•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.
The effect of forging on the low cycle fatigue (LCF) behavior of cast AZ31B was investigated. The forging process was conducted at a temperature of 450°C and speed of 390mm/min. Fully reversed (Rε = ...−1) strain controlled cyclic tests were performed on as-cast and forged materials under total strain amplitudes of 0.1–1.2%. Forging eliminated the dendritic morphology and reduced β-phase intermetallics observed in the cast alloy. Additionally sharp basal texture and bimodal grain structure were developed. Generally, the forged material was found to exhibit longer fatigue life, especially at lower strain amplitudes. This was attributed to the grain refinement and the developed basal texture that changed the alloy’s deformation behavior. Also, the forged material achieved significantly higher stresses at the same total strain amplitudes compared to cast AZ31B, and substantial cyclic hardening occurred during cyclic loading. At the same time, considerable tension-compression asymmetry was observed in the forged AZ31B during LCF testing. The Smith-Watson-Topper model and Jahed-Varvani energy model were employed and both models were found to accurately predict the experimentally obtained fatigue life of both alloy conditions. The Jahed-Varvani model accurately predicted fatigue life within a factor of 1.5 especially for the asymmetric behavior of forged AZ31B.
•Influence of corrosion and buckling on hysteretic loops of corroded bars.•Impact of corrosion on cyclic stress degradation of bars.•Combined impact of corrosion and buckling on number of cycles to ...failure.•Combined influence of corrosion and buckling on fracture mechanism.
The combined effect of inelastic buckling and chloride induced corrosion damage on low-cycle high amplitude fatigue life of embedded reinforcing bars in concrete is investigated experimentally. A total of forty-eight low-cycle fatigue tests on corroded reinforcing bars varied in percentage mass loss, strain amplitudes and buckling lengths are conducted. The failure modes and crack propagation are investigated by fractography of fracture surfaces using scanning electron microscope. The results show that the inelastic buckling, percentage mass loss and nonuniform corrosion pattern are the main parameters affecting the low-cycle fatigue life of reinforcing bars. It was found that the fatigue life of corroded reinforcing bars combined with inelastic buckling has a significant path dependency. The results show that in some cases the number of cycles to failure of corroded bars under constant amplitude fatigue test is increased.
The LCF behaviors of two Ni-based single-crystal superalloys at room temperature and 600 °C have been studied. Superalloy with 3 wt% Ru-containing (3Ru alloy) had more excellent fatigue performances ...than that of non-Ru superalloy (0Ru alloy). Microstructures observed by transmission electron microscope showed that 0Ru alloy exhibited slip bands with orthogonal slip directions after the test. (1ī1) 011 and (1ī1) 0ī1 slip systems were activated in plastic deformation process. While (111) 0ī1 and (1ī1) 0ī1 slip systems of 3Ru alloy were along the single slip direction. Therefore, adding alloying element Ru to the superalloy promoted uniform deformation and prolonged fatigue life.
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•Four kinds of fatigue crack nucleation modes were revealed for P/M FGH96 superalloy.•Microscopic mechanisms of the effects of the size and location of inclusions on LCF lifetime were investigated.•A ...modified model was proposed to quantify the inclusions effect on LCF lifetime.
Powder metallurgy (P/M) nickel-based superalloy FGH96 is widely used for turbine discs in aero-engines. With an increasing trend towards powder metallurgy routes for turbine discs, undesired non-metallic inclusions comparable to the grain size are unavoidably introduced during the manufacturing process. In this study, a series of strain controlled low cycle fatigue (LCF) tests were conducted on the specimens cut from a P/M FGH96 turbine disc at elevated temperature. The microscopic mechanisms of the effects of the size and inclusion location on LCF lifetime were investigated by using scanning electronic microscope (SEM) with energy dispersive X-ray spectroscopy (EDS) for the fractographic analysis of specimens. It is revealed that the P/M FGH96 superalloy is sensitive to surface defects, such as surface scratches and surface inclusion; while the specimen for which the crack nucleates from internal inclusion usually has a longer fatigue lifetime due to smaller stress concentration at the crack tip. Moreover, the inclusion involving its location and size has a significant influence on the P/M FGH96 superalloy’s LCF lifetime. Then, a modified model was proposed to quantify the inclusion effect on the LCF lifetime. Finally, a probabilistic model based on Bayesian approach was formulated to describe the scattering in LCF lifetimes induced by random inclusions.
•The relationship between stiffness and strength of BRBs is quantified.•The low-cycle fatigue behavior of BRB is considered.•The seismic performance of BRB-RCFs is investigated.•An optimal solution ...for the BRB core is suggested.
