Multiaxial Low Cycle Fatigue Tests Research Articles

A semi-empirical life-prediction model for multiaxial ratchetting-fatigue interaction of SUS301L stainless steel tubular welded joint

Based on the strain measured in the SUS301L stainless steel tubular welded joint in the asymmetrically stress-controlled multiaxial low-cycle fatigue tests by using the digital image correlation (DIC) method, the dependence of fatigue life on the loading level and loading path is well explained by discussing the multiaxial ratchetting-fatigue interaction in the fatigue failure zone of welded joint. To reflect the contribution of mean stress to the non-proportionality of loading path, a new non-proportionality factor is defined. And then the path-dependent damage parameters of pure fatigue damage and additional ratchetting one are proposed in a strain form and validated. Finally, a multiaxial life-prediction model is constructed through addressing the concurrent efforts of additional ratchetting damage and pure fatigue one to predict the multiaxial fatigue life of welded joint. The results demonstrate that the predicted multiaxial fatigue lives are well expected and all life data fall in the twice error bands regarding to the experimental ones.

Comparation of non-proportional hardening behavior and microscopic mechanism for CP-Ti under strain-controlled and stress-controlled multiaxial low cycle fatigue

Proportional and non-proportional multiaxial low cycle fatigue tests were performed on commercial pure titanium (CP-Ti), employing both strain-controlled and stress-controlled modes. CP-Ti exhibits a four-stage characteristic under higher applied strain amplitude. For other cases and under stress-controlled mode, the initial hardening dissipated, giving way to a typical three-stage cyclic softening characteristics. Two failure modes, fatigue failure and ratcheting failure, occur depending on the applied stress amplitude. CP-Ti exhibits obvious asymmetric strain response under symmetric stress-controlled mode depending on the loading level. CP-Ti demonstrates non-proportional hardening under both strain-controlled mode and stress-controlled mode with different microscopic mechanisms. The additional hardening due to non-proportional loading results in an increase in the axial response stress with increasing multiaxial strain ratio. Interactions between dislocations with different slip systems hinder the development of stable dislocation structures, leading to pronounced non-proportional hardening of CP-Ti under strain-controlled mode. As for stress-controlled mode, the significant growth of twin content improves the grain boundary strength, leading to the non-proportional hardening of CP-Ti.

Unravelling the effects of ratcheting and constraint on the cyclic behaviour of a martensitic steel under elevated temperature

This work examines important aspects of the high-temperature cyclic visco-plasticity behaviour, namely the ratcheting (cyclic creep) and constraint effects, under low-cycle fatigue (LCF) via experimental characterisation and physically based damage modelling. For this purpose, fully-reversed, load-controlled uniaxial and multi-axial saw-tooth (SWT) low cycle fatigue tests were carried out on a martensitic steel (FV566) at 600oC. Further, an improved physically-based viscoplasticity modelling framework is introduced, which links the material microstructural properties to the constitutive mechanical behaviour, thereby allowing us to explore the ratcheting and constraint effects not only at the macro level but also at the subgrain level. The Finite Element (FE) analyses were complemented by detailed microstructural characterisation of the tested samples to unveil the key mechanisms contributing to the cyclic visco-plasticity damage in the FV566 steel at high temperatures. Based on this investigation, the micromechanics origin of the softening was elucidated. Moreover, the micro-damage modelling results in conjunction with the physical characterisation offered an improved mechanistic understanding of fatigue notch strengthening behaviour in multi-axial LCF tests. Importantly, an additional cyclic degradation mechanism was captured in the presence of constraints, which relates to the precipitate coarsening behaviour.

Comparison of critical plane models based on multiaxial low-cycle fatigue tests of 316L steel

An endurance against multiaxial fatigue of 316L steel is studied. A fatigue life curve of torsional mode is substantially shifted to longer fatigue lives while tension/compression mode causes fracture the earliest. Fatigue life curves of both multiaxial modes are close to axial mode. Fourteen critical plane models for fatigue life predictions were applied and compared. Modified Smith-Watson-Topper models proved to be the most accurate. Criteria proposed by Fatemi and Socie, Li et al., Zhu et al. and by Wang and Brown provided satisfactory results as well. Smith-Watson-Topper model led to the most precise predictions of fatigue crack orientation.

Low-cycle fatigue behavior and life prediction of CP-Ti under non-proportional and multiaxial loading

Non-proportional multiaxial low-cycle fatigue (MLCF) tests were performed on CP-Ti at different multiaxial strain ratios. The optical microscopy observations and life prediction models verified that the critical plane of CP-Ti was the maximum principal strain plane and that it was effective under non-proportional loading. A non-proportional life coefficient and strain energy density coefficient are proposed. Typical MLCF life prediction models, such as the ESN, FS, KBM, and KBM-P models, are discussed under non-proportional loading. Finally, by associating the newly defined non-proportional life coefficient with the non-proportional hardening factor, two comprehensive non-proportional MLCF life prediction models are established, which have higher accuracies than those of previous models.

