汽轮机联轴器螺栓剪切疲劳试验研究

在常温空气介质下，利用薄壁圆筒试样进行剪切疲劳试验，得到具有95％置信度、5％误差限的联轴器螺栓材料40CrNiMoA钢的剪切循环应力－应变特性参数和应变疲劳寿命特性参数，给出该材料在常温下的单调和循环剪切应力应变特性曲线以及不同可靠度下的剪切疲劳寿命曲线。试验表明，该材料在循环扭转载荷下表现出循环软化特性，并且比用von Mises准则所预测的循环软化程度要大些。对不同应变下的循环稳定应力变化规律进行比较可得，无量纲化的不同应变水平下的峰谷值应力仅与无量纲化的循环周次有关。     

GS-20Mn5钢非对称循环应力-应变本构模型及其应用

在实际工程中,机械结构件承受反复载荷时,内部往往是非对称的应力应变状态。在非对称循环加载条件下,材料不仅会表现出循环软/硬化特性,还会表现出平均应力松弛行为。这会影响其在循环稳定状态下的力学性能,进而影响结构在相应工况下承载服役的强度安全性。针对大型压机本体结构常用GS-20Mn5钢进行了单向拉伸及应变比R为0.5,应变幅0.20%、0.25%、0.27%、0.30%和0.40%的非对称应变循环加载试验研究,分别构建了基于单向拉伸试验结果的A-F随动硬化模型,以及基于非对称循环加载的Landgraf模型来描述其平均应力松弛特性,将其应用到Ramberg-Osgood公式中,结合A-F非线性随动硬化模型,建立了非对称循环加载条件下对应于循环应力-应变曲线的本构模型,并确定了相应模型参数。针对承受非对称循环载荷的某大型锻造液压机上横梁,应用所建立的本构模型分别进行了安定性数值分析,评估了其在循环载荷下的弹塑性强度安全性。结果显示,与采用单向拉伸条件下的A-F模型时的计算结果相比,采用非对称循环应力-应变本构模型时上横梁的安定极限载荷提高了约7%。     

Effect of single-pulse laser irradiation energy on healing fatigue damage for copper film

In this study, the optimum laser process parameter was determined by fatigue experiment for the treated copper film specimens with thickness of 25 μm. The mechanism of healing fatigue damage was analyzed by the numerical simulation of temperature field induced by laser irradiation and the microstructure changes on the surface of specimens. The results showed that the fatigue damage of copper film specimens can be healed when the applied energy density is in the range from 4×10³ J/m² to 14×10³ J/m², and the fatigue life can increase by about 5 times when the applied energy density is 7×10³ J/m². The process hardening and grain refinement in the surface layer are mainly responsible for extending fatigue life.     

316L不锈钢non-Masing特性分析和疲劳寿命预测

对316L不锈钢在不同应变范围下分别进行了293 K和873 K试验温度下的低周疲劳试验,讨论了材料循环特性的幅值相关性和温度相关性,比较了不同条件下non-Masing特性,并利用能量方法进行了低周疲劳寿命预测。实验结果表明, 在不同条件下,循环初期会出现不同程度的循环硬化现象,随后会出现循环软化、饱和直至材料失效;与873 K 试验条件相比,材料在293 K温度下的non-Masing特性更为显著;在大应变范围下,两温度下材料的non-Masing特性更加明显。采用能量方法进行疲劳寿命预测时,预测结果均位于两倍分散带内,且基于non-Masing特性得到了比基于Masing特性更为精确的预测结果。在873 K温度和大应变范围下,显著的动态应变时效效应导致考虑non-Masing与Masing特性的预测结果相差不大。     

Effect of single overload on fatigue behavior in the welded haz of low alloy steel

Room temperature fatigue crack growth behavior was obtained for 4140 parent steel, parent heat treated (same as PWHT), as-welded HAZ and PWHT HAZ material under R≈O constant amplitude loading and single tensile overloads with an over load ratio (OLR: P_over/P_max) of 2.5. Double pass automatic submerged arc welding with AWS EM2 electrode was used. PWHT was performed at 650°C for one hour. Constant amplitude fatigue crack growth behavior was very similar for all four material conditions in the log-log linear Paris region. All material conditions responded favorably to the single tensile overloads with fatigue crack growth retardation ranging from 2.5×10⁵ to 4.5×10⁵ cycles which corresponded to life increases of 250 to 400 percent. SEM analysis indicated many similarities on the fatigue fracture surfaces with predominant ductile quasi-striation morphology.     

