Investigation of yield strength and single cycle overload on crack closure

Crack closure experiments were performed on 6063-T6 and 6061-T6 aluminium alloys, using a crack opening displacement gauge, for various overload ratios (1·67, 1·88 and 2·06). On the basis of these experiments some relationships are developed. The delay period after application of a single overload increases with increase in the magnitude of overload. The retardation is decreased with increase in prestrain for the same overload ratio. After the overload cycle the increase in U values is less in prestrained material than in the as-received material. Crack growth rate also decreases after application of an overload cycle; it attains a constant amplitude crack growth rate and crack closure value after a certain number of cycles. For the same stress ratio, the delay period in the 6063-T6 alloy was found to be more than in the 6061-T6 alloy. The experimental results when plotted on log-log graph paper, show that N_D/N_CAL vs overload ratio and crack length (when U = U_min) vs overload ratio fit a straight line, from which the power laws are developed.     

考虑变量时间动态变化的燃料全生命周期优化研究

通过对设计变量的敏感度分析,找到了对目标函数敏感度最大的变量,即影响目标函数的主要因素.对这些变量随时间变化情况进行拟合,用拟合后的时间变量函数代替静态模型中的固定系数,建立了考虑变量时间动态变化的全生命周期优化模型,完成了考虑变量时间动态变化的全生命周期优化.以广西木薯乙醇燃料全生命周期成本优化为案例,进行了案例分析.优化结果表明,考虑变量时间动态变化能更真实地反映最优值随时间变化而变化的关系.     

A continuum damage mechanics model on low cycle fatigue life assessment of steam turbine rotor

A nonlinear Continuum Damage Mechanics model (CDM) is proposed to assess the low cycle fatigue life of a steam turbine rotor, in which the effects of mean stress are taken into account and the damage is accumulated nonlinearly. The model is applied to a 200 MW steam turbine under cold start and sliding parameter stop. The results are compared with those from the linear accumulation theory that is dominant in life assessment of steam turbine rotors at present. The comparison results show that the nonlinear CDM model describes the accumulation and development of damage better than the linear accumulation theory.     

A new method for low cycle fatigue life analysis at high temperature

High temperature low cycle fatigue analysis is a problem of general concern in engineering. In this paper, which deals with prediction of high temperature low cycle fatigue (LCF) life at the edge of the leakage hole of an aero-turbine disk, the LCF tests have been carried out at 300°C for cylindrical specimens with and without circumferential notch. A new parameter, local equivalent strain energy density (LESED), was suggested. It can be used to describe and predict the high temperature LCF life of notched specimens which are in the multiaxial stress-strain state. Various mechanics parameters including LESED at the notch root of specimens have been calculated using an elastic-plastic finite element program. Comparing the LCF life curves obtained with different methods, it is evident that the LESED method can be used effectively not only for describing the LCF behavior of the notched specimens but also for predicting the LCF life of the notched specimens from the LCF life curves of the smooth specimens.     

Prediction of the fatigue life and damage accumulation in components at variable cyclic stresses

This paper compares the effectiveness of several hypotheses for evaluating the service life of components under variable cyclic loading, especially taking account of the differences between different structural materials and the severity of different loading spectra. The comparison is based on results obtained from extensive research carried out on specimens and components made from steel and aluminium alloys. A modified hypothesis to give improved predictions (see eqns. (11) and (12)) is developed.     

Effect of vibration loading on the fatigue life of part-through notched pipe

A systematic experimental and analytical study has been carried out to investigate the effect of vibration loading on the fatigue life of the piping components. Three Point bend (TPB) specimens machined from the actual pipe have been used for the evaluation of Paris constants by carrying out the experiments under vibration + cyclic and cyclic loading as per the ASTM Standard E647. These constants have been used for the prediction of the fatigue life of the pipe having part-through notch of a/t = 0.25 and aspect ratio (2c/a) of 10. Predicted results have shown the reduction in fatigue life of the notched pipe subjected to vibration + cyclic loading by 50% compared to that of cyclic loading. Predicted results have been validated by carrying out the full-scale pipe (with part-through notch) tests. Notched pipes were subjected to loading conditions such that the initial stress-intensity factor remains same as that of TPB specimen. Experimental results of the full-scale pipe tests under vibration + cyclic loading has shown the reduction in fatigue life by 70% compared to that of cyclic loading. Fractographic examination of the fracture surface of the tested specimens subjected to vibration + cyclic loading have shown higher presence of brittle phases such as martensite (in the form of isolated planar facets) and secondary micro cracks. This could be the reason for the reduction of fatigue life in pipe subjected to vibration + cyclic loading.     

Study on the influence of magnitude and instant of overload on fatigue

Experiments on single tensile overload applied at different crack length were performed on 6063-T6 Al alloy. Experimental results show that increasing the magnitude of overload increases crack growth retardation. It was also found that crack growth retardation decreases when overload is applied at larger crack length. A model for finding the number of delay cycles by considering overload ratio and instant of overload is developed.     

