Low Cycle Fatigue Properties Research Articles

Evaluating Low Cycle Fatigue Property of Nanoscale Zro2 Particles Strengthening Molybdenum Alloy Prepared by Liquid-Liquid Doping Method

Evaluating Low Cycle Fatigue Property of Nanoscale Zro2 Particles Strengthening Molybdenum Alloy Prepared by Liquid-Liquid Doping Method

Seismic resistance of the corroded bimetallic steel bar under different strain amplitudes

To evaluate seismic resistance of concrete structures with bimetallic steel bars (BSBs) over the entire life cycle, it is important to clarify effects of corrosion ratio and fatigue strain amplitude on low-cycle fatigue properties. The experimental results of this study indicated that corrosion had little effect on the shape of hysteretic curve of BSB specimens. Increases in the fatigue strain amplitude and corrosion ratio decreased the cycles to failure and total energy densities. For BSB specimens with ε = 0.6%, the stable stage in maximum stress curves gradually decreased with an increase in corrosion ratio. With increases in fatigue strain amplitude and relative number of cycles, the plastic strain range and plastic and elastic strain ratio increased. The effects of fatigue strain amplitude on low-cycle fatigue properties of the BSB specimens were significantly higher than those of the corrosion ratio. Predictive equations were suggested to quantify the effects of the fatigue strain amplitude and corrosion ratio on the cycles to failure and total energy density. For BSB specimens with ε = 0.6%, fracture source zones, fracture developing zones, and fracture zones could be observed. No obvious separation was found between the stainless steel cladding layer and the carbon steel substrate. When ε = 1.5 and 2.4%, the arc line in the fracture developing zone barely disappeared. There were separations between the cladding layer and the substrate. Furthermore, radial cracks were observed near the stainless steel cladding layer.

Grain-size-dependent microstructure effects on cyclic deformation mechanisms in CoCrFeMnNi high-entropy-alloys

The effect of grain size on strain-controlled low-cycle fatigue (LCF) properties in the CoCrFeMnNi high-entropy alloys (HEAs) was investigated towards the distinct microstructural developments during cyclic loading at a strain amplitude of ± 1.0%. A much more prominent secondary cyclic hardening (SCH) behavior at the final deformation stage was observed in the fine-grained (FG, 18 µm) than in the coarse-grained (CG, 184 µm) CoCrFeMnNi. In-situ neutron-diffraction and microscopic examination, strongly corroborated by molecular dynamic (MD) simulations, indicated that dislocation activities from planar slip to wavy slip-driven subgrain structures within the grains acted as the primary cyclic-deformation behaviors in the FG CoCrFeMnNi. Differently observed in the cyclic behavior of the CG CoCrFeMnNi was due to a transition from the planar dislocation slip to twinning. Our findings suggested that the fatigue-resistant HEAs can be designed via tuning the microstructure with an optimal range of grain size at a specific strain amplitude.

Prediction of fatigue crack initiation life in SA312 Type 304LN austenitic stainless steel straight pipes with notch

In the nuclear power plants, stainless steel is widely used for fabrication of various components such as piping and pipe fittings. These piping components are subjected to cyclic loading due to start up and shut down of the nuclear power plants. The application of cyclic loading may lead to initiation of crack at stress raiser locations such as nozzle to piping connection, crown of piping bends etc. of the piping system. Crack initiation can also take place from the flaws which have gone unnoticed during manufacturing. Therefore, prediction of crack initiation life would help in decision making with respect to plant operational life. The primary objective of the present study is to compile various analytical models to predict the crack initiation life of the pipes with notch. Here notch simulates the stress raisers in the piping system. As a part of the study, Coffin-Manson equations have been benchmarked to predict the crack initiation life of pipe with notch. Analytical models proposed by Zheng et al. [1], Singh et al. [2], Yang Dong et al. [25], Masayuki et al. [33] and Liu et al. [3] were compiled to predict the crack initiation life of SA312 Type 304LN stainless steel pipe with notch under fatigue loading. Tensile and low cycle fatigue properties were evaluated for the same lot of SA312 Type 304LN stainless steel as that of pipe test. The predicted crack initiation lives by different models were compared with the experimental results of three pipes under different frequencies and loading conditions. It was observed that the predicted crack initiation life is in very good agreement with experimental results with maximum difference of ±10.0%.

