•A strain based damage model is proposed to study the strain life behavior.•Effect of strain ratio on the strain life curve is captured adequately.•CDM-XFEM framework is developed to perform the LCF ...crack growth simulations.•Concept of two definitions of stress triaxiality is employed to mimic the experimental observations.•Developed framework predicts the remaining fatigue life easily and efficiently.
In this paper, a strain-based damage model is developed to account the effect of different strain ratios on the fatigue failure. Thereafter, continuum damage mechanics and extended finite element method based framework is proposed to simulate the low cycle fatigue crack growth simulations. The proposed damage model is imposed in the developed framework to evaluate the fatigue life of cracked specimen. Chaboche mixed hardening model is implemented to determine the stress–strain accurately in the cracked domain. The acquired stress-strains are incorporated in the proposed damage model to estimate the remaining fatigue life. The computed strain life curves at different strain ratios as well as crack growth curves are compared with the experimental results available in literature. The simulated results are found in good agreement with the experimental observations. This work ensures that the continuum damage mechanics based damage law is an excellent and readily applicable method to acquire the effect of strain ratios on the strain life. This work also provides comprehensive and significant modeling insights for computing the low cycle fatigue crack growth efficiently.
In the present work low-cycle fatigue experiments were carried out on thermo-mechanically processed AA6082 and AA7075 sheets to evaluate mechanical properties under cyclic loading. Different cooling ...rates imposed by use of tempered forming tools after solutionizing and subsequent aging treatment led to the formation of precipitates with differing sizes and morphologies. Specimens thermo-mechanically processed in tools with temperatures of 24 °C and 200 °C showed superior mechanical properties under both monotonic and cyclic loading. Different behavior was observed for the specimens formed in the tool with a temperature of 350 °C. Based on thorough analysis of prevalent microstructural features, processing-property-damage relationships are established pointing at the major impact of the thermal history on the final performance of the high-strength aluminum alloys in focus.
•Slower cooling rate resulted in a coarser microstructure in the oscillation build.•Higher tensile and yield strengths were found normal to the material build direction.•Cyclic softening rate was two ...times higher along the material build direction.•Higher fatigue life was found along the material build direction in low cycle fatigue regime.•No defects were found, and cracks were initiated either in α laths or at α/β interface.
This paper presents the cyclic deformation behaviour and fatigue properties of a wire + arc additive manufactured Ti-6Al-4V alloy in the as built condition under strain-controlled test condition. Higher local energy input used to build the material has resulted in a coarser primary columnar β grain structure along with a coarser α microstructure compared to Ti-6Al-4V alloys produced by other additive manufacture processes. Test specimens were manufactured in horizontal and vertical orientations with respect to the deposited layers. Isotropic fatigue property was observed at lower applied strain values. When the strain amplitude was above 0.6%, the vertical samples, where the loading axis was parallel with the primary columnar β grains, showed marginally higher fatigue life owing to the material being more ductile in this direction. Moreover, higher cyclic softening rate by a factor of two was measured in the vertical samples. No porosity defects were found in the material. Cracks were initiated from either the α laths or α/β interface due to cyclic slip localisation.
•A modified Coffin-Manson model considering the effect of stress triaxiality is proposed.•The parameters of the proposed model for Q345qC steel are calibrated.•The prediction accuracy of the proposed ...model is verified based on test results.
The ductile crack initiation of structural steels due to ultra-low cycle fatigue (ULCF) loading often governs the limit state in steel structures. This paper proposes a modified Coffin-Manson model for ULCF fracture of structural steels considering the effect of stress triaxiality. To this end, tests on coupon specimens and circular notched specimens made of Chinese Q345qC steel were conducted to form different stress triaxiality states. Based on test results and corresponding finite element analysis, the parameters of modified Coffin-Manson model were determined, and empirical formulas were established between model parameters and stress triaxiality. Finally, detailed finite element analysis was conducted on circular notched specimen tests to validate the proposed model. The results show that the proposed model can be used to predict ULCF fracture of structural steels at different stress triaxialities with high accuracy.
