Low Cycle Fatigue Resistance Research Articles

Post-fire mechanical behavior of iron-based shape memory alloy used for structural damping

This study sheds light on the post-fire mechanical behavior of Fe-SMA by conducting a series of tests at material level, with a particular focus on the low-cycle fatigue (LCF) performance. Three temperatures (500 °C, 750 °C, 1000 °C) and three cooling methods (air cooling, water spray quenching, water quenching) are considered to simulate different fire-cooling scenarios. After the heating-cooling cycle, 10 monotonic tensile specimens and 20 LCF specimens are tested. Essential material parameters for Manson-Coffin model and combined hardening model are also calibrated based on the experimental data. The failure mechanism and possible factors affecting the post-fire LCF performance of Fe-SMA are further explored by scanning electron microscope (SEM), metallographic observation (MO), electron backscattered diffraction (EBSD) and X-ray diffraction (XRD) analysis. Results show that Fe-SMA could exhibit even better LCF resistance after undergoing the considered fire-cooling procedure, especially those heated up to 750 °C and 1000 °C. Even under rapid cooling, no signs of brittle fracture are observed, which is superior to traditional structural steels. Different heating temperatures and cooling methods affect the grain sizes and microstructure of Fe-SMA, leading to variations in its post-fire LCF behavior.

Material characterization of iron-based shape memory alloys for use in self-centering columns

Iron-based shape memory alloys (FeSMAs) are emerging as a promising material for use in post-tensioning concrete structures to provide self-centering capabilities during a seismic event. Past experimental studies on FeSMA focused on strengthening or repairing existing structural components under gravity loading. In addition to the structural rehabilitation, FeSMA also have potential for use in self-centering columns subjected to seismic loads. However, the basic material properties, such as strength, ductility, recovery strain, actuation stress (i.e. prestress) stability, low-cycle fatigue resistance, and temperature dependence of FeSMA related to self-centering column applications have not been studied extensively to-date. To fill this knowledge gap and determine the feasibility of using FeSMA in self-centering columns, this study performed a comprehensive characterization and analysis of FeSMA both before and after actuation (i.e. thermal stimulation). The strength, ductility, energy dissipation, and recovery strain of FeSMA before actuation were tested at different temperatures from −40 °C to 50 °C. After actuation, the actuation stress, low-cycle fatigue resistance, and strain capacity of FeSMA were tested at different temperatures from −40 °C to 50 °C and prestrain levels from 4% to 30%, and under low-cycle fatigue loading with strain amplitudes from 0.5% to 1.0%. The results from this study demonstrated that FeSMA exhibit high ductility, cyclic actuation stress stability, and low-cycle fatigue resistance at temperatures from −40 °C to 50 °C. Furthermore, it was found that increasing the prestrain level can effectively increase the post-actuation strain amplitude at which the actuation stress reduces to zero. A prestrain level between 15% and 20% is recommended for application of FeSMA in self-centering columns. The research findings from this study demonstrated the feasibility of using FeSMA in self-centering columns subject to seismic loading.

Effect of Nanostructuring on Operational Properties of 316LVM Steel

In this study, high-pressure torsion (HPT) was used to process austenitic 316LVM stainless steel at 20 °C and 400 °C. The effects of HPT on the microstructure, mechanical, and functional properties of the steel were investigated. By applying both HPT modes on the 316LVM steel, a nanocrystalline state with an average size of the structural elements of ~46–50 nm was achieved. The density of the dislocations and twins present in the austenite phase varied depending on the specific HPT conditions. Despite achieving a similar structural state after HPT, the deformation temperatures used has different effects on the mechanical and functional properties of the steel. After HPT at 20 °C, the yield strength of the 316L steel increased by more than nine times up to 1890 MPa, and the fatigue limit by more than two times up to 550 MPa, when compared to its coarse-grained counter-parts. After HPT at 20 °C, the 316LVM steel exhibited better ductility, higher low-cycle fatigue resistance, greater resistance to corrosion, and improved in vitro biocompatibility compared to processing at 400 °C. The reasons for the deterioration of the properties after HPT at 400 °C are discussed in the article.

