Strain-controlled Low Cycle Fatigue Research Articles

Low cycle fatigue behaviour at 300 °C and microstructure of Al-Si-Mg casting alloys with Zr and Hf additions

The strain-controlled low cycle fatigue (LCF) behaviour at 300 °C and the microstructure of Al-7Si-0.3Mg casting alloys with additions of Zr and/or Hf are studied. Microstructural characteristics are investigated in detail by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It is found that the Zr- and/or Hf-rich primary particles and precipitates form in the alloys and have a significant effect on the fatigue properties. The cyclic softening characteristics are mainly attributed to the combination of the effects of the interaction of dislocations with nanobelt-like precipitates and an over-ageing phenomenon in the Mg2Si phase during the LCF test at 300 °C. The dislocations climbed over the nanobelt-like precipitates and tangled around the rectangle-like precipitates. Fatigue cracks initiated from the specimen surface, and crack propagation was characterized by dimple-like features coupled with fatigue striations. The best fatigue resistance of Al-7Si-0.3Mg-0.14Zr-0.44Hf casting alloy is related to the combination of morphological changes of the primary phase, the pinning effect of the Zr + Hf-rich precipitates and the different fracture characteristics.

Effects of Grain Size on the Fatigue Properties in Cold-Expanded Austenitic HNSs

Cold-expanded austenitic high nitrogen steel (HNS) was subjected to investigate the effects of grain size on the stress-controlled high cycle fatigue (HCF) as well as the strain-controlled low cycle fatigue (LCF) properties. The austenitic HNSs with two different grain sizes (160 and 292 μm) were fabricated by the different hot forging strain. The fine-grained (FG) specimen exhibited longer LCF life and higher HCF limit than those of the coarse-grained (CG) specimen. Fatigue crack growth testing showed that crack propagation rate in the FG specimen was the same as that in the CG specimen, implying that crack propagation rate did not affect the discrepancy of LCF life and HCF limit between two cold-expanded HNSs. Therefore, it was estimated that superior LCF and HCF properties in the FG specimen resulted from the retardation of the fatigue crack initiation as compared with the CG specimen. Transmission electron microscopy showed that the effective grain size including twin boundaries are much finer in the FG specimen than that in the CG specimen, which can give favorable contributions to strengthening.

A strain-gradient, crystal plasticity model for microstructure-sensitive fretting crack initiation in ferritic-pearlitic steel for flexible marine risers

A three-dimensional, strain-gradient, crystal plasticity methodology is presented for prediction of microstructure-sensitive length-scale effects in crack initiation, under fatigue and fretting fatigue conditions, for a ferritic-pearlitic steel used in flexible marine risers. The methodology, comprising length-scale dependent constitutive model and scale-consistent fatigue indicator parameters, is calibrated and validated for representative (measured) dual-phase microstructures under strain-controlled low cycle fatigue conditions. Prediction of the effects of length-scale on fretting crack initiation is based on a three-dimensional, crystal plasticity, frictional contact model to predict fretting crack location and initial growth path, accounting for the effects of crystallographic orientation. The length-scale dependent fatigue and fretting simulations predict (i) significant beneficial effect of reducing length-scale for low cycle fatigue life, (ii) complex cyclically- and spatially-varying effects and differences due to changing contact and grain length-scales, and (ii) that fretting damage generally decreases with decreasing (contact-grain) length-scale.

A study of low cycle fatigue life and its correlation with microstructural parameters in IN713C nickel based superalloy

Up to date, IN713C Nickel-based superalloy has been continued to be the best alloy candidate for turbocharger wheel applications due to its adequate fatigue property and resistance to degradation under harsh operating environments. Throughout this study, three different batches of as-cast IN713C nickel based superalloys with different microstructures including columnar, equiaxed and transition microstructures were investigated. Strain control Low Cycle fatigue (LCF) tests were conducted for the three different microstructures, achieving fatigue life between 100 and runout at 100,000 cycles, depending on the testing parameters. The fracture mechanics and failure mechanism were correlated to the alloy's microstructure, texture and chemical composition under various LCF conditions using optical microscopy, SEM, EDX and EBSD. In the current study an exact correlation between alloy's microstructure/microtexture and LCF endurance is established. The results showed that equiaxed microstructure has a superior fatigue life than the transition microstructure by 10% and columnar microstructure by > 200% at a given temperature and strain rate. This large discrepancy was mainly due to the grain size differences between the studied microstructures. Here, it was evidenced that the grain size controls the dendrites length. It is also demonstrated that all microstructures exhibited a longer fatigue life at room temperature than at 650 °C, doubling or tripling the fatigue life of the tested IN713C. Furthermore, the high presence of precipitates between dendritic arms in all three microstructures was found to have great influence on crack propagation path. It was apparent that segregated carbides in between dendritic arms caused secondary crack initiation and crack path undulations during the LCF tests.