Multiaxial low cycle fatigue of notched 10CrNi3MoV steel and its undermatched welds

It is a fundamental step to quantify the multiaxial fatigue damage considering the complex service conditions for notched components or welded structures. In our study, multiaxial Low Cycle Fatigue (LCF) tests of notched specimens were conducted on 10CrNi3MoV high strength steel and related undermatched welds under proportional and non-proportional cyclic loadings. The characteristic states of normal/shear stress, strain and energy around notch roots are analyzed considering the elasto-plastic material behavior under low cyclic loadings by the cyclic plasticity model. The predicted fatigue life curves of the test data were constructed by five strain-based and energy-based parameters combining with the critical plane approach and notch Theory of Critical Distance (TCD). According to Smith-Watson-Topper (SWT) damage parameters definitions, the paper proposed a simplified SWT model incorporating the effect of non-proportional loading, notch energy concentration characteristics and cyclic hardening behaviors for the LCF assessment of notch specimens. The proposed model shows good agreements between predictions and test data by comparing different multiaxial fatigue damage models for notch specimens.

Comparison of multiaxial low cycle fatigue behavior of CP-Ti under strain-controlled mode at different multiaxial strain ratios

Multiaxial low cycle fatigue (MLCF) tests were performed on commercial pure titanium (CP-Ti) at different multiaxial strain ratios (λ = 0.865, 1.73, 3.46, 5.19). A higher λ accelerated the initial cyclic hardening for the axial stress. For torsional stress responses, all specimens showed initial cyclic hardening characteristics. Optical microscopy (OM) observations of the fracture surfaces showed that the critical plane of CP-Ti is better aligned with the maximum principal strain plane rather than the maximum shear strain plane. The fatigue-life curves obtained from the tests were constructed using the Zamrik, ESN, SWT, FS, and KBM models. According to the modified critical plane, a KBM-P model based on the maximum principal strain plane is proposed. A unified life prediction KBM-U model incorporating the effect of λ is further proposed. Scanning electron microscopy (SEM) observations of fatigue fractures showed that fatigue cracks initiated from the outer surface of the specimen at all λ. Many small cleavage surfaces of different sizes and heights were observed in the crack propagation zone. At higher λ, these cleavage characteristics were accompanied by shear dimples.

Effect of Pre-deformation on the Multiaxial Low-Cycle Fatigue Resistance of 2024-T4 Aluminum Alloy

Abstract The effect of pre-deformation on the fatigue properties of 2024-T4 aluminum alloy is evaluated, including pre-tension of 1, 2, 3, and 4 % as well as pre-torsion of 10°, 20°, 30°, and 40°. Multiaxial low-cycle fatigue (LCF) tests are carried out under a constant amplitude sinusoidal wave loading with a tension-torsion ratio √3 in air at room temperature. The results indicate that the multiaxial LCF life of the specimen reduces with the pre-deformation level. The observed mechanical behavior and associated phenomena are directly linked to microstructure characteristics, such as dislocation density, surface defects, and microcracks. The analysis of mechanism reveals a competition between beneficial effects that are due to hardening and detrimental influences that are due to defects that are caused by the pre-deformation. However, the multiaxial loading makes the detrimental influence more prominent. The typical fracture morphology of multiaxial fatigue can be observed. With the consideration of the effect of pre-deformation on fatigue limit, a modified criterion is developed to predict the multiaxial fatigue life of 2024-T4 aluminum alloy.

Low cycle fatigue behavior of additively manufactured Ti-6Al-4V under non-proportional and proportional loading

Experimental tests were conducted on additive manufactured Ti-6Al-4V titanium alloy to investigate the mechanical and crack properties under multiaxial cyclic loading. Selective Laser Sintering technique (SLS) was employed to fabricate four types of cylindrical hollow specimens. The typology of each specimen is defined by the orientation of the layers and by the application of a stress-relieving heat treatment after the production process. Stress-strain cyclic curves of the materials were obtained to investigate the material cyclic plastic behavior, that resulted independent of specimen variety. Strain-controlled multiaxial low cycle fatigue tests under proportional and non-proportional loading paths were carried out on the specimens. Not heat-treated specimens exhibited a higher low cycle fatigue resistance both for proportional and non-proportional loading. Drastic initial softening was detected in the majority of the tests. Additional hardening was detected in part of non-proportional tests, which is atypical for this alloy. The mutual influence of applied load and microstructural characteristics on fatigue life are finally discussed.