Low-cycle fatigue properties and unified fatigue life prediction equation of hot-rolled twin-roll-cast AZ31 sheets with different thicknesses

This study investigates the low-cycle fatigue properties of AZ31 sheets with different thicknesses of 1, 1.5, 2, and 3 mm—which are fabricated by twin-roll casting and subsequent hot rolling—through fully reversed strain-controlled fatigue tests. As the thickness of the sheets decreases, their average grain size decreases, texture intensity increases, and tensile yield strength and elongation gradually increase. At strain amplitudes of greater than or equal to 0.6%, {10–12} twinning in compression and detwinning and subsequent slip in tension occur repeatedly, which forms asymmetric hysteresis loops. The different grain sizes of the sheets result in different compressive peak stresses during the fatigue tests. However, the overall cyclic deformation behavior is similar in all the sheets, and consequently, their fatigue lives exhibit an insignificant difference. The loading direction also has a negligible influence on both the cyclic deformation behavior and the fatigue life, which implies that the sheets exhibit in-plane isotropic fatigue properties. The stress amplitude and plastic strain amplitude vary considerably during the fatigue test. In contrast, the variation in total strain energy density is insignificant over the entire fatigue life, and therefore, it is a proper fatigue damage parameter for predicting fatigue life. A unified fatigue life prediction equation using the total strain energy density is established, and the fatigue lives predicted using the equation are found to be in good agreement with the experimentally determined values, regardless of the strain amplitude, sheet thickness, and loading direction.

The Low-Cycle Fatigue Behavior, Failure Mechanism and Prediction of SLM Ti-6Al-4V Alloy with Different Heat Treatment Methods.

Selective laser melting (SLM) is a promising additive manufacturing (AM) process for high-strength or high-manufacturing-cost metals such as Ti-6Al-4V widely applied in aeronautical industry components with high material waste or complex geometry. However, one of the main challenges of AM parts is the variability in fatigue properties. In this study, standard cyclic fatigue and monotonic tensile testing specimens were fabricated by SLM and subsequently heat treated using the standard heat treatment (HT) or hot isostatic pressing (HIP) methods. All the specimens were post-treated to relieve the residual stress and subsequently machined to the same surface finishing. These specimens were tested in the low-cycle fatigue (LCF) regime. The effects of post-process methods on the failure mechanisms were observed using scanning electron microscopy (SEM) and optical microscopy (OM) characterization methods. While the tensile test results showed that specimens with different post-process treatment methods have similar tensile strength, the LCF test revealed that no significant difference exists between HT and HIP specimens. Based on the results, critical factors influencing the LCF properties are discussed. Furthermore, a microstructure-based multistage fatigue model was employed to predict the LCF life. The results show good agreement with the experiment.

Development and experimental validation of a partially buckling-restrained brace with dual-plate cores

A new partially buckling-restrained brace with dual-plate cores (DPBRB) was proposed in this paper to improve the flexural stiffness of an all-steel BRB with a plate core. The core member was composed of two identical and separated plates in parallel. The restraining member consisted of steel channels and steel plates that were assembled with high-strength bolts. The restraining member was located at the periphery of the core member to restrain the edge part of the outside and the inside plates in the core member. The restraining member can be reused and easily disassembled by removing the bolts, which effectively improves the utilization. The test results and numerical simulation indicated that all specimens presented stable energy dissipation capacities and low-cycle fatigue properties. The yield segments exhibited inwards buckling deformation, and the deformations at the ends were more evident than those in the middle. Insufficient stiffness of the end of the restraining member caused large deformation and fatigue fracture of the end of the yield segment. A new all-steel assembled restraining plate was provided. The calculation formula for the design length of the restraining member was given based on the results of finite element analysis.