A microstructure sensitive modeling approach for fatigue life prediction considering the residual stress effect from heat treatment

A multiscale numerical method to study the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) properties of bearing steels is proposed in this study. The method is based on the microstructur sensitive modeling approach resulting from the integrated computational materials ensfginerrring concept, and further consider the effect of residual stress generated from the prior heat treatment processes. The microstructure features, including the grain size and shape distribution and inclusion size and shape description, are represented by the representative volume element (RVE) models. The matrix mechanical response to the cyclic loading is described by the crystal plasticity (CP) model. The CP material parameter set is calibrated inversely based on the strain-controlled low cycle fatigue tests. The results show that the residual stresses, especially those around the inclusion, have a great effect on the fatigue properties, which provides the key factor to give the correct prediction of the fatigue crack initiation site.

Low cycle fatigue of a directionally solidified nickel-based superalloy: Testing, characterisation and modelling

Low cycle fatigue (LCF) of a low-carbon (LC) directionally-solidified (DS) nickel-base superalloy, CM247 LC DS, was investigated using both experimental and computational methods. Strain-controlled LCF tests were conducted at 850°C, with a loading direction either parallel or perpendicular to the solidification direction. Trapezoidal loading-waveforms with 2s and 200s dwell times imposed at the minimum and the maximum strains were adopted for the testing. A constant strain range of 2% was maintained throughout the fully-reversed loading conditions (strain ratio R = −1). The observed fatigue life was shorter when the loading direction was perpendicular to the solidification one, indicating an anisotropic material response. It was found that the stress amplitude remained almost constant until final fracture, suggesting limited cyclic hardening/softening. Also, stress relaxation was clearly observed during the dwell period. Scanning Electron Microscopy fractographic analyses showed evidence of similar failure modes in all the specimens. To understand deformation at grain level, crystal plasticity finite element modelling was carried out based on grain textures measured with EBSD. The model simulated the full history of cyclic stress-strain responses. It was particularly revealed that the misorientations between columnar grains resulted in heterogeneous deformation and localised stress concentrations, which became more severe when the loading direction was normal to a solidification direction, explaining the shorter fatigue life observed.

Strain energy based low cycle fatigue damage analysis in a plain C-Mn rail steel

Strain-controlled low cycle fatigue tests were performed on 60kg/m 90-UTS plain C-Mn rail steel with fully pearlitic microstructure. Analysis of hysteresis loop shape and properties revealed that the steel deviated from the ideal Masing type material behaviour to a little extent. To evaluate the fatigue damage in rail steel, energy-based analytical approach proposed by Morrow based on Masing hypothesis was employed. The Morrow energy model resulted in reasonably good approximation of both average plastic strain energy and fatigue life in comparison to experimentally observed values. In addition to that the plastic strain energy density (ΔWp) was correlated to fatigue life (2Nf) through a simple power law and very well represented by Coffin-Mansion type relationship similar to conventional strain-life (Δεp – 2Nf) relationship. Finally, the concept of fatigue toughness, an energy parameter, was introduced to life prediction in rail steel which was proved to be a suitable and reliable alternative to strain-life approach in fatigue damage evaluation with high degree of accuracy.

Effect of dwell condition on fatigue behavior of a high-Nb TiAl alloy at 750 °C

The strain-controlled low cycle fatigue (LCF) and creep-fatigue interaction (CFI) tests of a newly developed Ti-45Al-8Nb-0.2W-0.2B-0.02Y (at.%) alloy were carried out at 750 °C in air. The hysteresis loop, cyclic stress response and life modeling as well as failure mechanism of the alloy were investigated in detail. It was revealed that the tensile and compressive mean stresses would generate when the dwell condition was introduced at minimum and maximum strain, respectively. In addition, the dwell condition, especially for the compressive dwell condition, would significantly decrease the fatigue life. The typical continuum damage accumulation(CDA) and modified CDA life models proposed in the present study were employed to predict both LCF and CFI life of the alloy, which showed that the modified CDA life model had a higher accuracy than the typical CDA life one. Moreover, only single crack initiation source was observed at 92% (i.e. 11/12) of LCF fracture while multiple crack initiation sources at 84% (i.e. 31/37) of CFI fracture. Apparently different from LCF specimen showing more transgranular appearance, CFI specimen shows more intergranular appearance.