A key point for the development of a renewable energy economy at a large scale is the possibility to use the current natural gas network and storage capacity to transport and store hydrogen. This ...study is dedicated to evaluate the integrity of materials used for underground aquifer storage regarding hydrogen embrittlement in a gaseous environment saturated with water vapour and containing few amounts of H2S. In this environment, the water vapour content in the gas stored may reach saturation at bottom well pressure and temperature. It is known that the presence of impurities in the gas, such as H2O, can promote or inhibit hydrogen embrittlement phenomenon, depending on the nature of the steel. This work investigates mechanical properties of a tempered martensite N80-Q steel, and a ferrite-perlite L360-NB C–Mn steel issued from a completion and a collect tube respectively. The testing environments are NG, NG + H2S, NG + 25%H2 and NG + 25%H2 + H2S saturated in water vapour at 8.5 MPa and room temperature. These environmental conditions aim at replicating the storage service conditions.
Fracture toughness and fatigue crack growth properties are assessed. So far, the mechanical behaviour of such steels under hydrogen gas pressure saturated with water vapour and low amount of H2S has poorly been addressed. Regarding toughness properties, the two steels present different behaviour: for the L360 NB crack has not propagated for any testing environments, while cracks propagated in all the tests for the N80 Q. Despite this difference, for the two steels, the toughness does not seem to be affected by hydrogen as the results obtained in NG + 25%H2 + H2O and NG + H2O are similar. Based on the literature C–Mn steels toughness is affected by the presence of dry hydrogen. Hence, the results presented here show that H2O inhibits hydrogen embrittlement as far as toughness is concerned. The L360NB, however, presented a slight decrease in toughness properties with the combination of NG + 25%H2 + H2S + H2O. FCG (fatigue crack growth) results, on the opposite, clearly highlight the influence of hydrogen on the mechanical behaviour of the two steels. The FCG rates are faster from a factor five (resp. 10) in NG + 25%H2 + H2O compared to NG + H2O for the N80 Q (resp. L360). In this article, toughness and FGC results on the two steels are discussed in terms of microstructure and mechanical loading modes, aiming to quantify and better understand the influence of H2O and H2S on the sensitivity of low alloy steels to hydrogen embrittlement.
•The hydrogen embrittlement of metallic components used for underground hydrogen storage is studied.•The fracture toughness and fatigue crack growth properties in 8.5 MPa H2 are discussed.•The effect of a gas saturated with water vapour and/or containing small amount of H2S is studied.•Water vapour mitigates hydrogen deleterious effect on fracture toughness of the two studied steels (ferritic and martensitic).•A synergetic hydrogen embrittlement effect between water vapour and H2S is highlighted.
Low-cycle fatigue (LCF) behavior and property characteristics of titanium alloy with a tri-modal microstructure, consisted of equiaxed α (αp), lamellar α (αl) and β transformed matrix (βt), were ...explored in this work. The results show that, at different strain amplitude (εta) levels, the cyclic hardening/softening is determined by the competition (εta < 0.9%) or superposition (εta ≥ 0.9%) effect of the variations of back stress and friction stress. As well, the fractography shows remarkably different features at different εta levels. When εta < 0.9%, there exists only one fatigue crack initiation site activated by the dislocation pile-ups at αp/βt and αl/βt interfaces. Besides, narrow fatigue striation space in fatigue crack propagation region implies a relatively slower crack propagation. When εta ≥ 0.9%, the additionally high-stress-induced crossed slip bands and coarse slip bands in αp cause the multiple fatigue crack initiations. Moreover, wider fatigue striation space in propagation region indicates a faster crack propagation. The above divisional LCF behavior and fracture features determine a two-part linear Coffin-Manson relationship. On the other hand, increasing αp content could delay the fatigue crack nucleation and propagation due to its positive effect for improving deformation compatibility. This will effectively increase the LCF life. However, increasing αl content would produce αl colonies, which promote the dislocation slip and reduce the slip reversibility. So, the crack nucleation and propagation will be facilitated and then the LCF life is decreased.
•Cyclic response of Cu-Al-Be SMA wires under multiple environments is investigated.•The ideal superelastic conditions are determined through heat treatment variation.•The evolution of the degradation ...of mechanical properties is characterized.•The effect of temperature, strain amplitude, grain size and time course are addressed.•A low-cycle fatigue model based on the Coffin – Manson relationship is proposed.
