In this study, the fatigue performance of additively manufactured Ti25Ta, produced by laser powder bed fusion (L-PBF) using pre-mixed powder is investigated. Ti25Ta shows promise as a biomedical ...implant alloy, due to its high strength to elastic modulus ratio. However, the fatigue response of L-PBF Ti25Ta is yet unknown and understanding fatigue behaviour is crucial for cyclically loaded implants.
The Ti25Ta alloy was produced employing single melt and remelt scanning strategies. It was shown that the remelt strategy had a positive effect on reducing the amount of remaining partially melted Ta particles from 2.07 ± 0.01 vol % to 0.22 ± 0.01 vol % while only slightly increasing the porosity from 0.15 ± 0.01 vol % to 0.37 ± 0.01 vol %. Furthermore, it was found that the remelt strategy resulted in alloy strengthening and a randomised orientation of the α′ lath microstructure.
Machined fatigue samples were tested in the low-cycle fatigue regime under strain-controlled conditions. The alloy demonstrated a superior yield stress normalised fatigue performance compared with commercially pure (CP) Ti, and Ti–6Al–4V ELI, and was second only to pure Ta. However, the Ti25Ta L-PBF material retains less than half the elastic modulus of all the compared materials. The remelt samples showed an increased stress response due to their higher strength and an increased elastic modulus, however a reduced number of cycles to failure. This was attributed to reduced ductility and increased crack propagation rate. It is believed that remelt scan parameter optimisation can further enhance the performance of this alloy.
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•L-PBF Ti25Ta shows promising fatigue performance for biomedical applications.•Micro-CT analysis shows remelt scanning reduces remaining Ta particles.•Fatigue crack initiation is solely caused by process induced defects.•Reduced ductility of remelted Ti25Ta leads to lower fatigue life.
This paper deals with the cyclic deformation behavior and microstructural evolution in a low-carbon carbide-free bainitic steel with two different microstructures. Low-cycle fatigue tests were ...performed at room temperature at various strain amplitudes under total strain control. The variations of the amount of retained austenite and the substructures versus the number of fatigue cycles were evaluated by the X-ray diffraction technique and electron microscopy. Fatigue test results demonstrate that the two microstructures exhibit very similar cyclic stress responses, i.e. initial cyclic hardening followed by cyclic softening or by cyclic saturation and softening till failure, depending on the strain amplitude applied. Parametric studies of the microstructure–property relationship indicate that the major cause for the initial cyclic hardening is neither martensitic transformation nor increased dislocation density. Based on these results and considering the initial high density of dislocations, which are pre-existent and mobile in the starting microstructure and which are entangled, rearranged or annihilated with cycling, the mechanisms responsible for the initial cyclic hardening followed by softening are analyzed.
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•Cyclic plastic strain does not cause notable austenite-to-martensite transformation.•Initial cyclic hardening is not due to strain-induced martensitic transformation.•Dislocation density decreases with cycling from the very first cycle.•Interactions of pre-existent high-density dislocations cause initial cyclic hardening.
•A model is proposed to predict Ultra Low Cycle Fatigue and ductile fracture in structural steel.•The model is verified against 66 experiments from two grades of low carbon structural steel.•The ...model shows superior performance to three other fracture criteria.
A new criterion is proposed to simulate the initiation of ductile fracture due to Ultra-Low Cycle Fatigue (ULCF). Building on previous research, the new fracture criterion broadens the scope of ULCF models to account for a broader range of multi-axial stress and strain states that may be encountered in steel structures. The model formulation, supported by observations from finite element plasticity simulations of void growth and fractographic analyses of fracture surfaces, describes damage accumulation as a function of plastic strain, stress triaxiality, and the Lode stress parameter. The damage rate model is integrated over arbitrary cyclic loading histories of local strains and stresses to predict ULCF fracture. The proposed criterion (termed the Stress Weighted Ductile Fracture Model – SWDFM) is supported by a series of 66 coupon scale experiments on two grades of low-carbon structural steel (A572 and A36). These tests interrogate a range of positive and negative stress triaxiality with absolute values between 0.1 and 1.6, Lode stress parameters between 0 and 1, and monotonic and cyclic loading histories. The SWDFM criterion, which requires the calibration of only two material parameters for the materials investigated in this study, is evaluated against the experimental test data using an average-error assessment as well as a cross-validation analysis. Three other ULCF rupture criteria are similarly assessed and compared. The SWDFM is shown to accurately predict ULCF initiation over a wide range of stress states and loading histories, suggesting that the model represents well the micro-mechanical mechanisms of ULCF.
In this article, mechanical and low-cycle fatigue (LCF) properties of piston aluminum alloys were characterized. In addition, the effect of the reinforcement including nano-clay-particles and the ...heat treatment was also investigated. For this purpose, nano-particles were added to the aluminum alloy and then samples were heat-treated. Fully-reversed strain-controlled LCF tests were conducted besides tensile experiments. The strain amplitudes were set to 0.20%, 0.25%, 0.30%, 0.35%, 0.40% and 0.45% under the strain rate of 1%/s. They were repeated at four different temperatures (25 °C, 200 °C, 250 °C and 300 °C). Obtained results showed that there were no significant differences between mechanical properties such as the yield stress, the ultimate stress and the elastic modulus of specimens due to heat treat and nano-clay-particles. However, a little increment of the elongation of the heat-treated reinforced sample was observed. The reinforcement was not an effective parameter on the LCF lifetime at lower temperatures. However, the LCF lifetime of the base material decreased significantly by the reinforcement at 300 °C.