Buckling-restrained braces (BRBs) have been widely adopted in engineering structures for seismic protection owing to their satisfactory energy-dissipating performance at favorable costs. However, the low-cycle fatigue fracture of BRBs may cause structures to fail in unexpected failure modes under strong earthquakes. Moreover, as the two fundamental parameters of a BRB, strength and stiffness exhibit a matching relationship. In this study, the effect of the core plate geometric construction on the strength and stiffness of a BRB was first quantified. Then, a total of 27 seven-story inverted V-type BRB-reinforced concrete frames (BRB-RCFs) with three story shear ratios p and nine corresponding stiffness ratios k were designed. The seismic responses of the structures considering low-cycle fatigue behavior under 22 far-field ground motions were evaluated, in terms of inter-story drift ratios, BRB cumulative plastic deformation, BRB damage indices, etc. The correlations between the seismic performance indices were analyzed. In addition, the effects of story shear ratios and stiffness ratios on the structural seismic response were assessed, and the optimal design for a BRB core was suggested. The analytical results can provide fundamentals to develop the design approach for BRB components and BRB-RCF systems.
•An ideal model is developed to study the association between surface roughness and fatigue properties of AM alloys.•The maximum surface height and correlation length jointly influence the surface ...fatigue crack evolution process.•A parameter “G” is proposed to quantify the effect of surface roughness on the fatigue properties of AM alloys.
Additive manufacturing (AM) is a complex process involving a multiscal physical phenomena (solid–liquid-gas), often resulting in poor surface quality. Although surface treatments such as polishing or chemical treatment can improve surface quality, localized sub-grain stress concentrations induced by surface roughness anomalies are still easily formed, which can lead to the reduction and dispersion of fatigue properties in AM alloys. In this study, a numerical simulation model is proposed to study the association between surface roughness and fatigue properties of AM alloys. Based on the continuum damage mechanics modeling framework, an idealised grain/grain boundary model is generated by employing the Voronoi tessellation meshing technology. The numerical simulations are performed for the model with different surface geometric states. The correlation between surface roughness geometric features and fatigue properties is analyzed and discussed. A parameter “G” which comprehensively considers the influence of the maximum surface height (Rz) and correlation length (Lcor) is proposed to quantify the influence of surface roughness geometric features on the fatigue properties of AM alloys. This finding can be of great significance in improving the surface integrity of components to increase service life.
•We studied the LCF behavior of FSW joint of AZ31 alloy.•LCF fractured at the boundary between NZ and TMAZ at the advancing side owing to grain coarsening and texture softening.•Decreasing rotation ...rate improved fatigue life of FSW joints.•LCF behavior was well expressed by the Coffin-Manson and Basquin relations.•Basal slip was the main deformation mechanism associated with coordinated twinning.
Effect of the rotation rate on the low-cycle fatigue (LCF) of friction-stir welding joint of AZ31 magnesium (Mg) alloys was studied. The rotation rate had a significant impact on the grain size but only slightly affected the texture in the nugget zone (NZ) and thermo-mechanically affected zone (TMAZ). Grain coarsening and texture softening resulted in the fracture at the boundary between the NZ and TMAZ at the advancing side. Decreasing rotation rate enhanced the fatigue life. The LCF behavior was well expressed by the Coffin-Manson and Basquin relations. The basal slip was the main deformation mechanism associated with coordinated twinning.
Thermo-mechanical creep-fatigue (TMCF) behaviors of a TiAl alloy were investigated by conducting various creep, creep-fatigue, and thermo-mechanical tests and performing detailed microstructural ...analysis. A new creep model was proposed to unify primary strain hardening and tertiary accelerating damage effects. The TMCF failure of TiAl alloys is accompanied by the crack initiation from surface oxides/carbides and mixed transgranular and intergranular cracking. It revealed that introducing thermo-mechanical cycles significantly reduces the creep damage rate during the creep dwell stage and the fatigue process, i.e., the creep-delaying effect, which is attributed to the non-proportional strengthening effect induced by thermal cycles. A life model based on the average minimum creep strain rate was proposed to predict the creep-dominated TMCF life. This model incorporates the complex creep-fatigue interactions on creep damage and demonstrated good agreement with experimental results under varying loading conditions. The present work provides new insights into understanding the creep-dominant thermo-mechanical damage evolution of TiAl alloys.
The influences of temperature on the microstructure evolution, tensile properties, especially low-cycle fatigue (LCF) behaviors and damage mechanisms of Al-Si piston alloy have been investigated in ...this paper. The results show that the alloy exhibits cyclic softening at high-temperature. Fatigue cracks usually initiate from primary silicon phase and preferentially grow along particles in a slightly zigzag path at relatively low temperature. With temperature increasing, however, the ductile tearing fracture through micro-cracks can be found. In order to evaluate the fatigue life, considering the temperature and loading conditions, a comprehensive 3-parameter model based on hysteresis energy has been proposed; at a constant temperature the fatigue life can be controlled by two parameters, i.e., the intrinsic fatigue toughness W0 (the resistance to crack propagation) and the fatigue cracking exponent β (the resistance to fatigue cracking), which dominate the LCF damage mechanisms (from fatigue-induced particle cracking to rapid fatigue crack growth). For the current Al-Si alloy, the combined effect of W0, β and temperature T can lead to an optimal fatigue life at a critical temperature. This model provides a new clue for optimizing and designing the high-temperature materials.
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