Anisotropic in-plane fatigue behavior of rolled magnesium alloy with {10−12} twins

The effects of initial {10−12} twins on the in-plane fatigue characteristics of a rolled Mg alloy were investigated by introducing these twins through precompression along the rolling direction of the material and then performing strain-controlled low-cycle fatigue tests along the rolling direction, 45° to the rolling direction, and transverse direction. The as-rolled sample with a strong basal texture was found to exhibit the same fatigue behavior in all the loading directions, whereas the precompressed samples exhibited anisotropic in-plane fatigue behaviors because the specific crystallographic orientation of the twinned region resulted in a variation in the active deformation modes during cyclic loading in the applied loading directions. An increase in the amount of precompression was also found to enhance the anisotropy of the in-plane fatigue behavior via an increase in the area fraction of the twinned region. The variation in the dominant deformation mechanism with a change in the loading direction was investigated by analyzing the crystallographic angle relationship and activation stresses, and the resultant evolutions of the cyclic flow curve and fatigue life were discussed in detail.

Low-cycle fatigue behavior of fiber-laser welded, corrosion-resistant, high-strength low alloy sheet steel

Advanced laser welding techniques such as fiber laser welding are desirable for joining high-strength steels in ground vehicle applications since they can greatly increase productivity. Weld fatigue is a primary design consideration for implementation of laser-welded sheet into ground vehicle structures. However, much of the previous fatigue work on fiber laser welded sheet steel has utilized stress-controlled fatigue testing. Therefore, this paper presents an optimized fiber laser welding procedure for butt welds of high-strength low-alloy steel sheet, and subsequent fully-reversed, strain-controlled low-cycle fatigue testing on weld material. Existing low cycle fatigue models adequately describe fatigue behavior of the laser welds. The results show that weld low cycle fatigue strength is significantly increased relative to base metal due to formation of martensite in the weld caused by rapid cooling inherent to laser welding. Weld fatigue life was shorter than base metal when plastic strains were high, but was close to base metal at lower plastic strains. Fatigue specimens containing weld failed at both base metal and in the weld indicating that there is some competition between weld cyclic fatigue strength and stress concentration effect at the weld fusion zone.

Effect of Grain Size on Low Cycle Fatigue Life in Compressor Disc Superalloy GH4169 at 600 °C

Nickel-based superalloy GH4169 in Chinese series (similar to Inconel 718 in the U.S. and NC19FeNb in France) has been widely used for aero-engine discs due to its excellent fatigue, oxidation and corrosion resistance at high temperature. The effect of grain size on the low cycle fatigue (LCF) lifetime was investigated in this study. Smooth specimens cut from an actual compressor disc in a civil aero-engine were conducted to strain-controlled LCF tests at 600 °C, and results showed a significant scattering in the LCF lifetime. Then grain sizes of specimens were estimated using optical microscope (OM) and Image-Pro Plus (IPP) software. A negative correlation between the grain size and LCF lifetime was found by inducing the grain refinement strengthening theory. Afterwards, a modified Smith-Watson-Topper (SWT) model was developed to describe the dependence of LCF lifetime on grain size. Finally, the possible microscopic mechanism of the grain size-dependent LCF behavior of GH4169 superalloy was discussed based on fracture analysis.

Fatigue Deformation Behavior of Fe-Mn-C-(Al) TWIP Steels

The effects of Al on the monotonic deformation behavior of Fe-Mn-C twinning-induced plasticity (TWIP) steels have been extensively investigated; however, how the addition of Al affects the fatigue properties of these steels is unknown. The present paper deals with the cyclic deformation properties of Fe-22Mn-0.6C-0Al and Fe-22Mn-0.6C-3Al steels by means of total strain-controlled low-cycle fatigue tests. The total strain amplitude ranges from 0.002 to 0.01. The evolved microstructures of fatigued samples were observed by transmission electron microscopy. Results show that the addition of Al has a significant effect on the cyclic deformation behavior, fatigue lifetime and evolved microstructure of Fe-Mn-C TWIP steel.

