finally the corresponding improvement suggestions based on these were also proposed.

Effect of Overaging on the Cyclic Deformation Behavior of an AA6061 Aluminum Alloy

The present work encompasses the effect of overaging on the strain-controlled low-cycle fatigue (LCF) behavior of an extruded AA6061 aluminum alloy at varying strain amplitudes. During the T7 aging treatment, the size of precipitates increased from 60 nm under T6 conditions to 220 nm after aging for 48 h at 200 °C, leading to a decrease in the monotonic tensile strength. During the LCF tests, nearly symmetrical hysteresis loops can be observed in the mid-life cycle under all test conditions, whereas the first-cycle hysteresis loops were moderately inflected under long-aging conditions. With increasing aging time, the cyclic peak stresses decreased and the plastic strain increased. Nearly ideal Masing behavior was exhibited under T6 conditions, while it was lost under T7 overaging conditions. The cyclic stress responses were similar under all tested conditions, involving stabilization at low strain amplitudes and softening at high strain amplitudes, with initial hardening for the first few cycles. Compared to the T6 condition, the fatigue life increased with increasing T7 aging time. Various LCF parameters were estimated based on the Coffin-Manson and Basquin relationships and on the LCF experimental results. The relationship between the fatigue life, strength, and microstructure of the investigated AA6061 aluminum alloy under various aging conditions was discussed.

Enhanced low-cycle fatigue and crack propagation resistance of an Al-Cu-Mg-Si forging alloy by non-isothermal aging

Low-cycle fatigue (LCF) and fatigue crack propagation (FCP) behavior of an Al-Cu-Mg-Si forging alloy with different non-isothermal aging (NIA) degrees were investigated. The main question addressed was about the interactions between precipitates and dislocations and their influences on LCF and FCP resistance. Strain-controlled LCF and ΔK-controlled FCP were tested. The characterization of precipitates and fractography were performed on TEM/HRTEM and SEM, respectively. The results showed that under cyclic loading, shearable precipitates were suffered from particle shearing mechanism associated with slip localization, resulting in cyclic softening behavior and low FCP rates; non-shearable precipitates were suffered from bypassing mechanism associated with homogeneous slip, resulting in cyclic hardening behavior and high FCP rates. The mixed precipitation characteristics obtained by complete NIA treatment, consisting of shearable and non-shearable precipitates, led to cyclic stability behavior and low FCP rates, which means a simultaneous enhancement of LCF and FCP resistance.