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.

Multiaxial low cycle fatigue life of Ti-6Al-4V under non-proportional loading with mean strain

The mean strain effect on the multiaxial low cycle fatigue behavior of Ti-6Al-4V was investigated. Strain-controlled multiaxial low cycle fatigue tests at room temperature were carried out using a tubular specimen in tension and torsion loadings. Strain paths employed were four types of proportional and non-proportional loadings. Failure lives are affected by not only the non-proportionality of stress and strain histories but also the mean stress and strain. This study discusses the failure life properties based on the test results obtained and also shows strain parameters for the life evaluation under non-proportional loading with mean strain. The proposed strain parameters, where Findley and Walker models were applied within a non-proportional strain range, were suitable parameters to evaluate the life for Ti-6Al-4V under non-proportional straining with mean strain.

Analysis of multiaxial low cycle fatigue of notched specimens for type 316L stainless steel under non-proportional loading

The paper analyzes multiaxial low cycle fatigue tests of notched specimens under proportional and non-proportional loading conditions. Strain controlled multiaxial low cycle fatigue tests were carried out using circumferentially notched round-bar specimens of type 316L stainless steel, with different stress concentration factors. The experimental results show that the crack initiation site is shifted from the notch tip. Based on this finding, a new model for life evaluation is proposed by taking into account the strain gradient in the proximity of the notch tip and the effective maximum strain range. The new model allows evaluating the fatigue life in a narrow scatter band.

Multiaxial Fatigue of 6061-T6 Aluminum Alloy under Corrosive Environment

Aluminium alloys are widely used in the fields of automobile, machinery and naval construction. To investigate the effect of non-proportional loadings and corrosive environment on the fatigue resistance of 6061-T6 aluminum alloy, a set of uniaxial and multiaxial low cycle fatigue tests were carried out. Firstly, the results of uniaxial tests showed that the alloy exhibited cyclic hardening then cyclic softening. With the increase of stress amplitude the cyclic softening became pronounced. The increasing of plastic deformation was basically cyclically stable with small plastic strain amplitude accumulation when the stress amplitude was lower than 200MPa ,while it was increasing rapidly when the stress amplitude was higher than 220MPa. Secondly, it was observed that non-proportional cycle additional hardening of 6061-T6 aluminum alloy was little. While the fatigue life was badly affected by the loading paths. Thirdly ,the fatigue corrosion interactions were also talked about in details by performing the tests under the same loading conditions with corrosive environment. The experiment proved that the seawater corrosion has huge impact on fatigue life under pH 3. Finally, a multi-axial fatigue life prediction model was used to predict the fatigue life with or without the corrosive environment which showed a good agreement with experimental data.

Microstructural study of multiaxial low cycle fatigue

This paper discusses the relationship between the stress response and the microstructure under tension-torsion multiaxial proportional and nonproportional loadings. Firstly, this paper discusses the material dependency of additional hardening of FCC materials in relation with the stacking fault energy of the materials. The FCC materials studied were Type 304 stainless steel, pure copper, pure nickel, pure aluminum and 6061 aluminum alloy. The material with lower stacking fault energy showed stronger additional hardening, which was discussed in relation with slip morphology and dislocation structures. This paper, next, discusses dislocation structures of Type 304 stainless steel under proportional and nonproportional loadings at high temperature. The relationship between the microstructure and the hardening behavior whether isotropic or anisotropic was discussed. The re-arrangeability of dislocation structure was discussed in loading mode change tests. Microstructures of the steel was discussed in more extensively programmed multiaxial low cycle fatigue tests at room temperature, where three microstructures, dislocation bundle, stacking fault and cells, which were discussed in relation with the stress response. Finally, temperature dependence of the microstructure was discussed under proportional and nonproportional loadings, by comparing the microstructures observed at room and high temperatures.

Multiaxial fatigue behavior of AISI 316L subjected to strain-controlled and ratcheting paths

To investigate the material behavior of a stainless steel grade AISI 316L, subjected to loading conditions typical for the primary cooling circuit of a light water nuclear reactor, a set of uniaxial, torsional and multiaxial low cycle fatigue and strain-controlled ratcheting tests has been carried out. The main goal of these experiments is to analyze how the cyclic deformation response is influenced by strain amplitudes, loading paths and ratcheting, and to estimate how fatigue life is affected by these elements. Finally, the performance of three advanced multiaxial fatigue criteria (i.e. Smith–Watson–Topper, Fatemi–Socie and Jiang) has been critically evaluated for all the considered loading cases.