焊接参数对轨道车辆焊接结构疲劳寿命的影响

焊接构件疲劳寿命普遍受到应力集中和残余应力的影响,通过优化焊接结构、提高焊接质量、进行焊后处理可以减小应力集中,消除残余应力,从而提高焊接构件的疲劳寿命。采用IIW标准对设计的高速货车转向架进行疲劳寿命分析,研究了焊接参数、焊接质量以及焊后处理对转向架疲劳寿命的影响,发现残余应力和焊后处理对其疲劳寿命影响较大。所以,尽可能降低焊接构件中残余应力能有效提高构件的疲劳寿命。     

Effect of discharge current on cycle life of a rechargeable lithium battery

The cycle life of a rechargeable cell depends on operating conditions such as the depth of discharge, specific voltage limits during charge and discharFor a rechargeable Li/MoS₂ cell, a strong dependence of cycle life on discharge current was found. For discharge rates ranging from 0.5 to 20 h, a     

Effect of hold-time on low cycle fatigue behaviour of nitrogen bearing 316L stainless steel

Axial strain controlled low cycle fatigue (LCF) behaviour and creep–fatigue interaction behaviour of 316L(N) stainless steel (SS) in solution annealed condition has been investigated at various strain amplitudes. Creep–fatigue interaction behaviour of the material has been evaluated by employing tension and compression holds at peak strain. Tension holds up to 90 min were studied. Under all the testing conditions, the material showed initial hardening followed by stress saturation. Hold-time conditions generally showed lower cyclic stress response compared to continuous cycling. The decrease in cyclic stress response with hold-time is attributed to enhanced recovery of the substructure and increase in the grain boundary damage accumulated during the stress relaxation period. The fatigue life is observed to be lower in tensile-hold conditions and the endurance decreased with increase in the duration of the hold. The factors that contribute to the decrease in fatigue life with hold-time have been identified from metallographic studies as the development of creep cracks and cavities and crack initiation and propagation assisted by oxidation. Creep–fatigue damage values based on the damage summation rule have been computed and compared with the RCC-MR bi-linear creep–fatigue interaction diagram suggested for nitrogen alloyed 316L SS at 600°C.     

Effect of pre-deformation on the fatigue crack initiation life of X60 pipeline steel

It is impossible to keep petroleum and natural gas transmission pipelines free from defects in the manufacturing, installation and servicing processes. The damage might endanger the safety of pipelines and even shorten their service life; gas or petroleum release due to defects may jeopardise the surrounding ecological environments with associated economic and life costs.Pre-tensile deformation of X60 steel is employed to experimentally simulate the influence of dents on the fatigue crack initiation life. The investigation indicates that the fatigue crack initiation life of pre-deformed X60 pipeline steel can be assessed by a previously proposed energetic approach. The threshold for crack initiation increases with the pre-deformation due to a strain hardening effect, while the fatigue resistant factor exhibits a maximum with pre-deformation owing to its special dependence on fracture strain and fracture strength. The result is expected to be beneficial to the understanding of the effect of damage on the safety of pipelines and fatigue life prediction.     

A benchmark for life cycle air emissions and life cycle impact assessment of hydrokinetic energy extraction using life cycle assessment

As the demand for renewable energy increases, it becomes important to critically examine the environmental impacts of renewable energy production. Often, the approach has been trial and error in renewable energy with respect to its impact on the environment. Hydrokinetic Energy Extraction (HEE) has been seen as a potentially “benign” form of renewable hydropower. This paper provides a benchmark for initial measurement of HEE environmental impacts, since negative outcomes have been present with previously assumed “benign” renewable hydropower. A Gorlov system was used to represent a HEE system. Life Cycle Assessment (LCA) was utilized to compare the environmental impacts of HEE with small hydropower, coal, natural gas and nuclear power. Environmental Protection Agency (EPA) criteria air emissions were quantified and compared over the life cycle of the systems. Life cycle air emissions were used in combination with TRACI to compare the systems. The Gorlov system was found to have the lowest life cycle impact with a system lifetime comparison, and did compare closely with small hydropower.     

Effect of strain rate on low cycle fatigue with hold time in 9Cr rotor steel

Abstract

One of the main mechanisms of turbine rotor damage is cyclic thermal stress produced during transition conditions, such as warm-up and shutdown processes. In this paper, the cyclic damage with ramp rate is studied in terms of material properties. To evaluate thermal stress damage during the cyclic operation, experiments were carried out on low cycle fatigue with hold time and a range of strain rates for COST FB2, 9Cr ferritic steel. Initially, the experimental fatigue life was evaluated using the time fraction and ductility exhaustion methods. The two analysis methods are compared with real life cases of different hold times and strain rates, and the accuracy is discussed. Next, to predict the plastic deformation curve for low cycle fatigue with hold time more accurately, an attempt was made to simulate cyclic hysteresis loop behaviour numerically using a time dependent viscoplastic constitutive model, and this was compared with the experimental hysteresis loop curve for each cycle number.     

