Low cycle fatigue behavior of constraint connections

The general influence of constraint effects on reversed low-cycle fatigue performance was investigated. Constraint effects are being considered to have contributed to the failure of welded Steel Moment Resisting Frames (SMRFs), were cracks initiated in constraint regions during recent earthquakes. However, this study revealed that high constraint enhanced the resistance to crack initiation during cyclic loading by reducing the local strains at the notch tip. Furthermore, changes in toughness due to various constraint severities had almost no influence on the low cyclic performance of steel under constraint. This applies to low and high stress levels as well as low and high constraint, covering the whole range of possible scenarios of low-cycle and even towards high-cycle fatigue. While high toughness is still important for enhancing the fracture strength to accommodate the maximum imposed stresses/strains in earthquake loading, it most likely does not play a key role in cyclic performance.     

Low cycle fatigue behavior of Cu-Cr-Zr alloy with different cold deformation

The present study addressed the cyclic deformation behavior and fatigue properties of Cu-0.69Cr-0.07Zr alloy with different cold deformation (ε = 64%, 75%, and 84%) using low cycle fatigue test. Low cycle fatigue tests were conducted under fully-reversed conditions at different total strain amplitudes. The microstructure changes and fatigue fracture characteristics were analyzed by scanning electron microscope (SEM) and transmission electron microscope (TEM). The main findings suggest that the Bauschinger effect was significantly stronger with larger deformation at low total strain amplitude. And it was proved that the relationship between the total strain amplitude and the low cycle fatigue life of Cu-Cr-Zr alloy with different deformation can be expressed by the Manson–Coffin–Basquin formula. Further, the reason for the fatigue life was shorter and the cyclic softening rate decreased faster at high applied total strain amplitude was that the dislocation density decreased due to the rearrangement of the dislocations.     

Low cycle fatigue behavior and failure mechanism of a dual-phase steel

Low cycle fatigue behavior and failure mechanism of a dual-phase steel was investigated by using LCF tests, interrupted LCF tests, SEM, TEM and XRD peak broadening. LCF tests were performed at constant strain amplitudes of ±0.002, ±0.004, ±0.006 and ±0.1. Microscopic investigations were carried out on gauge surfaces of the interrupted LCF specimens at various stages of their fatigue life. It was observed that at high strain amplitudes damage was started at fractured martensite particles and passed through areas with high density of martensite. At low strain amplitudes damage was started at separated ferrite/martensite interface and passed through areas with low density of martensite. All specimens showed fatigue hardening during LCF tests. It was also observed that the rate of hardening was affected by strain amplitude. The results show that XRD peak broadening is sensitive to the strain amplitude and the stage of damage in the specimens.     

Low cycle fatigue behavior of a directionally solidified cobalt base superalloy

An investigation has been made of the low cycle fatigue behavior and microstructure evolution of a directionally solidified cobalt-base superalloy at room temperature, 700 and 850°C under the control of different total strain amplitudes. The results show that at room temperature the cyclic hardening of the alloy appears during the first few cycles, and then a long saturation stage begins. At 700°C, the alloy exhibits a pronounced initial hardening, and a secondary hardening after a short saturation. At 850°C, the alloy shows a continuous cyclic hardening until fracture. Examination by TEM indicates that the initial hardening of the alloy at room temperature is caused by the pile-ups of the stacking faults at the stacking fault intersections, while the saturation is related to the formation of the hexagonal close-packed (HCP) zones and twins. The mechanism of initial hardening at 700°C is similar to that at room temperature, while the stress saturation is due to interaction obstacle to stacking-fault becoming weaker, because of thermal activation. The secondary hardening is attributed to the formation of sessile dislocation tangles. The continuous cyclic hardening at 850°C is related to the interaction between the precipitates (M₂₃C₆)and dislocations.     

无铅焊料Sn-3.8Ag-0.7Cu的低周疲劳行为

测量了Sn-3.8Ag-0.7Cu无铅焊料试样的循环滞后回线、循环应力响应曲线、循环应力-应变和应变寿命关系,研究了焊料在总应变幅控制下的低周疲劳行为结果表明:该焊料合金在总应变幅较高(1%)时发生连续的循环软化,而在总应变幅较低(≤0.4%)时则表现为循环稳定.线性回归分析表明,该焊料的低周疲劳寿命满足Coffin-Manson经验关系式,由此给出了焊料在室温下的低周疲劳参数.采用扫描电镜观测和分析了焊料在疲劳前后的组织特征.     