Post-fire hysteretic and low-cycle fatigue behaviors of Q345 carbon steel

A series of material tests are carried out to study the cyclic loading behavior of Q345 carbon steel after fire. A total of 60 specimens are examined, which are heated to 500, 750 or 1000 °C, then cooled by air cooling, water spray cooling or water immersion cooling, followed by either incremental-amplitude cyclic loading or constant-amplitude low-cycle/extremely low-cycle fatigue (LCF/ELCF) loading. The typical fracture failure mechanism of the specimens is analyzed by scanning electron microscope (SEM) and metallurgical microscopy (MM). The material parameters of Ramberg–Osgood model, Masson–Coffin model and a nonlinear combined hardening model are calibrated to facilitate the assessment of the residual seismic performance of the steel after fire. Among other important findings, the test results show that water immersion cooling with a rapid cooling speed following a medium or large fire (heated to 750 °C or 1000 °C) have a significant impact on the cyclic behavior of the Q345 steel. In these cases, the formation of martensite with high strength and poor ductility results in an increase in the cyclic stress, a decrease in the ductility and a sharp reduction of the fatigue life. These phenomena indicate that the seismic performance of steel structures may be significantly compromised after rapid cooling from a medium or large fire. Other heating-cooling processes have less pronounced effects on the hysteretic and low-cycle fatigue properties of the material.

Low cycle fatigue properties assessment for rotor steels with the use of miniaturized specimens

The article deals with the development of advanced techniques of low cycle fatigue testing (LCF) using miniature test samples, which has the potential to be used to assess the degree of material degradation of large in-service components in the power industry. For this purpose, the results of a wide portfolio of test samples geometries have been investigated. Using the advanced techniques involving a special sampling device, a DIC virtual extensometer and test rig, a new possibility in strain-controlled LCF testing is presented. The results indicated that cyclic-strain and strain-life curves of proposed minisample geometries are almost of the same shape for both investigated rotor steels. Also the cyclic parameters obtained indicate a good consistency of results despite of a wide range of sample volumes and utilization of two different strain-control methods.

Low-cycle fatigue properties of bimetallic steel bars after exposure to elevated temperature

The BSB (bimetallic steel bar) has a broad application prospect in RC (reinforced concrete) structures servicing in corrosive environment. To determine seismic resistance of RC structures, it is important to clarify the low-cycle fatigue properties of reinforcement. Fire has lasting effects on mechanical properties of BSB. To value the service performance of RC structures after fire, the low-cycle fatigue properties of BSB are investigated experimentally. After elevated temperature, the substrate and cladding layer are still tightly bonded. When exposure temperature is relatively high (800 °C and 900 °C), there is martensite formed because of the quenching treatment. When the exposure temperature is lower than 700 °C, the hysteresis curves perform the full rhombus. When the exposure temperature ranges from 700 °C to 900 °C, the hysteresis curves become slender and the maximum tensile and compressive stresses increase. The models for predicting the cycles to failure and total dissipated energy density are proposed, which can predict the low-cycle fatigue properties of BSB after exposure to elevated temperatures. The comprehensive discussion between metallographic structures and low-cycle fatigue properties is performed. When the exposure temperature ranges from 300 °C to 700 °C, BSB specimens perform the ductile fracture failure. The intergranular embrittlement of martensite leads to the intergranular failure of BSB specimens exposed to 800 °C and 900 °C.

Improving the low-cycle fatigue properties of laser-welded Al–Zn–Mg–Cu alloy joints using double-sided ultrasonic impact treatment

To improve the low-cycle fatigue properties of Al–Zn–Mg–Cu 7075 aluminum alloy laser-welded joints, a post-weld treatment method for double-sided ultrasonic impact treatment (DSUIT) was used to treat the joints. The mechanism of different DSUITs on the microstructure and fatigue properties of welded joints was analyzed. The results showed that DSUIT reduced the welding defects and roughness of the joint surface, and the dendrite structure at the upper and lower surfaces of the joints was broken to form a plastic deformation layer (PDL) with an approximate thickness of 100 μm. Furthermore, the texture strength and grain size at the PDL were reduced. A beneficial residual compressive stress was introduced into the upper and lower surfaces of the joints after the DSUIT. When the number of cycles was 2 × 106, the maximum fatigue strength of the joints was 103.02 MPa after DSUIT, indicating an increase of 111.8% of the untreated joints of 48.62 MPa. Moreover, observance of the fatigue fracture of the joints revealed that the PDL produced by the DSUIT inhibited the initiation and propagation of cracks at the surface. Therefore, the DSUIT effectively improved the low-cycle fatigue properties of the joints.