High-temperature low cycle fatigue, creep–fatigue and thermomechanical fatigue of steels and their welds

High-temperature low cycle fatigue (LCF) is influenced by various time-dependent processes such as creep, oxidation, phase transformations and dynamic strain ageing (DSA) depending on test conditions of strain rate and temperature. In this paper, the detrimental effects of DSA and oxidation in high-temperature LCF are discussed with reference to extensive studies on 316L(N) stainless steel and modified 9Cr–1Mo steel. DSA has been found to enhance the stress response and reduce ductility. It localizes fatigue deformation, enhances fatigue cracking and reduces fatigue life. High-temperature oxidation accelerates transgranular and intergranular fatigue cracking in modified 9Cr–1Mo steel and during long hold time tests in austenitic stainless steel. In welds, microstructural features such as presence of course grains in the HAZ and formation of brittle phases due to transformation of δ ferrite during testing influence crack initiation and propagation and fatigue life. Thermomechanical fatigue (TMF) studies are suggested as more closer to the actual service conditions. In 316L(N) stainless steel, TMF lives under out-of-phase cycling are found to be lower than those under in-phase conditions in the low-temperature regimes, while the converse holds good when the upper temperature encompassed the creep-dominant regime.     

铜锌合金及20CrMnTi钢中温塑性加工的规律和机理的研究

本文研究了H68黄铜和20CrMnTi 钢的动态应变时效(DSA)和动态退火(DA)对其机械性能的影响。试验是在300—1100K 的温度范围内进行的,应变速度为1.1×10^-2s^-1到1.1×10^-4s^-1,平均晶粒尺寸为10μm。试验结果表明:H68黄铜的最大均匀应变量和加工硬化参数均在某一温度范围内出现一个峰值,而20CrMnTi 钢则在高温区和低温区分别形成一个峰值。试验的另一个结果是发现动态应变时效现象与动态退火现象在靠近临界温度时有一定的重叠。建议在研究中温塑性加工的规律时,有必要考虑这两种现象的影响。     

镍基合金GH536材料高温低循环疲劳特性研究

对镍基合金 GH5 36材料在三种不同温度下的低循环疲劳特性进行了研究。给出了材料的应变 -寿命关系和应力应变关系。讨论了材料的疲劳特性随温度的变化。结果表明 ,在中等应变幅值之下 ,材料的疲劳寿命随温度的增加而降低     

不锈钢多轴非比例加载低周疲劳的研究

对３１６Ｌ不锈钢进行了单轴及多轴非比例加载低周疲劳试验及其微结构的观察,分析了非比例循环附加强化及低周疲劳寿命对应变路径依赖性的微观机理,基于微观机理的研究结果,以位错结构特征参量的统计平均值给出了加载路径的非比例度定义,建立了新的多轴非比例加载低周疲劳寿命估算公式。     

基于灰色理论镍基单晶合金多轴非比例加载低周疲劳研究

基于灰色理论研究DD3镍基单晶合金高温多轴非比例加载低周疲劳特性,对等效应变范围、温度、应变路径角、拉/扭载荷相位角和轴向应变比等影响疲劳寿命的因素进行灰色关联度分析,并引入损伤参量Q表征非对称循环特性和拉/扭多轴效应,以参量Q、等效应变范围Δεe和Mises等效应力范围Δσe构造疲劳损伤参量,建立低周疲劳寿命GM(1, N)预测模型。结果表明,各影响因素与多轴低周疲劳寿命的关联度等级依次为等效应变范围、温度和应变路径角为一级,拉/扭载荷相位角为二级,轴向应变比为三级;680 ℃和850 ℃温度下的GM(1, N)疲劳寿命模型的预测寿命与试验寿命的绝对关联度分别为0.97、0.86,平均相对误差分别为4.9%、6.0%;两种温度的试验数据几乎分别落在1.93、2.13倍偏差分布带内。     

耦合温度效应晶体塑性的TWIP钢变形行为研究

为探究TWIP钢高温条件下的塑性变形机理,建立了耦合温度效应的晶体塑性本构模型,考虑温度对TWIP钢滑移和孪生的影响,提出了耦合温度效应的流动法则和硬化法则。结合在500 ℃和750 ℃条件下的原位SEM高温拉伸试验,建立了描述TWIP钢热变形过程的晶体塑性有限元模型。模拟获得不同温度条件下的应力应变曲线、应变硬化率和孪晶体积分数与试验结果相吻合,验证了该模型的正确性。进而,基于所建立的模型研究了温度对TWIP钢塑性变形过程滑移、孪生演化及应变硬化过程的影响规律,结果表明：滑移阻力、孪生阻力和应变硬化率随温度的升高呈不均匀降低的趋势,且断后伸长率呈现降低的趋势,由25 ℃时53.4%降低至750 ℃时16.5%。同时,随温度升高,孪生受到抑制,但滑移受温度的影响较小,表现为滑移主导的塑性变形机制。     