Low cycle fatigue and creep behavior of recrystallized Molybdenum near room termperature

The deformation behavior of pure recrystallized molybdenum under cyclic and static loads was investigated in the temperature range between 30°C and 10O°C, for stress amplitudes between 100 MPa and 250 MPa and for static loads up to 200 MPa. The results show that in spite of the low test temperatures and stress levels the Mo material exhibits considerable plastic strains which depend sensitively on frequenez and small changes in temperature.The activation energy deduced for the static and dynamic deformation is less than 0.98 eV which indicates thermally activated processes, to be explained by a dislocation kink model as described in the literature. The low-temperature fatigue behavior appears strongly influenced by creep-fatigue interaction phenomena.     

Low cycle fatigue behavior of a eutectic 80Au/20Sn solder alloy

The eutectic 80Au/20Sn solder alloy is widely used in high power electronics and optoelectronics packaging. In this study, low cycle fatigue behavior of a eutectic 80Au/20Sn solder alloy is reported. The 80Au/20Sn solder shows a quasi-static fracture characteristic at high strain rates, and then gradually transforms from a transgranular fracture (dominated by fatigue damage) to intergranular fracture (dominated by creep damage) at low strain rates with increasing temperature. Coffin-Manson and Morrow models are proposed to evaluate the low cycle fatigue behavior of the 80Au/20Sn solder. Besides, the 80Au/20Sn solder has enhanced fatigue resistance compared to the 63Sn/37Pb solder.     

Low cycle fatigue behaviour of HAYNES 230 alloy

Abstract

HAYNES® 230® alloy is a commercial solid–solution strengthened Ni-base superalloy that has found wide spread use in the hot sections of gas turbines. Recently, the alloy has been considered for other applications, e.g. in concentrated solar power plants. One common key performance criterion for all these applications is the ability of the material to withstand large numbers of thermal cycles due to start-ups, operatings, and shut-downs. Such thermal cycles may trigger low-cycle fatigue (LCF) due to differential thermal expansion and contraction. This paper reports isothermal LCF performance data of 230 alloy standard mill-annealed plate product for a wide range of temperatures 427–982°C and strain ranges (up to 1·5%), relevant to many of the aforementioned applications. Low-cycle fatigue lives are analysed in terms of the contributions of cyclic elastic and plastic strain components to fatigue damage. The fit parameters are discussed in the context of the alloy’s tensile properties.     

Low cycle fatigue of a titanium 829 alloy

The fatigue behaviour of titanium 829 in its oil quenched (‘basketweave’) and air cooled (‘aligned’) microstructural forms has been examined at 600°C and room temperature under fully reversed, total strain controlled conditions. Identical endurances are observed for each microstructure together with a low transition life. Similarly, almost perfect cyclic stability is exhibited irrespective of microstructure, temperature, strain range and rate. This is tentatively attributed to the ability of the aligned colonies present in both microstructures to accomodate plasticity. It is argued that a reduction in strain rate shortens life due to environmental effects. Multiple crack initiation is generally associated with facet-like features, with later growth surfaces bearing striations only after fatigue at 600°C.     

Low cycle fatigue behaviour of a plasma-sprayed coating material

Single crystal nickel-base superalloys employed in turbine blade applications are often used with a plasma-sprayed coating for oxidation and hot corrosion resistance. These coatings may also affect fatigue life of the superalloy substrate. As part of a larger programme to understand the fatigue behaviour coated single crystals, fully reversed, total-strain controlled fatigue tests were run on a ‘free standing’ NiCoCrAiY coating alloy, PWA 276, at 0.1 Hz. Fatigue tests were conducted at 650°C, where the NiCoCrAiY alloy has modest ductility, and at 1050°C, where it is extremely ductile, showing tensile elongation in excess of 100%. At the lower test temperature, deformation-induced disordering softened the NiCoCrAlY alloy, while at the higher test temperature cyclic hardening was observed which was linked to gradual coarsening of the two-phase microstructure. Fatigue life of the NiCoCrAlY alloy was significantly longer at the higher temperature. Further, the life of the NiCoCrAlY alloy exceeds that of coated, [001]-oriented PWA 1480 single crystals at 1050°C, but at 650°C the life of the coated crystal is greater than that of the NiCoCrAlY alloy on a total strain basis.     