Shape memory alloys (SMAs) are smart materials with self-centering characteristics that can be used to reduce the earthquake damage to structures. In this paper, the optimal thermal treatment to obtain the ideal mechanical properties of a CuAlBe SMA wires for seismic protection applications is discussed. For this purpose, the uniaxial cyclic and monotonic stress-strain response of a 1.4 mm-diameter wire subjected to heat treatments applied for different durations is evaluated. Wire specimens are tested in multiple environments at several strain amplitudes and loading protocols. The variation of mechanical properties over time is also addressed. Test results show that a minimum treatment time of 25 min at 750 °C (∼200 μm) is necessary for obtaining superelasticity at all ambient temperatures tested. Also, as the heat treatment time increases, the residual strain, functional fatigue, strength, energy dissipation and hysteresis degradation decrease, but the superelasticity limit and low cycle fatigue life are increased. However, a 35 min treatment (∼400 μm) causes the loops to become unstable at high temperatures and the training process is not efficient. The best conditions for seismic protection purposes are achieved with a 30 min treatment (∼300 μm), due to its maximum recoverable strain of 6.5 %, 3.6 % equivalent viscous damping, negligible residual strain and stable hysteresis under all temperatures evaluated and over time. A low-cycle fatigue model based on the Coffin - Manson relationship is also proposed.
The paper presents a novel approach towards developing fatigue design curve under combined loading involving low cycle fatigue (LCF) and high cycle fatigue (HCF), in a type 316LN austenitic stainless ...steel. The total strain life curve used for fatigue design is modified taking into account the effect of varying load history. The methodology relies on the test data obtained to previous studies by authors pertaining to LCF‐HCF interaction using a sequential pattern at 923 K. Modified design curves are generated at 923 K where the effect of varying degree of prior LCF exposure at strain range of 0.12% is accounted for, on HCF.
•Clarify the influence of cold expansion on the fatigue performance of GH4169 alloy under multiple conditions.•Analyzed the evolution of the residual stress of cold expansion during fatigue and ...thermal aging.•Using quantitative fracture analysis to determine the reason why cold expansion improves fatigue performance.
Low-cycle fatigue properties before/after cold expansion under multiple conditions were comparatively researched. The results showed the fatigue cycles of the cold-expanded specimens are more than doubled compared to the original, and the fatigue gain decreased as the stress increased. Surface smoothing, surface compressive residual stress field, and cold work-hardened structure with the sub-grain refinement were recognized as the main reasons for increasing the initiation life. The relatively stable compressive residual stress under 400 °C aging and 820 MPa/RT limited the crack propagation, making the width of the fatigue band and the crack propagation speed of cold-expanded specimen smaller than hole-reamed at the same depth.
•Cyclic softening, cyclic hardening–softening and cyclic hardening–softening–secondary hardening behaviors were investigated.•A novel factor was introduced to describe the transition of back stress ...evolution for a wide range of cases.•The isotropic hardening yield surface was modified by two terms of Chaboche model considering the contribution of the secondary hardening.•The predicted results by the proposed model matched well with the experimental data.
In this study, the stress–strain responses of G115 martensitic steel, Inconel alloy 750H, and 316H austenitic steel were thoroughly investigated at elevated temperatures via low cycle fatigue tests. The evolutions of effective stress and back stress were determined by stress partition method and were related to cumulative plastic strain. The mobile dislocation density, dislocation structure, and sub-grain structure were discussed to reveal the effect of microstructure on stress–strain responses during fatigue process. Furthermore, a unified elastic–plastic framework was established by introducing a peak plastic strain-dependent relaxation factor, a cumulative plastic strain-modified kinematic hardening model, and a modified isotropic hardening model. The validity of the unified model was discussed based on the maximum stress evolution, stress rate factor, and hysteresis loop. Good consistencies were observed between the experimental and predicted results and showed its strong capability to integrate continuous softening, hardening–softening, and hardening–softening-secondary hardening behaviors into a constitutive model.