Fracture prediction of Fe-SMA under monotonic and low cycle fatigue loading

A total of 18 Fe-SMA notched round bars were tested to study their monotonic and low-cycle fatigue (LCF) fracture behavior, followed by FE simulation to calibrate necessary parameters for void-growth-model (VGM) and cyclic-void-growth model (CVGM). Subsequently, Fe-SMA shear-panel-type and bar-type dampers are considered for further validation. The CVGM is shown to predict the LCF fracture of these Fe-SMA dampers with good accuracy under large strain amplitudes, but may underestimate their LCF resistance under small strain amplitude. A phenomenological modified CVGM is developed to provide a more accurate and consistent fracture prediction for Fe-SMA under a spectrum of strain amplitudes.

Low-cycle fatigue behavior of differently matched welded joints of quenched and tempered steel

The appropriate matching of base and filler material is a complex task, where yield strength matching is the most general aspect. As the strength properties of structural steels have significantly improved in the past decades, the matching problem has become more relevant today. The mismatch phenomenon significantly affects the behavior of welded joints under dynamic and cyclic loading. Among cyclic loading, low-cycle fatigue (LCF) often occurs in welded steel constructions; furthermore, the LCF resistance of these advanced steels and their welded joints is limitedly known. In this paper, welding experiments are presented for the analysis of the LCF behavior of differently matched butt-welded joints made from two grades of quenched and tempered (Q + T) high-strength steels. For S690QL steel, matched and overmatched consumables were applied, while for S960QL steel, matched and undermatched filler materials were used. Material tests were performed to determine the mechanical properties of the different welded joints. In both examined steel grades, the welded joints tolerated a smaller number of cycles until failure than the base materials. In the case of S690QL, the LCF resistance of the matched welded joints was higher than the overmatched filler material. At S960QL in the higher strain amplitude range, the number of cycles to failure was higher at the same total and plastic strain amplitudes when undermatched filler material was used; however, an opposite ratio can be observed at lower strain values, compared to the matching filler material.

Two-fold improvement of low cycle fatigue resistance of steel by bidirectional transformation-induced plasticity

A new deformation mechanism via bidirectional transformation between γ-austenite and ε-martensite [B-transformation-induced plasticity (TRIP)] provides a great possibility to develop fatigue-resistant steel due to its high reversibility in dislocation motion. The Fe-18Mn-11Cr-7.5Ni-4Si B-TRIP steel was designed by modifying the previously developed Fe-15Mn-11Cr-7.5Ni-4Si steel. Strain-controlled fatigue test was conducted at the total strain range of 1%, and newly developed steel demonstrated two-fold fatigue life (21,994 cycles in average) compared with the previous one. Microstructural analysis after fatigue fracture showed that the enhanced irreversible α’-martensitic transformation at the crack tip in the previous steel negatively affected fatigue durability by locally invaliding B-TRIP.

Emerging steel frames with Fe-SMA U-shaped dampers for enhancing seismic resilience

Post-earthquake financial loss of structures induced by residual inter-story deformation (RID) has been recently noticed and various strategies have been proposed. A newly-emerged energy-dissipation material, namely, iron-based shape-memory-alloy (Fe-SMA), has gain favor in civil engineering community due to its extraordinary low-cycle fatigue resistance property. Besides, Fe-SMA is believed to help improve structure’s post-earthquake residual inter-story deformation control capacity due to its moderate strain hardening behavior and pseudo-elasticity. This study explores the performance of emerging steel frame systems equipped with energy-dissipation devices fabricated with Fe-SMA. In particular, a brace with built-in Fe-SMA-based U-shaped strips (USSs) is employed serving as the energy-dissipation devices in the proposed system. In this paper, the basic mechanical properties of Fe-SMA are discussed firstly, followed by a description of the working principle of the USSs. The hysteresis behavior of two types of USSs, fabricated with Fe-SMA and conventional mild steel (Q235), were investigated experimentally. Based on the test results, numerical models for the prototype braces equipped with USSs are established using ABAQUS and OpenSEES, and the necessary key material parameters are calibrated. Subsequently, system-level analysis is performed on 5-story prototype steel buildings. Specially, two types of steel frames, i.e., frames with high-strength-steel columns and frames with conventional structural steel-based columns, are established. Concurrently, braces with Fe-SMA-based USSs as well as mild steel-based USSs are considered. The results demonstrated that the Fe-SMA-based energy-dissipation devices along with the steel frames with high-strength-steel columns can provide the most satisfied RID control effectiveness compared with the other yielding structural systems investigated in this paper.