Low Cycle Fatigue Damage of Mod.9Cr-1Mo Steel under Non-Proportional Multiaxial Loading

Abstract This study discusses multiaxial low cycle fatigue damage of Mod.9Cr-1Mo steel under proportional and non-proportional loadings at room and high (823K) temperatures. Strain-controlled multiaxial low cycle fatigue tests were carried out using a hollow cylinder specimen including interruption tests. Strain paths employed were a push-pull straining, a reversed torsion straining and a circle straining. Behaviors of cyclic deformations and failure lives and an evaluation of the failure life are discussed. Surface cracks and microstructures in fatigued specimens are also observed by a digital microscope and a transmission electron microscope (TEM) in order to investigate mechanisms of cyclic deformation, failure and reduction in failure life due to non-proportional loading.

改良9Cr-1Mo鋼の非比例多軸低サイクル疲労強度評価

This study discusses multiaxial low cycle fatigue strength of Mod.9Cr-1Mo steel under proportional and non-proportional loadings at room and high (823K) temperatures. Strain-controlled multiaxial low cycle fatigue tests were perfoemed using a hollow cylinder specimen including interruption tests. Strain paths employed were a push-pull straining, a reversed torsion straining and a circle straining. The former two are the proportional loadings where principal directions of stress and strain are unchanged. The latter one is the non-proportional loading where axial and shear strains have 90 degree phase difference and principal directions of stress and strain are changed continuously in a cycle. Behaviors of cyclic deformations and failure lives and an evaluation of the failure life are discussed. Surface cracks and microstructures in fatigued specimens are also observed by a digital microscope and a transmission electron microscope (TEM) in order to investigate mechanisms of cyclic deformation, failure and reduction in failure life due to non-proportional loading.

A Damage-coupled Multi-axial Time-dependent Low Cycle Fatigue Failure Model for SS304 Stainless Steel at High Temperature

Based on the time-dependent strain cyclic characteristics and fatigue behaviors of SS304 stainless steel under multi-axial cyclic loading at 700 ◦C, and in the frame of unified visoco-plastic cyclic constitutive model and continuum damage mechanics theory, the damage-coupled multi-axial time-dependent constitutive model and fatigue failure model were proposed. In the model, the evolution equation of damage was introduced in and the time-dependent effects, e.g. holding time, loading rate, were taken into account. The model was applied to the simulation of whole-life cyclic deformation behaviors and prediction of LCF life for SS304 stainless steel in multiaxial time-dependent low cycle fatigue tests. It is shown that the simulated results agree well with experimental ones.

Multiaxial Low Cycle Fatigue Test Method for Cruciform Specimen of Nickel-Base Single Crystal Superalloy

This paper presented the multiaxial low cycle fatigue test method for cruciform specimens of YH61 nickel base single crystal superalloy . The elastic compliances were obtained in tensile and torsion tests using single crystal bar specimens whose specimen axis was aligned to the crystal growth directions of [100] and [110]. Elastic finite element analysis was performed to determine specimen shape and relationship between the specimen axis and crystal growth direction. Multiaxial low cycle fatigue tests were performed using the cruciform specimens whose x axis was aligned to the [001] direction and the y axis to [010], [110] and [210] directions both x and y axis were aligned to the<110> direction. Mises equivalent strain was not a suitable parameter and Mises equivalent stress was proved to be available for the correlating the multiaxial fatigue lives of the single crystal superalloy.

OS0712 非比例多軸負荷におけるSUS316L鋼切欠材の低サイクル疲労寿命評価(OS7-3 解析・評価・き裂II,OS-7 多軸応力下における疲労損傷および疲労き裂進展2)

This study discusses multiaxial low cycle fatigue life of notched specimen under non-proportional loadings at room temperature. Strain controlled multiaxial low cycle fatigue tests were carried out using smooth and circumferentially notched round-bar specimens of type 316L stainless steel. Four kinds of notched specimens were employed with elastic stress concentration factors, K_t, are 1.5, 2.5, 4.2 and 6.0. The strain paths include proportional and non-proportional loadings. The former employed was a push-pull straining and a reversed torsion straining. The latter was achieved by strain path where axial and shear strains has 90° phase difference but their amplitudes are the same based on von Mises' criterion. The notch dependency of multiaxial low cycle fatigue life and the life estimation are discussed with employing inelastic finite element analysis. The lives depended on both K_t and strain path. The strain parameter for the life estimation is discussed with the non-proportional strain parameter proposed by an author with introducing K_t. The proposed parameter gave a satisfactory evaluation of multiaxial low cycle fatigue life for notched specimen of type 316L stainless steel under proportional and non-proportional loadings.