Tensile Mean Strain Research Articles

Effects of Temperature on the Evolution of Yield Surface and Stress Asymmetry in A356-T7 Cast Aluminium Alloy.

As the electrification of vehicle powertrains takes prominence to meet stringent emission norms, parts of internal combustion engines like cylinder heads are subjected to an increased number of thermal load cycles. The cost-effective design of such structures subjected to cyclic thermo-mechanical loads relies on the development of accurate material models capable of describing the continuum deformation behaviour of the material. This study investigates the effect of temperature on the evolution of flow stress under cyclic loading in A356-T7 + 0.5% Cu cast aluminium alloy commonly used in modern internal combustion engine cylinder heads. The material exhibits peak stress and flow stress asymmetry with the stress response and flow stress of the material under compressive loading higher than under tension. This peak and flow stress asymmetry decrease with an increase in temperature. To compare this stress asymmetry against conventional steel, cyclic strain-controlled fatigue tests are run on fully pearlitic R260 railway steel material. To study the effect of mean strain on the cyclic mean stress evolution and fatigue behaviour of the alloy, tests with tensile and compressive mean strains of +0.2% and −0.2% are compared against fully reversed (Rε = −1) strain-controlled tests. The material exhibits greater stress asymmetry between the peak tensile and peak compressive stresses for the strain-controlled tests with a compressive mean strain than the tests with an identical magnitude tensile mean strain. The material exhibits mean stress relaxation at all temperatures. Reduced durability of the material is observed for the tests with tensile mean strains at lower test temperatures of up to 150 °C. The tensile mean strains at elevated temperatures do not exhibit such a detrimental effect on the endurance limit of the material.

Effect of Tensile Mean Strain on Fatigue Behavior of Al-Li Alloy 2099

In this study, the effect of tensile mean strain on the fatigue behavior of aluminum alloy 2099-T83 was experimentally examined for the first time. In order to elucidate the effect of tensile mean strain on fatigue lives, strain-control fatigue testing was conducted at three strain ratios (R = − 1, 0.1, 0.7). Experimental results show that the application of greater mean strains during cyclic deformation resulted in shorter fatigue lives, particularly at lower strain amplitudes. At these lower strain amplitudes, the mean stress response to deformation did not fully relax as it did when subjected to larger strain amplitudes, where the effect of mean strains on fatigue performance was negligible. The Morrow and Smith–Watson–Topper mean strain correction models were implemented using monotonic and fully reversed fatigue properties in order to account for the damage associated with the relaxed mean stresses. A new model is presented that incorporates a mean stress sensitivity material property to capture, within a scatter band of three, the effects of tensile mean strains on the fatigue behavior of AA2099.

Superior fatigue life of Fe-15Mn-10Cr-8Ni-4Si seismic damping alloy under asymmetric cyclic loading with tensile mean strain

An Fe–15Mn–10Cr–8Ni–4Si (in wt%) alloy was recently developed for hysteretic seismic dampers that protect constructions against earthquakes. The alloy was found to exhibit a stable hysteresis response and an extraordinarily long low-cycle fatigue (LCF) life, Nf, under symmetric cyclic loading (tension/compression strain ratio Rε = −1), owing to reversible cyclic deformation associated with ε-martensitic transformation. The LCF life and failure microstructure of the alloy were studied at Rε values ranging from − 1–0.5, so that the strain domain of the cyclic deformation changed from symmetric tension–compression to asymmetric loading with tensile mean strain. The shift of the strain to the tensile domain, which is known to frequently have a negative effect on fatigue resistance because it induces tensile mean stress, brought about only a moderate fluctuation in Nf (Nf = 9200, 9500, 7500, and 8400 cycles at Rε = −1, −0.2, 0.2, and 0.5, respectively). The stress–strain hysteretic response remained stable, and the mean stress observed in early cycles was relaxed to zero during cyclic deformation. This stress relaxation is associated with strengthening of austenite texture with (111)// loading direction and suppression of the martensite fraction, which the reversible cyclic deformation mechanism seems to sustain.