The role of stress-state on the low cycle fatigue resistance of non-hydrided beta-treated Zircaloy-4

Reactor operation results in cyclic stresses that produce fatigue. Increased strain to failure under monotonic loading has been observed for Zircaloy-4 under a stress-state of shear defined by a low triaxiality. Triaxiality (η) is defined as the ratio of hydrostatic stress to von Mises stress and typically ranges from values of -0.33 for compression, to 0 for torsion loading and to 0.9 for notched, axial loaded specimens. Low Cycle Fatigue (LCF) testing of Zircaloy-4 was performed under uniaxial and torsion loading in air at room-temperature. Both smooth and notched specimens were tested to vary the value of positive triaxiality. An increase in LCF resistance with less notch sensitivity is observed under torsion loading. Cyclic stress-strain curves are developed that show a higher strain to failure and lower flow stress under torsion loading. This suggests a lower triaxiality produces increased strain to failure capability under both cyclic and monotonic loading that is shown to generally follow a Rice-Tracey model for void nucleation, growth and coalescence. Fractography examinations are used to understand the mechanism for crack extension.

Heterogeneous-structured Ti-6Al-4V with enhanced mechanical properties in monotonic and cyclic deformation modes

This study aimed to develop heterogeneous-structured (HS) Ti-6Al-4V composed of coarse and fine grains to enhance the alloy's mechanical properties. This microstructure was induced by selective activation of dynamic globularization by using water-quenching and subsequent rolling below the β-transus temperature. Microstructures and mechanical properties of the HS alloy in monotonic and cyclic deformation modes were compared with those of coarse-grained (CG) and ultrafine-grained (UFG) counterparts. HS Ti-6Al-4V had a higher mechanical strength and better resistance to low-cycle fatigue (LCF) fracture without the loss of ductility compared with commercial mill-annealed CG alloy, and HS alloy had an engineering advantage of low-cost mass production in contrast to UFG counterpart, despite their similar mechanical properties under both monotonic and cyclic deformations. The enhanced mechanical properties of HS alloy were attributed to the presence of UFG region and the partial transformation of transverse to basal texture with a high intensity. Such conditions gave rise to an active slip transfer, low internal stress, and high strain compatibility, thereby leading to the high LCF resistance.

Experimental and Numerical Studies on Fe–Mn–Si Alloy Dampers for Enhanced Low-Cycle Fatigue Resistance

Experimental and Numerical Studies on Fe–Mn–Si Alloy Dampers for Enhanced Low-Cycle Fatigue Resistance

Origin of superior low-cycle fatigue resistance of an interstitial metastable high-entropy alloy

In this study, the deformation behaviors and related microstructural evolutions were investigated in either monotonic or cyclic deformation modes in an interstitial metastable high-entropy alloy. These investigations aimed to reveal the mechanisms underlying the superior low-cycle fatigue (LCF) life of this alloy. A thermomechanical process was applied to induce fine-grained (FG) and coarse-grained (CG) microstructures in Fe–30Mn–10Co–10Cr–0.4C (atomic percentage) alloy. Their superior combination of strength and ductility was attributed to the appearance of deformation-induced ε-martensite and the presence of carbon. The CG alloy showed a greater volume fraction of ε-martensite than the FG alloy in the monotonic deformation mode, and vice versa in the cyclic mode. Such a disparity was interpreted in light of the back-stress effect of the relaxed γ-grain boundaries in the latter mode. Meanwhile, the γ-to-ε phase transformation under cyclic loading at low strain amplitudes (0.4%) barely led to an improved fatigue life as compared with that at higher strain amplitudes (≥ 0.55%). The high reversibility of partial dislocation motions under cyclic loading and delaying the formation of dislocation cells through the martensitic transformation could explain why the alloys investigated in this study exhibited a superior LCF life compared with high-entropy alloys reported in previous studies.