Effect of microstructure on low-cycle fatigue property of Ti6321 alloy

The low-cycle-fatigue behavior of Ti6321 alloy with bimodal and lamellar microstructure have been investigated by analyzing of strain-cycles curve under different stress amplitudes. The results show that under the stress amplitude of 600 MPa, the fatigue life of lamellar microstructure is 6644 times, and the fatigue life of bimodal microstructure is 16252, which is more than twice that of the former. What’s more, two dislocation models of initiation and growth of cracks were demonstrated by analysis of fatigue fracture. The bimodal microstructure have the higher dislocation density and the shorter effective dislocation slip distance, which could hinder the initiation and propagation of fatigue cracks. The findings of low-cycle-fatigue property can provide data support and theoretical guidence for the safety assessment of titanium alloy pressure structures.

Effect of tempering temperature on monotonic and low-cycle fatigue properties of a new low-carbon martensitic steel

The relationship between tempering temperature and low cycle fatigue (LCF) is not clear. New low-carbon martensitic steels under different tempering temperatures (320 °C, 350 °C, 380 °C) are tested for tensile mechanical properties, LCF, and microstructure. The results show that the monotone mechanical properties of martensite steels decrease with the increase of tempering temperature. The microstructure shows that the width of the laths increases, the residual austenite is decomposed, and the dislocation density decreases. A damage hysteresis fatigue life prediction model considering tempering temperature is proposed and the fatigue performance is evaluated. The fatigue performance parameters show that the fatigue damage capacity (W0) decreases and the damage transition index (β) increased. The established fatigue life model can predict the fatigue life at different tempering temperatures.

Anisotropic Low Cycle Behavior of the Extruded 7075 Al Alloy.

The quasi-static and low cycle fatigue tests of extruded 7075 Al alloy (Φ200 mm) were investigated in three directions: the extrusion direction (ED), the radial direction (RD), and 45° with ED (45°). Grain morphology analysis, texture measurement, and fatigue fracture characterization were conducted to discuss the relationship between microstructure and mechanical properties. The experimental results showed that the ED specimen had higher ultimate tensile strength (UTS) and low cycle fatigue (LCF) properties, which were mainly attributed to the following three causes. First, the grain boundaries (GBs) had an obvious effect on the crack growth. The number of GBs in the three directions was different due to the shape of the grains elongated along the ED. Second, the sharp <111> texture and the small Schmidt factor along the ED explained the higher ultimate tensile strength (UTS) of the ED specimens. Third, fatigue fracture observation showed that the ED specimen had a narrow fatigue striation spacing, which indicated that the plastic deformation of the ED specimen was the smallest in each cycle. In addition, two fatigue prediction models were established to predict the LCF life of extruded 7075 Al alloy, based on the life response behavior of the three directions under different strains.

Low Cycle Fatigue Characteristics of Oxygen-Free Copper for Electric Power Equipment

The effect of heat treatment on tensile and low cycle fatigue properties of the oxygen-free copper for electric power equipment was investigated. The heat treatment at 850 °C for 20 min, which corresponds to the vacuum brazing process, caused the grain growth and relaxation of strain by recrystallization, and thus, the residual stress in the oxygen-free copper was reduced. The tensile strength and 0.2% proof stress were decreased, and elongation was increased by the heat treatment accompanying recrystallization. The plastic strain in the heat-treated specimen was increased compared with that in the untreated specimen under the same stress amplitude condition, and thus, the low cycle fatigue life of the oxygen-free copper was degraded by the heat treatment. Striation was observed in the crack initiation area of the fractured surface in the case of the stress amplitude less than 100 MPa regardless of the presence of the heat treatment. With an increase in the stress amplitude, the river pattern and the quasicleavage fracture were mainly observed in the fracture surfaces of the untreated specimens, and they were observed with striations in the fracture surfaces of the heat-treated ones. The result of the electron backscattered diffraction (EBSD) analysis showed that the grain reference orientation deviation (GROD) map was confirmed to be effective to investigate the fatigue damage degree in the grain by low cycle fatigue. In addition, the EBSD analysis revealed that the grains were deformed, and the GROD value reached approximately 28° in the fractured areas of heat-treated specimens after the low cycle fatigue test.