A combined critical distance and highly-stressed-volume model to evaluate the statistical size effect of the stress concentrator on low cycle fatigue of TA19 plate

Titanium alloy has been widely used for compressor discs in aero-engines due to its high specific strength. As a common geometry feature in compressor discs, bolt hole with stress concentration can cause compressor discs to fail under low cycle fatigue (LCF). In this study, the statistical size effect of such stress concentrators was investigated by LCF experimentations and theoretical predictions for titanium alloy plate specimens with a central circular hole (CHP). Four different scales of test sections without the influence of geometrical size were specially designed and experimental tests were conducted under LCF loading at 180°C. The strain-controlled LCF tests of smooth specimen were also carried out at the same temperature. It is found that the predicted LCF lifetime from the theory of critical distance (TCD) correlates well with experimental results for the 100%-scale CHP specimens. However, it fails to accurately predict the dependence of LCF lifetime on the statistical size effect for CHP specimens at the other scales i.e., 40%-scale, 60%-scale, and 80%-scale. Furthermore, a novel methodology combining the TCD and highly-stressed-volume models is proposed to evaluate the statistical size effect, which exhibits much improved accuracy than the TCD method alone. The new model not only inherits the versatility of the TCD method but providing an essential assessment of the statistical size effect.

Deformation and damage mechanisms of a bimodal 12Cr-ODS steel under high-temperature cyclic loading

The present study reports on the high-temperature low-cycle fatigue (LCF) behavior of a ferritic bimodal 12Cr oxide dispersion strengthened steel. The fully reversed strain-controlled LCF tests were conducted at a constant total strain rate with different axial strain amplitude levels. The measured cyclic stress response showed four distinct stages which include instant initial cyclic softening followed by gradual cyclic hardening thereafter continuous linear cyclic softening and finally crack initiation and growth stage. The rate and amount of these stages depend on both strain amplitude and testing temperature. The cyclic stress-strain and strain-life relationships were obtained through the test results, and related LCF parameters were calculated. Microstructural investigations, using electron microscopy, were carried out to shed light on the deformation mechanisms. At 550°C, no appreciable microstructural evolution occurred, which is consistent with the manifestation of Masing behavior. A propensity towards cross-slip and hence the formation of three-dimensional dislocation structures increased with increase in loading amplitude and testing temperature, which assisted in accumulating higher inelastic strain and led to slightly higher cyclic hardening at these conditions. Furthermore, W-enriched Laves phase particles precipitated at grain boundaries. The evolution of cell structures with strain amplitude at 650°C resulted in a small deviation from Masing behavior. The damage studies revealed multiple surface initiated transgranular cracks under higher strain amplitudes and a single surface initiated transgranular crack under lower strain amplitudes. The different stages of the crack path, whose durations depend on the testing conditions, revealed distinct fatigue fracture features.

Experimental fatigue characterization and elasto-plastic finite element analysis of notched specimens made of direct-quenched ultra-high-strength steel

The low-cycle fatigue behavior of a direct-quenched ultra-high-strength steel was experimentally characterized and numerically modeled. Fatigue and cyclic parameters were obtained by conducting strain-controlled low-cycle fatigue tests on uniform-gage specimens. Surface residual stresses were minimized and axial deflection eliminated by optimization of machining parameters and post-machining electro-polishing. The steel material showed cyclic softening and decrease in yield strength. Cyclic softening, stabilized response, and the cyclic stress–strain curve were numerically simulated using finite element analysis with a model capable of describing nonlinear kinematic-isotropic hardening. The results showed good agreement with experimental values and validated the model’s ability to simulate the softening and cyclic stabilization of the material under investigation. The same numerical method was then used in elasto-plastic stress–strain analysis of notched specimens made of the same material to make fatigue life predictions. Estimated lives were compared with predictions made by analytical approximations such as the linear rule, Neuber’s rule, and the strain energy density method and verified by experimental data. Finite element analysis using stabilized cyclic response yields the most accurate predictions and, thus, provides an effective tool for the fatigue analysis of this material.

Behavior of 321 stainless steel under engineering stress and strain controlled fatigue

Cyclic deformation and fracture behavior of the 321 stainless steel are investigated at 633K under engineering-stress and strain controlled low cycle fatigue (LCF) cycling, and the fatigue life data are compared with ASME section-III Division-1 S-N curve. In stress-controlled LCF tests, ratcheting-strain accumulation is observed even under zero mean-stress, and the direction of ratcheting is found to strongly depend on initial-ramp direction of first-cycle. In addition, plastic-strain range and striation width are observed to be higher than those in strain-controlled tests. The stress-life curve obtained from stress-controlled tests is 2–3 orders of magnitude lower in comparison to stress-life curve derived from strain-controlled LCF tests as per the ASME procedure. The discrepancy is attributed to the differences in deformation and damage development in stress and strain-controlled cycling.