Fe-Mn-Si alloy U-shaped dampers with extraordinary low-cycle fatigue resistance

This study presents a new type of Fe-Mn-Si alloy U-shaped dampers (FMS-UDs) that exhibit extraordinary low cycle fatigue (LCF) performance. The basic material properties of Fe-Mn-Si alloy were first evaluated by a series of material tests considering both the monotonic and cyclic loading schemes, where normal carbon steel specimens were also tested for comparison. After the material tests, a total of 30 U-shaped dampers were examined, considering various parameters such as material type, geometry, loading direction and strain rate. Following the experimental program, finite element models were established to help interpret the test result. The material tests showed that Fe-Mn-Si alloy exhibited non-obvious yield plateau and moderate strain hardening under monotonic loading. Spring-back phenomenon, non Masing behavior, and excellent LCF performance were generally observed under cyclic loading. The damper tests showed that the FMS-UDs exhibited slightly “narrower” hysteresis responses compared with normal steel dampers. The LCF life and the total energy dissipation of the FMS-UDs are up to 7 times that of their steel counterparts, depending on the parameters considered. From a modeling point of view, the hysteretic behavior of the Fe-Mn-Si alloy could be adequately captured by a combined kinematic/isotropic hardening model. By using the calibrated parameters from the material tests, the hysteresis behavior and stress distribution of the FMS-UDs can be well represented. Critical locations which are prone to fatigue failure of the FMS-UDs were accurately captured by the numerical model, and the predictions agree very well with the test results.

Damage Accumulation Phenomena in Multilayer (TiAlCrSiY)N/(TiAlCr)N, Monolayer (TiAlCrSiY)N Coatings and Silicon upon Deformation by Cyclic Nanoindentation.

The micromechanism of the low-cycle fatigue of mono- and multilayer PVD coatings on cutting tools was investigated. Multilayer nanolaminate (TiAlCrSiY)N/(TiAlCr)N and monolayer (TiAlCrSiY)N PVD coatings were deposited on the cemented carbide ball nose end mills. Low-cycle fatigue resistance was studied using the cyclic nanoindentation technique. The obtained results were compared with the behaviour of the polycrystalline silicon reference sample. The fractal analysis of time-resolved curves for indenter penetration depth demonstrated regularities of damage accumulation in the coatings at the early stage of wear. The difference in low-cycle fatigue of the brittle silicon and nitride wear-resistant coatings is shown. It is demonstrated that when distinguished from the single layer (TiAlCrSiY)N coating, the nucleation and growth of microcracks in the multilayer (TiAlCrSiY)N/(TiAlCr)N coating is accompanied by acts of microplastic deformation providing a higher fracture toughness of the multilayer nanolaminate (TiAlCrSiY)N/(TiAlCr)N.

Low-cycle fatigue behavior for stainless-clad 304 + Q235B bimetallic steel

Stainless-clad bimetallic steel is a high-performance steel with the advantages of cost and mechanical resilience. As the low-cycle fatigue characteristics of a material is essential for the seismic performance of structures, this paper investigates the low-cycle fatigue properties of a stainless-clad 304 + Q235B bimetallic steel plate through a series of strain-controlled fatigue tests to gain a comprehensive understanding of the cyclic response, failure mode and fatigue strength of this material. Fatigue test results indicate that most of the fatigue cracks initiate and propagate in the side of substrate layer Q235B carbon steel, and no interface debonding is found till the fracture occurrence of the test coupons. The different applied strain ratios have negligible effects on the fatigue life of the 304 + Q235B bimetallic steel due to the effect of the mean stress relaxation. The level of the observed cyclic hardening shows positively related with the cycled strain amplitude. Comparison of the Coffin-Manson strain-life curves between different steel grades further proves this 304 + Q235B bimetallic steel possesses excellent low-cycle fatigue resistance. Finally, the calibration and verification of the nonlinear isotropic/kinematic hardening model parameters for this bimetallic steel are conducted and provide a good description of the experimental results.

On the changes in the low-cycle-fatigue life and cracking mechanism of P91 cross-weld specimens at elevated temperatures

A significant reduction in the fatigue life of cross-weld P91 specimens was observed in tests at 538 and 566 °C compared to that at 25 °C. The remarkable reduction of the fatigue strength was directly correlated to the shift of the fatigue cracking locations from the BM region at 25 °C to the interface between the intercritical heat-affected zone (ICHAZ) and BM at elevated temperatures. The low-cycle fatigue (LCF) resistance deterioration mechanisms were attributed to synergic interactions between a high rate of microstructure deterioration accompanied by severe plastic deformation and lower strain hardenability in the ICHAZ region at elevated temperature.

Constitutive and damage modelling of selective laser melted Ti-6Al-4V lattice structure subjected to low cycle fatigue

Based on low cycle fatigue (LCF) experiment data of SLM Ti-6Al-4V, a comprehensive finite element model of the combined isotropic and kinematic hardening and continuous damage for SLM Ti-6Al-4V is established. The simulation results of the stress-strain hysteresis loop and the LCF life are in good agreement with the experiment results under different strain amplitude levels. Based on the reliable achieved model of SLM Ti-6Al-4 V, the finite element model of the Gyroid and Diamond lattice structure is established for LCF analysis. It is found that the Diamond lattice structure has higher LCF resistance.