On the application of a micromechanical small fatigue crack growth model to predict fretting fatigue life in AA7075-T6 under spherical contact

This work presents a method for assessing the fretting fatigue life by estimating the fatigue crack growth rate from the regime of microcracks to the final failure, which is achieved using a two-threshold small fatigue crack growth model. The propagation thresholds are associated with the interaction of the "monotonic plastic zone" and the "cyclic plastic zone" with the microstructure of the material. The predicted fatigue life and the estimated non-propagating cracks agree very well with the experimental fretting fatigue tests with spherical contact in 7075-T6 aluminium alloy.     

Computational evaluation of strain-rate-dependent deformation behavior of rubber and carbon-black-filled rubber under monotonic and cyclic straining

The molecular chain network model for elastic deformation behavior and the reptation theory for viscoelastic deformation behavior are used to derive a constitutive equation for rubber. The new eight-chain-like model contains eight standard models consisting of Langevin springs and dashpot to account for the interaction of chains with their surroundings. Monotonic and cyclic deformation behavior of rubber with relaxation under different strain rates have been examined. The results reveal the roles of the individual springs and dashpot, and the strain rate dependence of materials in the monotonic and cyclic deformation behaviors, particularly softening and hysteresis loss, that is, the Mullins effect, occurring in stress-stretch curves under cyclic deformation processes. The validity of the results is checked through comparison with experimental results. The deformation behaviors of a plane strain rubber unit cell containing carbon-black (CB) under monotonic and cyclic straining are investigated by computational simulation using the proposed constitutive equation and homogenization method. The results reveal the substantial enhancement of the resistance of CB-filled rubber to macroscopic deformation, which is caused by the marked orientation hardening due to the highly localized deformation of rubber. The role of strain rate sensitivity on such characteristic deformation behaviors as increases in the resistance to deformation, hysteresis loss, and the effects of the distribution morphology and the volume fraction of CB on the deformation behavior is clarified. The increases in the volume fraction and in the aggregation of the distribution of CB substantially raise the resistance to deformation and hysteresis loss.     

Fatigue of bitumen and bituminous mixes

This paper gives details of an investigation into the fundamental fatigue properties of bitumen and bituminous mixes. Fatigue tests carried out under constant bending stress, at varying temperatures between −13·5°C and +25°C, show that the material exhibits fatigue properties over wide ranges of stress and that for a particular temperature and speed of loading the log stress-log number of cycles to failure relationship is linear between 10⁴ and 10⁸ cycles. The life under constant stress is highly dependent on the temperature of the test, a low temperature giving a longer life at a particular stress; it is also dependent to some extent on the speed of loading, but taking into account the stiffness of the material which depends on temperature, speed of loading, rheological characteristics and composition of the mix, it has been found that when the logarithm of the strain is plotted against the logarithm of the number of cycles to failure all experimental results at different speeds and temperatures for one mix lie with a certain amount of scatter about a straight line. It appears therefore that the factor affecting the fatigue life is one of strain rather than stress, and the effects of temperature and speed can be accounted for by their effect on the stiffness of the specimen. This has been confirmed by tests under constant torsional strain at different temperatures between −20°C and +40°C, but at the higher temperatures under this type of loading the fatigue life includes a considerable crack-propagation time. Similar results have been obtained from mixes containing different amounts of aggregate, but as the quantity of aggregate in the mix is reduced so the life for a given strain increases, suggesting that the criterion of failure may be one of tensile strain in the bitumen present in the mix. Some tests have also been carried out on pure bitumen specimens at different temperatures.

Careful examination of the fatigue cracks and failure surfaces shows that in nearly all cases failure originates on the principal tensile plane. The effects of such factors as surface conditions, void content and rest periods have also been studied.     

Study of anomalous hardening in NiAL single crystals at intermediate temperatures by FEA

The hardening model based on the dislocation mechanics is employed to study the experimentally observed high tensile elongations of NiAl along the [1 1 0] orientation at intermediate temperatures. In the hardening model proposed, a mobility of dislocation is assumed to be restricted to glide through the slip plane by forest dislocation and thermally activated cross-slip event. Overall deformation behavior of NiAl was greatly influenced by temperature-dependent dislocation mobility that both experimental and simulated yield stresses decreased as temperature increased. The results of simulation showed anomalous hardening behaviors analogous to those of experiment at certain circumstances. This behavior occurred due to the hardening contributions generated by cross-slip events that disable the dislocation motion in the primary slip systems. By comparing simulation results with experiments, it is confirmed that the proposed hardening model can represent anomalous tensile elongations due to the hardening by forest dislocations and cross-slip events.     