Fatigue Modeling for Superelastic NiTi Considering Cyclic Deformation and Load Ratio Effects

A cumulative energy-based damage model, called total fatigue toughness, is proposed for fatigue life prediction of superelastic NiTi alloys with various deformation responses (i.e., transformation stresses), which also accounts for the effects of mean strain and stress. Mechanical response of superelastic NiTi is highly sensitive to chemical composition, material processing, as well as operating temperature; therefore, significantly different deformation responses may be obtained for seemingly identical NiTi specimens. In this paper, a fatigue damage parameter is proposed that can be used for fatigue life prediction of superelastic NiTi alloys with different mechanical properties such as loading and unloading transformation stresses, modulus of elasticity, and austenite-to-martensite start and finish strains. Moreover, the model is capable of capturing the effects of tensile mean strain and stress on the fatigue behavior. Fatigue life predictions using the proposed damage parameter for specimens with different cyclic stress responses, tested at various strain ratios (R e = e min /e max) are shown to be in very good agreement with the experimentally observed fatigue lives.

Effect of initial twins on the stress-controlled fatigue behavior of rolled magnesium alloy

The effect of initial twins on the fatigue behavior of rolled AZ31 alloy was investigated by introducing {10–12} twins into the alloy through pre-deformation and then conducting fully reversed stress-controlled fatigue tests along the rolling direction (RD) and the direction normal to the rolling plane (ND). It was found that the fatigue life of the alloy is higher under RD loading than under ND loading, but when initial twins exist, the fatigue resistance along the RD decreases, whereas the ND fatigue resistance increases, eventually reversing the fatigue resistance behavior under the ND and RD loading conditions. The tensile mean strain generated under ND loading is also changed to compressive strain by initial twins, whereas the reverse is true under RD loading. The fatigue strength of samples with and without twins is inversely proportional to the logarithm of their plastic strain energy density, which corresponds to the inner area of the hysteresis loop at half-life, irrespective of the loading direction.

A modified energy-based approach for fatigue life prediction of superelastic NiTi in presence of tensile mean strain and stress

Fatigue behavior of superelastic NiTi (i.e. Nitinol) is experimentally investigated and an energy-based fatigue model is proposed to account for the effects of tensile mean strain and stress on fatigue resistance. Several zero and nonzero mean strain tests at various strain amplitudes were conducted on cylindrical fatigue specimens with uniform gage sections. Experimental results revealed significant detrimental effects of the tensile mean stress on the fatigue life of the superelastic NiTi. On the stress plateau, where a change in the mean strain did not significantly influence the stress value, fatigue lives were observed to be similar. A modified energy-based model is proposed to account for the effects of mean stress and strain on the fatigue behavior of superelastic NiTi. Among all the models investigated in this study, the proposed energy approach was able to better correlate the fatigue data with and without mean strains/stresses.

Mean strain effects on cyclic deformation and fatigue behavior of polyether ether ketone (PEEK)

The fatigue behavior of polyether ether ketone (PEEK) thermoplastic was investigated under uniaxial strain-controlled cyclic loading condition with various mean strains (Rε = 0, 0.2, and 0.25). The mean stress responses of PEEK from all mean strain fatigue tests were found to be fully-relaxed. As a result, minimum mean strain effect on fatigue behavior of PEEK was observed. Fractography analyses utilizing a scanning electron microscope were performed on PEEK specimens under tensile mean strain cyclic loading. Two distinct stages of fatigue, crack initiation and physically small crack growth, were observed on all the fracture surfaces. Moreover, two types of inclusions, unmelted particles and void-like defects, were identified to be the cause of crack initiation. Void-like defects were found to have more detrimental effect on fatigue life of PEEK when compared to particles of similar size. The effect of inclusions on fatigue behavior of PEEK was observed to be more significant than that of the tensile mean strain.