Enhanced synergy of strength-ductility and low-cycle fatigue resistance of high-entropy alloy through harmonic structure design

Compared with homogeneous structured materials, heterogeneous structured materials exhibit unprecedented performances. In the present work, a series of harmonic structured CoCrFeMnNi high-entropy alloys (HEAs) with different shell fractions were prepared by mechanical milling (MM) and spark plasma sintering (SPS) process. The effects of shell fraction on mechanical properties and low-cycle fatigue (LCF) performances were investigated at room temperature. Results show that the harmonic structured HEAs with a shell fraction between 20% and 40% demonstrate a combination of superior strength-ductility synergy and good LCF resistance. Development and rearrangement of dislocations dominantly occur in the coarse-grained areas (cores) during LCF process, while the ultrafine-grained areas (shells) show good cyclic stability. The enhanced strain hardening leads to a good balance of high strength and high ductility in the harmonic structured HEAs. Moreover, the enhanced ductility also restrains LCF failure.

Low cycle fatigue behavior of Inconel 706 at 650 °C

Low cycle fatigue (LCF) behavior of 2- and 3-step-aged Inconel 706 (IN706) specimens that were prepared from the center and the periphery of forged disc was examined at 650 °C and an R ratio of −1. The resistance to LCF of IN706 alloy was greater in approximately half order at 650 °C than that of IN718 alloy for the same aging condition. Elevated temperature Coffin–Manson relationship between 2- and 3-step-aged IN706 specimens was similar with each other, despite a considerable difference in microstructure. The fractographic analysis suggested that cluster of carbides provided the sites for crack initiation for both 2- and 3-step-aged specimens. The mode of fatigue crack growth for each specimen was intergranular and transgranular, but with different magnitude for each mode. Depending on the location of specimen preparation form the forged block of IN706 alloy, no notable difference in LCF resistance was found. The role of grain boundaries, clusters of carbides and bands of acicular η platelets on the initiation and propagation mechanism of IN706 alloy was discussed based on detailed fractographic and micrographic analyses.

Effect of Hydrogen, Hydride Orientation and Temperature on Low-Cycle Fatigue Resistance of Zr-1%Nb Fuel Rod Claddings

Low-cycle fatigue testing was conducted on annular samples with an outer diameter of 9.13 mm, a wall thickness of 0.68 mm and a width of 2.7 mm, namely: non-hydrogenated samples (cut out of standard Zr‑1%Nb cladding tubes); hydrogenated samples with a hydrogen concentration of 50 ... 400 ppm; samples cut out from hydrogenated dummy claddings after hydride reorientation tests performed according to various test modes. The tests were conducted at the temperatures of 25, 180, 350, 400 and 450 °С. The results obtained demonstrate that with increasing the hydrogen content in Zr-1%Nb alloy claddings the fatigue life increases.

Low cycle fatigue behaviour of a single crystal Ni-based superalloy with a central hole: Effect of inhomogeneous rafting microstructure

To revealing the role of unevenly microstructural rafting caused by stress gradient on the fatigue resistance of engineering parts made by single crystal (SC) Ni-based superalloys, a stress-control low cycle fatigue (LCF) experiments were performed at 980 °C with a 0.1 stress ratio of central-hole specimens that underwent different pre-rafting treatments. An image process algorithm and field emission scanning electron microscope (FE-SEM) were employed to given a quantitative description of the rafting field near the central hole experimentally. Within half radius of the central hole, the rafting state defined in the present work presents obvious gradient, which disappears with the deterioration of rafting conditions. The rafting morphology tends to be uniform that weakens the effect of inhomogeneous rafting microstructure on LCF resistance as the rafting state exceeds 0.6. The worsening of fatigue resistance is ascribed to the comprehensive degradation effect of rafting, even resulting in 90% reduction of fatigue life compared with the virgin state. Moreover, it was found that rafting changed the initial orientation of fatigue cracks, whereas the crack always propagated along the boundary of micro twins at the late life stage. Finally, an analytical model was proposed to given a quantitative linkage between the inhomogeneous rafting microstructure and the reduction of fatigue resistance.