Study of the influence of mixed mode overload on mode I fatigue crack propagation

The effect of mode I and equivalent mixed mode and mode II overload on subsequent mode I crack propagation was studied on compact tension shear specimens of D16AT Al alloy. It was observed that the retardation period is considerably affected by the contribution of the mode II component. Fractographic analysis of the fracture surfaces was carried out by observing the fracture surfaces under a Scanning Electron Microscope.     

Simulation of ratcheting and low cycle fatigue

Fatigue failure in the extremely low cycle and low cycle regimes is characterised by plasticity and high strains. Ratcheting as a secondary phenomenon of cyclic plasticity can accelerate fatigue damage or even act as the failure mechanism itself. The combination of the two damage phenomena and their interaction are of special interest. This paper will focus on the design by analysis of welded and non-welded pressure vessel components with respect to combined ratcheting and low cycle fatigue damage mechanisms. A combination of the local strain approach with a ratcheting check based on the non-linear kinematic constitutive law of Ohno and Wang is proposed. The method is verified by numerical fracture mechanics based on the cyclic J-integral. An algorithm for the determination of component fatigue curves is proposed.     

Effect of inside diameter on design fatigue life of stationary hydrogen storage vessel based on fracture mechanics

Design fatigue life of stationary hydrogen storage vessel constructed of the practical materials of low alloy steels was analyzed based on fracture mechanics in hydrogen and air of 45, 85 and 105 MPa using cylindrical model with inside diameter (Di) of 150, 250 and 350 mm. Design fatigue life of five typical model materials was also analyzed to discuss the effect of Di on the design fatigue life by hydrogen-induced crack growth of the vessel. K_IC of all the practical materials qualified the leak before burst. Design fatigue life generally increased slightly with increasing Di in air, while design fatigue life by K_IH was much shorter than that in air. Hydrogen influence on design fatigue life increased with increasing Di due to that K_I at initial crack increased with increasing Di. The design fatigue life data of the model materials under the conditions of Di, pressure, ultimate tensile strength, K_IH, fatigue crack growth rate and regulations in both hydrogen and air were proposed quantitatively for materials selection and development for stationary hydrogen storage vessel.     

Assessment of fatigue life of engineering structures

The possibility to extend the life of any existing plant beyond the original design demands a number of theoretical and experimental analyses to be done. Due to generally limited information regarding loading history and low availability of specimens, sampled from plant components in the actual damage state, a number of analytical as well as experimental problems arise. One of the prospective sources of investigation or/and recalculation is the cumulative fatigue damage area. In this paper a new energy based method to fatigue life calculations is discussed.     

Estimation of fatigue damage and fatigue life of components under random loading

To examine the applicability of methods proposed in the estimation of fatigue damage and fatigue life of components under random loading, a batch of specimens made of 7075-T651 aluminium alloy has been studied and some of the results are reported in the present paper. The paper describes different methods and rules in the calculation of fatigue damage, especially when random loading is considered. The methods and rules are then verified by the results of a series of low-cycle fatigue tests. It is concluded that a ‘plastic work interaction damage rule’ proposed by Morrow gives us better fatigue damage prediction than the widely used Palmgren-Miner rule, and a formula derived in random vibration books can be used to predict the fatigue damage very easily. It is also found that fatigue lives of the tested specimens can be fitted appropriately by normal or log-normal probability density function. The fatigue reliability of a component made of the same material can therefore be estimated based on the above information.     

Micro-crack growth behavior and life in high temperature low cycle fatigue of blade root and disc joint for turbines

Low cycle fatigue tests were carried out at a temperature of 600 °C using a component specimen of 12%-Cr steel, which simulates a blade root and disc joint for turbines. The growth behavior of micro-cracks in the joint region of the specimens was investigated to clarify the damage mechanism of blade-root joints used in high temperature environments and to improve life assessment methods using finite element analysis. Micro-crack growth behavior similar to that in smooth bar specimens was observed in the specimens tested under conditions of relatively high total strain. Micro-cracks initiation was observed at the notch region of the specimens at an early stage. The crack growth rate increased with surface crack length. The fatigue life of the component specimens under this condition was similar to that of smooth bar specimens. Meanwhile, the component specimens tested under conditions of relatively low total strain showed a different growth behavior. No cracks were observed at the notch region and some micro-cracks were initiated at the edge of the contact region of the specimens in the early stages. Almost no increase in the crack growth rate was observed. Life of the component specimens under this condition was shorter than that of the smooth bar specimens. This might be attributed to fretting fatigue at the contact edge and to mean stresses.     

基于生命周期微网的经济性分析

针对微网经济分析指标体系不系统、不科学等问题,提出了基于生命周期综合考虑各分布式能源经济运行状况、对环境和不同种类负荷造成影响等因素,确定了各分布式能源等年值投资、拆卸、治污、负荷停电赔偿费用以及碳贸易、可靠性、政府补贴效益等来量化微网成本与效益的经济性指标。通过对算例4种微网方案进行经济性分析,验证了理论可行性,同时研究结果为微网投资决策提供了科学依据。