Fatigue behavior of an austenitic steel of 300-series under non-zero mean loading

Strain-controlled fatigue data of an austenitic stainless steel subjected to uniaxial state at different strain ratios with tensile and compressive mean strain are experimentally investigated to understand the fatigue phenomena when non-zero mean straining is involved. The fatigue test result indicates that mean stresses relax to very low level and the material experiences elastic-plastic response throughout the life. Moreover, the material has higher fatigue resistance under tensile mean strain loading condition than that of completely reversed loading and compressive mean strain cycling at the same strain amplitude which might be attributed to the micro-structural deformation mechanism. The capabilities of several damage parameters to characterize the non-zero mean strain effect on fatigue life are examined based on fatigue data and are found inappropriate for the as-received material. Therefore, a fatigue damage approach with a power law relationship between modified damage parameter and reversals to failure, considering a mean strain function in stress-strain-based fatigue damage module, is introduced to the material which shows better correlation with the uniaxial fatigue data under mean strain loading compared to that of some established fatigue models.

FATIGUE LIFE PREDICTION OF ROLLED AZ31 MAGNESIUM ALLOY USING AN ENERGY-BASED MODEL

Fatigue behavior of rolled AZ31 magnesium alloy, which shows an anisotropic deformation behavior due to the direction dependent formation of deformation twins, was investigated by carrying out stress and strain controlled fatigue tests. The anisotropy in deformation behavior introduced asymmetric stress-strain hysteresis hoops, which make it difficult to use common fatigue life prediction models, such as stress and strain-based models, and induced mean stress and/or strain even under fully-reversed conditions; the tensile mean stress and strain were found to have a harmful effect on the fatigue resistance. An energy-based model was used to describe the fatigue life behavior as strain energy density was stabilized at the early stage of fatigue life and nearly invariant through entire life. To account for the mean stress and strain effects, an elastic energy related to the mean stress and a plastic strain energy consumed by the mean strain were appropriately considered in the model. The results showed that there is good agreement between the prediction and the experimental data.

The effect of strain ratio on morphology of dislocation in low cycle fatigued SAF 2205 DSS

The effects of strain amplitude and strain ratios in low cycle fatigue (LCF) at 0.25 Hz on the final stable dislocation structure of a failure specimen are investigated in a SAF 2205 duplex stainless steel. The morphology of the fatigued dislocation microstructure in α phase consists of dislocation cell structures and persistent slip bands (PSBs). No dislocation loop patches or labyrinths were observed in the α grains. Instead, persistent slip bands (PSBs) and wormlike PSB loops embedded in a highly dense dislocation matrix were found, and the dislocation density increases with the increase of the strain amplitude and tensile mean strain, which changes the strain ratio. The morphology of fatigued dislocation structure in the γ phase is characterized by the strain-induced martensite.

Low-cycle fatigue-induced martensitic transformation in SAF 2205 duplex stainless steel

The low-cycle fatigue (LCF) behavior of SAF 2205 duplex stainless steel at the strain amplitudes of 0.9% and 1.5%, combined with strain ratios R=−1.0 and −0.2, exhibits a mixed mode of cyclic hardening and cyclic softening as the cycle life increases until failure. The microstructure of the as-received metal is composed of ferrite (α) and austenite (γ) phases with 51α/49γ, vol.%. The evolution of the α/γ phase in the samples treated by all LCF tests has been revealed under the high-resolution transmission electron microscope. Dislocation cell structures and persistent slip bands (PSB) can be observed in the α phase in the samples of all LCF tests. The tangled dislocations accumulated at the stacking faults in the γ phase can be seen in the sample of strain amplitude of 0.9% with R=−1. At the same strain amplitude of 0.9% but with R=−0.2, the microstructure of the γ phase transforms to the distinct ɛ-martensite intersected mutually, which is promoted by a higher 0.6% tensile mean strain than the previous strain ratio, R=−1, having 0% mean strain. Further, at a relatively higher strain amplitude of 1.5% with R=−1, γ-austenite has transformed to the strain-induced thin lath-like α′-martensite sheaths. At the same strain amplitude with R=−0.2, the thin lath-like α′-martensite sheaths grows into thicker and longer strain-induced martensite bundles as the tensile mean strain increases from 0% to 1.0%. The strain-induced martensite present at the failure region correlates on intimate terms with the maximum microhardness distribution at the γ-austenite grains of the failure region at strain amplitude of 1.5% with R=−0.2. It suggests that the localized transformation of retained austenite into martensite at a tip of a fatigue crack improves the fatigue resistance by either hindering the crack propagation or reducing the crack growth rate.