Abstract
According to the AISC Seismic Provisions for Structural Steel Buildings (AISC341‐16) and EC8, the inelastic rotation demand at the design story drift is limited to 0.08 rad for I‐shape shear ...links in eccentrically braced frames (EBFs). Numerical studies on EBF archetypes show that the single‐sided inelastic rotation demands can be much higher than the limiting value. In addition, these links can fail due to low‐cycle fatigue (LCF) which depends on the loading history. A mid‐spliced end‐plated detachable replaceable link has recently been developed to promote easy replacement of end‐plated links. In this paper, a frictional mid‐spliced shear link is developed to increase the inelastic link rotation capacity and LCF life of shear links. The proposed link utilizes a splice connection at the mid‐length, where frictional faying surfaces are introduced to dissipate energy. Slip at the mid‐splice connection causes a relative vertical displacement between the link ends which eventually reduces the rotation demands on the I‐shape members. Experimental and numerical studies were conducted to study the proposed link concept. Three conventional and eight frictional mid‐spliced links were tested using a nearly full‐scale test setup. The results showed that the proposed links have a pinched link shear versus link rotation response. The links were able to sustain a link rotation demand of 0.23 rad together with a significant increase in their LCF life. Numerical studies were conducted to investigate the link rotation, interstory drift, and residual interstory drift of EBF archetypes equipped with the proposed frictional link.
An analytically formulated structural strain method is presented for performing fatigue evaluation of welded components by incorporating nonlinear material hardening effects by means of a modified ...Ramberg‐Osgood power law hardening model. The modified Ramberg‐Osgood model enables a consistent partitioning of elastic and plastic strain increments during both loading and unloading. For supporting 2 major forms of welded structures in practice, the new method is applied for computing structural strain defined with respect to a through‐thickness section in plate structures and cross section in piping systems. In both cases, the structural strain is formulated as the linearly deformation gradient on their respective cross sections, consistent with the “plane sections remain plane” assumption in structural mechanics. The structural strain‐based fatigue parameter is proposed and has been shown effective in correlating some well‐known low‐cycle and high‐cycle fatigue test data, ranging from gusset‐to‐plate welded plate connections to pipe girth welds.
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.
A microstructure‐based homogenization model is proposed for simulating the cyclic plasticity and predicting the low‐cycle fatigue (LCF) crack initiation life of GH4169 superalloy. Classical crystal ...plastic model (CPM) with a simple softening model is used at the grain level. Then, the transition from grain level to polycrystal level is based on the conservation of virtual work between the two levels. The Eshelby's formulation is applied in the model. Especially, local influences of grain interactions are considered by introducing the external Eshelby's tensor. Relatively precise macroresults and microresults as the finite element method can be provided by the present model with less computational cost. Grain volume averaged fatigue indicator parameters (FIPs) with considering the effect of inclusions are formed to predict the LCF crack initiation life, and a fold‐line fitting model is proposed to substitute for the cycle‐by‐cycle simulation. Predicted lives fit well with the experimental data for both the strain loading and stress loading simulations. Scatter of the life can also be predicted by the model and overwhelming influences of the incubation stage on the variability of LCF initiation life can be captured. It is shown that the inclusions and the inhomogeneous plastic strain are responsible for the scatter of the incubation stage.
•Strain ratio effect on low-cycle fatigue of 7050-T6 aluminium alloy is studied.•Mean stress relaxation rates have been modelled via second order logarithm functions.•Softening degree has been ...correlated with the strain amplitude and strain ratio by a linear relationship.•Low-cycle fatigue lifetime expectancy reduces with increasing values of strain ratio.
This paper aims at investigating the strain ratio effect on cyclic deformation behaviour of 7050-T6 aluminium alloy. Low-cycle fatigue tests are conducted under strain-controlled conditions at three strain ratios (−1, 0, and 0.5) with strain amplitudes in the range 0.60–1.75%. Microstructure is analysed by optical microscopy, transmission electron microscopy and X-ray diffraction, and fracture surfaces are examined by scanning electron microscopy and transmission electron microscopy. The results show that the material exhibits a cyclic strain-softening behaviour whose degree increases with increasing values of strain ratio and decreasing values of strain amplitude. Under non-zero mean strain, full relaxation of mean stress is only observed at higher strain amplitudes. A second order logarithmic model is proposed to account for the mean stress relaxation rates. Lifetime expectancy is reduced as the strain ratio raises which is consistent with the progressive increment of fatigue striation spacing observed in the fractographic analyses.
•The low-cycle fatigue (LCF) design rule of pipelines in ASME B31 code is studied.•The structural strain method generalizes ASME B31 design rule for welded pipelines.•The structural strain method can ...analyze LCF behavior of irregular pipe tee joint.•Different aspects of the structural strain method are discussed in detail.
The welded pipe components are prone to low-cycle fatigue (LCF) at welds, especially under extreme loadings. ASME B31.8 specifies the fatigue design rules of the welded pipelines. In this work, the LCF fatigue analysis approach stipulated in the ASME B31 code is revisited to figure out its underlying mechanism, limitations, and scope, by analyzing two sets of LCF fatigue data of piping structures using different strain information. A structural strain method is proposed to generalize the pseudo-stress idea implied in the ASME code, which correlates large amounts of high- and low-cycle fatigue data of weldments into the master E-N curve.
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