Effects of SiC orientations and particle sizes on the low cycle fatigue properties of SiCp/A356 composite

Herein, effects of SiC orientations and sizes on fatigue behaviors of SiCp/A356 composites are investigated. Results reveal longitudinal (LT) samples possess superior fatigue properties compared to transverse (TR) samples. Adding 4.5 μm SiC, deformation mechanisms transfer from dislocation pile-ups along Si to along both SiC and Si, contributing to SiC decohesion and degradation of fatigue resistances. Adding 20 μm SiC, SiC decohesion prevails at TR sample since angle θ between SiC orientation and loading direction locates 45°-135°, while broken SiC particles are predominant at LT sample as θ < 45° or θ > 135°. These further decline their fatigue life.

Low Cycle Fatigue Properties of FGH720Li Superalloy

Low cycle fatigue (LCF) properties of a powder metallurgy(PM) nickel base superalloy FGH720Li were systematically studied in this work, including smooth LCF and notched LCF tested at various temperatures and different stress. The relationship between the fatigue life and applied stress was analyzed both for smooth fatigue and notch fatigue tests. The effects of loading frequency and stress ratio on LCF behavior were also studied. As an important influencing factor of the fatigue life in powder metallurgy superalloy, the effect of inclusions on LCF life was also investigated. The results showed that the fatigue properties of FGH720Li alloy was excellent, when tested at the temperature of 450°C and applied stress of 1230MPa, the fatigue life could exceed 5×104 cycles. When tested at 650°C and 1150MPa, the average fatigue life was still beyond 2×105 cycles.

The fatigue properties, microstructural evolution and crack behaviors of low-carbon carbide-free bainitic steel during low-cycle fatigue

The low-cycle fatigue properties, microstructural evolution and crack growth behaviors of a low-carbon carbide-free bainitic steel subjected to austempering at 300, 320 and 350 °C have been investigated. The results indicated that the fatigue lifetimes of the specimens increase with decreasing the austempering temperature at lower total strain amplitudes, but a comparable fatigue lifetime or even an opposite result is observed at higher total strain amplitudes. During fatigue loading, the blocky retained austenite with inhomogeneous carbon distribution partially transforms into martensite, and the transformation amount increases with the total strain amplitude, especially for the specimens austempered at high temperatures. The transformed martensite and the untransformed austenite satisfy a Kurdjumov-Sachs (K–S) orientation relationship, which can increase the cooperative deformation capacity between the both phases. However, with continuous cycling, more martensite is formed, and these newly formed martensite grains are more likely to form micro-voids/cracks due to their brittleness, which reduces the crack initiation lifetime, and become the fast path for crack propagation, which deteriorates the crack propagation lifetime. Alternatively, more crack branching, crack deflection and severely twisted bainitic ferrite laths are observed for the specimens austempered at high temperatures, which helps to delay/hinder crack growth and increase the crack propagation lifetime, especially at high total strain amplitudes. This suggests that the fatigue lifetime can be closely associated with the interactions of the crack initiation and propagation caused by mechanically-induced martensite from retained austenite.

Effect of cold expansion on high-temperature low-cycle fatigue performance of the nickel-based superalloy hole structure

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.

On the influence of ϰ-carbides on the low-cycle fatigue behavior of high-Mn light-weight steels

High-manganese light-weight steels offer a great potential for structural purposes in the mobility sector due to their reduced density combined with outstanding mechanical properties. As many of the envisaged applications of these materials will suffer from cyclic loading during their service life, the fundamentals of their fatigue behavior must be studied extensively. In the present work, the low-cycle fatigue (LCF) properties of an austenitic Fe-29.8Mn-7.65Al-1.11C light-weight steel were investigated. Two microstructurally different conditions are directly compared: a homogenized, fully austenitic condition and an aged, austenitic, ϰ-carbide-containing condition. Results elaborated by total strain-controlled fatigue tests are discussed considering microstructural insights revealed by scanning electron microscopy and synchrotron diffraction analysis. Although fatigue properties are improved upon aging due to ϰ-carbide formation for lower strain amplitudes as compared to homogenized counterparts, the cyclic deformation behavior is characterized by cyclic softening at increased strain levels. Shearing of the ϰ-carbides and their mechanical dissolution due to the onset of plastic deformation are found to be the underlying mechanisms. Determination of LCF parameters for both conditions was realized by evaluation of the fatigue life based on the Coffin-Manson law and Basquin equation, eventually providing explanations for the Masing and non-Masing behavior of the aged and homogenized condition, respectively, deduced from analysis of half-life hysteresis loops.