Strain-controlled low cycle fatigue properties of a rare-earth containing ME20 magnesium alloy

The present study was aimed to evaluate the strain-controlled cyclic deformation characteristics and low cycle fatigue (LCF) life of a low (~0.3wt%) Ce-containing ME20-H112 magnesium alloy. The alloy contained equiaxed grains with ellipsoidal particles containing Mg and Ce (Mg12Ce), and exhibited a relatively weak basal texture. Unlike the high rare earth (RE)-containing magnesium alloy, the ME20M-H112 alloy exhibited asymmetrical hysteresis loops somewhat similar to the RE-free extruded Mg alloys due to the presence of twinning-detwinning activities during cyclic deformation. While cyclic stabilization was barely achieved even at the lower strain amplitudes, cyclic softening was the predominant characteristics at most strain amplitudes. The ME20M-H112 alloy showed basically an equivalent fatigue life to that of the RE-free extruded Mg alloys, which could be described by the Coffin-Manson law and Basquin's equation. Fatigue crack was observed to initiate from the near-surface imperfections, and in contrast to the typical fatigue striations, the present alloy showed some shallow dimples along with some fractions of quasi-cleavage features in the crack propagation area.

Cycle symmetrization, asymmetrical behaviour and ratchetting during low cycle fatigue of high temperature alloys

Strain-controlled (low-cycle fatigue type) experiments were performed on 12 different high temperature alloys (ferritic, austenitic, superalloys, single-crystal material, a directionally solidified alloy and two titanium alloys) in the range 550–950°C. Symmetrical cycling conditions were suddenly changed to offset strain cycling at the same strain range, but in the tension or compression direction. Up to 50 cycles were allowed to determine the subsequent approach to symmetrical stress limits for each material (elastoplastic shakedown). Comparison is made with limited reported tests and related load-control tests involving strain ratchetting . Shakedown behaviour is shown to depend both on the path of approach, defined by stress–strain properties, but overall trends of stress versus cycle plots are best made by assuming an effective strain range which is less than the imposed experimental strain range.

Modeling the Creep of Hastelloy X and the Fatigue of 304 Stainless Steel Using the Miller and Walker Unified Viscoplastic Constitutive Models

Hastelloy X (HX) and 304 stainless steel (304SS) are widely used in the pressure vessel and piping industries, specifically in nuclear and chemical reactors, pipe, and valve applications. Both alloys are favored for their resistance to extreme environments, although the materials exhibit a rate-dependent mechanical behavior. Numerous unified viscoplastic models proposed in literature claim to have the ability to describe the inelastic behavior of these alloys subjected to a variety of boundary conditions; however, typically limited experimental data are used to validate these claims. In this paper, two unified viscoplastic models (Miller and Walker) are experimentally validated for HX subjected to creep and 304SS subjected to strain-controlled low cycle fatigue (LCF). Both constitutive models are coded into ansys Mechanical as user-programmable features. Creep and fatigue behavior are simulated at a broad range of stress levels. The results are compared to an exhaustive database of experimental data to fully validate the capabilities and performance of these models. Material constants are calculated using the recently developed Material Constant Heuristic Optimizer (macho) software. This software uses the simulated annealing algorithm to determine the optimal material constants through the comparison of simulations to a database of experimental data. A qualitative and quantitative discussion is presented to determine the most suitable model to predict the behavior of HX and 304SS.

Low Cycle Fatigue Behaviour of Ex-Service P92 Steel at Elevated Temperature

The influence of strain amplitude and strain rate on low cycle fatigue (LCF) behaviour of the ex-service P92 steel at temperature of 600°C has been examined. Fully reversed strain-controlled LCF tests were conducted at the strain amplitude between ±0.4% and ±0.8% employing strain rate of 2.4x10-3s-1 to 2.4x10-5s-1. The material has been found to exhibit continuous cyclic softening behaviour throughout the LCF tests, and there was no saturation stage observed. The number of cycles to failure decreased with lowering the strain rate and increasing strain amplitude. Mathematically, the relationship between time to failure and strain rate can be expressed by a power law relation. Elastic-plastic finite element (FE) analysis was carried out to obtain the hysteresis loop and cyclic stress response of the material. The computation results are shown to be in good agreement with the experimental data. Fractographic examinations of the fatigued specimens were performed using scanning electron microscopy (SEM). Under the LCF condition at higher strain rates, the crack propagated intergranularly which due to fatigue while at lower strain rates the crack may be propagated in both inter- and transgranular manner.