Low-cycle fatigue life of SiC-particulate-reinforced Al-Si cast alloy composites with tensile mean strain effects

The low-cycle fatigue behaviour of a SiC-particulate-reinforced Al-Si cast alloy with two different volume fractions has been investigated under strain-controlled conditions with and without tensile mean strains. The composites and the unreinforced matrix alloy showed cyclic hardening behaviour. The composite having a higher volume fraction of the SiC particles exhibited a more pronounced strain-hardening rate. For the tensile mean strain tests, the initial high tensile mean stress relaxed to zero for the ductile Al-Si alloy, resulting in no influence of the tensile mean strain on the fatigue life of the matrix alloy. However, tensile mean strain for the composite caused tensile mean stresses and reduced the fatigue life. The pronounced effects of mean strain on the low-cycle fatigue life of the composite compared to the unreinforced matrix alloy were attributed to the initial large prestrain causing non-relaxing high tensile mean stress in the composite with limited ductility and cyclic plasticity. Fatigue damage parameter using strain energy density accounted for the mean stress effects quite satisfactorily. Predicted fatigue life using this damage parameter correlated fairly well with the experimental life within a factor of 3. Moreover, the fatigue damage parameter indicated the inferior life in the low-cycle regime and superior life in the high-cycle regime for the composite, compared to the unreinforced matrix alloy.

Finite-element visualization of fatigue crack closure in plane stress and plane strain: McClung, R.C., Thacker, B.H. and Roy, S. Int. J. Fract. July 1991 50, (1), 27–49

The effects of strain amplitude and strain ratios in low cycle fatigue (LCF) at 0.25 Hz on the final stable dislocation structure of a failure specimen are investigated in a SAF 2205 duplex stainless steel. The morphology of the fatigued dislocation microstructure in α phase consists of dislocation cell structures and persistent slip bands (PSBs). No dislocation loop patches or labyrinths were observed in the α grains. Instead, persistent slip bands (PSBs) and wormlike PSB loops embedded in a highly dense dislocation matrix were found, and the dislocation density increases with the increase of the strain amplitude and tensile mean strain, which changes the strain ratio. The morphology of fatigued dislocation structure in the γ phase is characterized by the strain-induced martensite.

Experimental studies of ratcheting of pressurized pipe: Scavuzzo, R.J., Lam, P.C. and Gau, J.S. J. Pressure Vessel Technol. (Trans. ASME) May 1991 113, (2), 210–218

The effect of initial twins on the fatigue behavior of rolled AZ31 alloy was investigated by introducing {10–12} twins into the alloy through pre-deformation and then conducting fully reversed stress-controlled fatigue tests along the rolling direction (RD) and the direction normal to the rolling plane (ND). It was found that the fatigue life of the alloy is higher under RD loading than under ND loading, but when initial twins exist, the fatigue resistance along the RD decreases, whereas the ND fatigue resistance increases, eventually reversing the fatigue resistance behavior under the ND and RD loading conditions. The tensile mean strain generated under ND loading is also changed to compressive strain by initial twins, whereas the reverse is true under RD loading. The fatigue strength of samples with and without twins is inversely proportional to the logarithm of their plastic strain energy density, which corresponds to the inner area of the hysteresis loop at half-life, irrespective of the loading direction.