Low cycle fatigue modeling for nickel-based single crystal superalloy

The low cycle fatigue (LCF) characteristics of nickel-based single crystal (SC) superalloy have been experimentally and numerically investigated. The effects of crystallographic orientation, load ratio and stress concentration are studied. In order to model the effect of crystallographic orientation, a new orientation factor, which is relevant to the yield strength, is constructed. On the other hand, a new asymmetrical loading factor is introduced to describe the effect of load ratio. The LCF model for SC superalloy smooth specimen is established with these new damage parameters. The effect of the strain gradient on the LCF life of SC superalloy is further studied, which is applied to the evaluation of the LCF life of SC superalloy notched specimen. The LCF model proposed is validated by the experimental data of SC superalloy DD3 and PWA1480.     

Stress relaxation behavior and low cycle fatigue behavior of bulk SAC 305

Bulk Sn_96.5Ag₃Cu_0.5 samples were mechanically tested to investigate the effect of temperature, frequency and applied stress on the low cycle fatigue and stress relaxation behavior and the corresponding microstructure. Samples were tested under a variety of parameters including applied stresses between 8 and 80 MPa, temperatures of 25, 50, 100 and 150 °C and frequencies of 1, 0.1 and 0.01 Hz, respectively. Samples used for the stress relaxation behavior exhibited plastic behavior with increased softening behavior with increased stress levels, increased temperature and lower frequencies. Bayesian analysis revealed that stress relaxation behavior could be expressed in general by the following expression: ?σ = AN ^B In the previous expression, Bayesian analysis showed that the testing frequency has an exponential dependency while the temperature has a power law dependency on the parameters A and b. The results of the low cycle fatigue study showed that life decreased with increased applied stress, decreased frequency and increased temperature. Bayesian analysis revealed that the low cycle fatigue behavior could be described by the following expression: ?σ = G(logN) ^m . Additionally, Bayesian analysis showed that the testing frequency and temperature both have a power law dependency on the parameters G and m.     

Low cycle fatigue resistance of SA533 pressure vessel steels

Abstract

Low cycle fatigue (LCF) tests were conducted on SA533 steels with different levels of sulphur content at room temperature and 300 °C. The fatigue limit shows little or no dependence on sulphur content, but significantly depends on testing temperature. At 300 °C, the fatigue limit is at 0.2% strain amplitude, slightly higher than the 0.1% strain amplitude obtained at room temperature. The fatigue limit of SA533 steel subjected to LCF tests at 300 °C was improved by the combined effects of dynamic strain aging (DSA) and grain size reduction. DSA and grain size reduction increase the steel strength and, accordingly, improve the LCF limit at 300 °C. But, concurrently, the carbide/nitride precipitates in SA533 steel lead to a decrease in steel strength. Grain size reduction and precipitation compete with each other. Grain size reduction is dominant and the net effect is reflected in the increase in hardness value. Fatigue life could be predicted either by means of the strain-life equation or the SWT (Smith, Watson and Topper) parameter for specimens under low cycle fatigue conditions. The strain-life equation for SA533 steel in air is independent of the sulphur content, but significantly dependent on the testing temperature.     

Low cycle fatigue behaviour of particulate reinforced metal matrix composites

The low cycle fatigue (LCF) resistance of two different 6061 Al/20 vol% alumina particulate metal matrix composites (MMCs) in a peaked-aged condition has been evaluated under fully reversed strain control testing. Test results were combined with scanning electron and optical microscopy investigations to determine the effects of reinforcement particles and strain amplitude on the LCF behaviour of these MMCs. Both materials show three stages of response to LCF: initial fast hardening or softening in the first few cycles; gradual softening for most of the fatigue life; and a rapid drop in the stress carrying capability prior to failure. Both MMCs exhibit short LCF life which follows a Coffin-Manson relationship. All tested specimens demonstrate ductile fracture morphology at final failure. The experimental results are discussed in respect of strain amplitude, matrix composition and reinforcement shape and crack initiation.     

Low cycle fatigue behavior of Ti6Al4V thermochemically nitrided for its use in hip prostheses

Titanium and its alloys have many attractive properties including high specific strength, low density, and excellent corrosion resistance. Besides, titanium and the Ti6Al4V alloy have long been recognized as materials with high biocompatibility. These properties have led to the use of these materials in biomedical applications. Despite these advantages, the lack of good wear resistance makes difficult the use of titanium and Ti6Al4V in some biomedical applications, like articulating components of prostheses. Some surface treatments are available in order to correct these problems, like thermal surface treatment by means of nitrogen gaseous diffusion at high temperature. Nitrogen enters into the material by diffusion, creating a surface layer of increased hardness. Low cycle fatigue behavior in air of Ti6Al4V alloy has been studied. Results show a reduction of low cycle fatigue life up to 10% compared to the not-treated material. Studies suggest it is not related to the titanium nitride surface layer, but to microstructural changes caused by the high temperature treatment.     