扭转预应变对45钢低周疲劳性能的影响

介绍４５钢在不同扭转预应力变下的低周疲劳性能,包括循环应力－应变特性,循环硬化软化特性及低周疲劳寿命的研究工作。结果表明,扭转预应变降低周疲劳寿命,循环应力随扭转预应变的增大而增大。     

Low-cycle fatigue of a friction stir welded 2219-T62 aluminum alloy at different welding parameters and cooling conditions

Strain-controlled low-cycle fatigue tests and microstructural evaluation were performed on a friction stir welded 2219-T62 aluminum alloy with varying welding parameters and cooling conditions. Cyclic hardening of friction stir welded joints was appreciably stronger than that of the base material. The cyclic stress amplitude increased, and plastic strain amplitude and fatigue lifetime slightly decreased with increasing welding speed from 60 to 200 mm/min but were only weakly dependent of the rotational rate between 300 and 1,000 rpm with air cooling. Friction stir welded joints with water cooling had higher stress amplitude and fatigue life than that with air cooling. Fatigue failure of the joint occurred in the HAZ where the soft zone was present, with crack initiation from the specimen surface or near-surface defect and crack propagation characterized by typical fatigue striations.     

An Investigation into the Effect of Normal Load Frequency on Fretting Fatigue Behavior of Al7075-T6

Most previous studies on fretting fatigue have been accomplished under constant normal loading and less attention has been paid to cyclic normal loading. An innovative test apparatus was specially designed and manufactured for fretting fatigue tests under cyclic loading in this work and the fretting fatigue behavior of Al7075-T6 was investigated at different normal load frequencies. A finite element model was developed to study the effect of normal load frequency on the contact stress distribution. It was found that the cyclic normal load has a more damaging effect on fretting fatigue life compared to constant normal load, particularly at lower frequencies. The results showed that at the normal load frequency of f = 1 Hz, fatigue life decreased by 52% in the high cycle fatigue regime and 28% in the low cycle fatigue regime. The experimental results also indicated that at the normal load frequency of 80 Hz, the fretting fatigue life converged to its corresponding life under constant normal load condition. The fracture surface and the fretting area of the specimens were examined using both optical and scanning electron microscopy (SEM). The experimental observations showed that the dominant partial slip condition with a wider slip region compared to constant normal loading, severe delamination, and higher oxidation rate due to the normal load release at each cycle, are the most important reasons for significant reductions in fretting fatigue life, under cyclic normal loading, especially for low normal load frequencies.     

概率循环应变—寿命曲线及其估计方法

引入了概率循环应变—寿命曲线描述低周疲劳循环应变—寿命试验数据的分散性。应用线性回归方法和极大似然法原理 ,发展了曲线及其置信限的估计方法。方法采用经验证的良好假设分布即对数正态分布模拟疲劳寿命的分散性。基于Coffin Manson律 ,将曲线近似表征均值和均方差循环应变—寿命曲线的形式。任意可靠度下的疲劳分析可方便地根据正态分布进行。与现有独立随机变量观点不同 ,材料常数视为关联随机变量。1Cr18Ni9Ti不锈钢管道焊接头试验数据分析验证了方法的有效性。     

FCC-BCC phase transformation in rectangular beams subjected to plastic straining at cryogenic temperatures

FCC metals and alloys are frequently used in cryogenic applications, nearly down to the temperature of absolute zero, because of their excellent physical and mechanical properties including ductility. These materials, often characterized by the low stacking fault energy (LSFE), undergo at low temperatures three distinct phenomena: dynamic strain ageing (DSA), plastic strain induced transformation from the parent phase (γ) to the secondary phase (α^′) and evolution of microdamage. The FCC-BCC phase transformation results from metastability of LSFE metals and alloys at very low temperatures. The phase transformation process leads to creation of two-phase continuum where the parent phase coexists with the inclusions of secondary phase. Such heterogeneous material structure induces strong strain hardening related to two distinct mechanisms: interaction of dislocations with the inclusions and increase of tangent stiffness as a result of mixture of two phases, each characterized by different parameters. The strain hardening model is based on micromechanics considerations (first mechanism) and on the Hill concept (1965) including the Mori-Tanaka (1973) homogenization scheme (second mechanism). Identification of parameters of the constitutive model is based on the available experimental data. The model is used to describe phase transformation in rectangular beams subjected to elastic-plastic bending at cryogenic temperatures. Several examples of rectangular beams with FCC-BCC phase transformation induced functionally graded (FGM) microstructure are presented.