High cycle fatigue behavior of as-extruded ZK60 magnesium alloy

Tensile and high cycle fatigue properties of hot extruded ZK60 magnesium alloy have been investigated, in comparison to that of hot-extruded plus T5 heat-treated ZK60 magnesium alloy which was named as ZK60-T5. High cycle fatigue tests were carried out at a stress rate (R) of −1 and a frequency of 100 Hz using hour-glass-shaped round specimens with a gage diameter of 5.8 mm. The results show that tensile strength greatly improved and elongation is also slightly enhanced after T5 heat treatment, and the fatigue strength (at 10⁷ cycles) of ZK60 magnesium alloy increases from 140 to 150 MPa after T5 heat treatment, i.e., the improvement of 7% in fatigue strength has been achieved. Results of microstructure observation suggest that improvement of mechanical properties of ZK60 magnesium alloy is due to precipitation strengthening phase and texture strengthening by T5 heat treatment. In addition, fatigue crack initiations of ZK60 and ZK60-T5 magnesium alloys were observed to occur from the specimen surface and crack propagation was characterized by striation-like features coupled with secondary cracks.     

High-temperature low cycle fatigue behavior of a gray cast iron

The strain controlled low cycle fatigue properties of the studied gray cast iron for engine cylinder blocks were investigated. At the same total strain amplitude, the low cycle fatigue life of the studied material at 523 K was higher than that at 423 K. The fatigue behavior of the studied material was characterized as cyclic softening at any given total strain amplitude (0.12%–0.24%), which was attributed to fatigue crack initiation and propagation. Moreover, this material exhibited asymmetric hysteresis loops due to the presence of the graphite lamellas. Transmission electron microscopy analysis suggested that cyclic softening was also caused by the interactions of dislocations at 423 K, such as cell structure in ferrite, whereas cyclic softening was related to subgrain boundaries and dislocation climbing at 523 K. Micro-analysis of specimen fracture appearance was conducted in order to obtain the fracture characteristics and crack paths for different strain amplitudes. It showed that the higher the temperature, the rougher the crack face of the examined gray cast iron at the same total strain amplitude. Additionally, the microcracks were readily blunted during growth inside the pearlite matrix at 423 K, whereas the microcracks could easily pass through pearlite matrix along with deflection at 523 K. The results of fatigue experiments consistently showed that fatigue damage for the studied material at 423 K was lower than that at 523 K under any given total strain amplitude.     

Low cycle fatigue and cyclic softening behaviour of martensitic cast steel

The paper presents the results of research on fatigue properties in a small number of cycles to failure of high-chromium martensitic GX12CrMoVNbN9-1 (GP91) cast steel designed for the casts working under the so-called supercritical parameters. The tests of fatigue life were carried out at three temperatures: room temperature, 550 °C and 600 °C, for five levels of controlled amplitude of total strain ɛ_ac: 0.25%; 0.30%; 0.35%; 0.50% and 0.60%. Cyclic softening was observed in the fatigue tests at all temperatures without a stabilization period. The fatigue lifetime curves at each temperature were obtained based on Basquin and Coffin–Manson equations. It has been shown that the influence of temperature on fatigue life depends on the level of strain and is the biggest for the lowest levels of strain covered in the research. The results of fatigue tests are connected with the quantitative changes in the microstructure of the cast steel. Quantitative tests were carried out using thin foils by means of TEM. Disappearance of martensitic laths, increase in the subgrains size, decrease in the dislocations density and coagulation of carbides were observed after low cycle fatigue. The scale of these changes in the microstructure is dependent on the temperature and level of strain.     

螺纹钢筋疲劳性能测定方法研究

为解决英标B500B螺纹钢在疲劳试验过程中断裂在试验机夹具或在2 d(d为钢筋公称直径)范围内而导致试验失败的问题,采用Abaqus软件模拟疲劳试验时夹持部分的受力分布并分析"断头"产生的原因,通过改变样品表面状态、夹持方式、夹持力等方法分析影响疲劳试验的具体因素。结果表明,试样受力不均而导致局部应力集中是"断头"形成的主要原因。通过使用砂轮机打磨样品的横肋,用砂布介质作为保护套和减少夹持力等方法,可改善试样夹持部位的应力集中情况,有效避免"断头"现象,使得试验成功率得到极大程度提高。