A phenomenological model of fatigue crack growth in perfectly plastic infinite plates under completely reversed uniaxial loading

The fatigue failure of a thin infinite center-cracked plate under completely reversed uniaxial loading is considered. A two-stage fatigue crack model including the incubation and crack propagation stages is constructed. The stress distribution in the vicinity of the crack tip is described using the concept of a conventional elastic crack. The crack-tip plastic zone is simulated by a Dugdale thin plastic zone, and the condition for the movement of the failure front is given by criteria of damage mechanics. It is shown that the fatigue crack growth rate in perfectly plastic materials with a plastic zone of constant length is a power-law function of the stress intensity factor range. This relationship is quadratic when the length of the plastic zone is not constant Published in Prikladnaya Mekhanika, Vol. 41, No. 12, pp. 116–127, December 2005.     

On shakedown theory for elastic-plastic materials and extensions

The idea that an elastic-plastic structure under given loading history may shake down to some purely elastic state (and hence to a safe state) after a finite amount of initial plastic deformation, can apply to many sophisticated material models with possible allowable changes of additional material characteristics, as has been done in the literature. Despite some claims to the contrary, it is shown; however, that the shakedown theorems in a Melan-Koiter path-independent sense have been extended successfully only for certain elastic-plastic hardening materials of practical significance. Shakedown of kinematic hardening material is determined by the ultimate and initial yield stresses, not the generally plastic deformation history-dependent hardening curve between. The initial yield stress is no longer the convenient one (corresponding to the plastic deformation at the level of 0.2%) as in usual elastic-plastic analysis but to be related to the shakedown safety requirement of the structure and should be as small as the fatigue limit for arbitrary high-cycle loading. Though the ultimate yield strength is well defined in the standard monotonic loading experiment, it also should be reduced to the so-called “high-cycle ratchetting” stress for the path-independent shakedown analysis. A reduced simple form of the shakedown kinematic theorem without time integrals is conjectured for general practical uses. Application of the theorem is illustrated by examples for a hollow cylinder, sphere, and a clamped disk, under variable (including quasiperiodic dynamic) pressure.     

Investigation on 3D fatigue crack propagation in surface-cracked specimens

In the present work the fatigue crack growth in AISI304 specimens is investigated experimentally. In 3D finite element analysis the virtual crack closure technique is applied to calculate distributions and variations of the stress intensity factor along the surface crack front. It is confirmed that the stress intensity factor along the surface crack front varies non-uniformly with crack growth. Crack growth rate is proportional to the stress intensity factor distribution in the 3D cracked specimen. The fatigue crack growth in surface cracked specimens can be described by the Forman model identified in conventional compact tension specimens. For crack growth in the free specimen surface the arc length seems more suitable to quantify crack progress. Geometry and loading configuration of the surface cracked specimen seem to not affect the fatigue crack growth substantially.     

三维结构安定分析的直接算法

基于线性随动强化理论和Von. Mises屈服准则,对蒙板结构直接安定分析法进行了扩展,建立了结构的三维安定直接分析法。根据投射原理,推导出结构发生塑性安定的存在条件,便于调整控制加载步长和载荷历程。采用逐次增量加载方式,确定出背应力的偏移范围,克服了原始直接分析法不能获得安定极限的缺陷,并得到安定极限条件下结构中残余应力与应变的分布状况。该数值方法将弹塑性问题分解为弹性问题和特征应变决定的残余问题,节约计算时间,提高计算效率,将该算法应用于相关算例,并与有关数值结果相比较,验证了该算法的有效性。     

A coupled kinetic-constitutive approach to the study of high temperature crack initiation in single crystal nickel-base superalloys

A rate-dependent crystallographic constitutive theory coupled with a mass diffusion model has been used to study crack initiation in single crystal nickel-base superalloys, exposed to an oxidising environment and subjected to mechanical loading. The time to crack initiation under constant load has been predicted using a strain-based failure criterion. A notched compact tension (CT) specimen containing a single casting defect, idealised as a cylindrical void close to the notch surface, has been studied. Finite element analysis of the CT specimen revealed that, due to the strong localisation of inelastic strain at the void, a microcrack will initiate in the vicinity of the void rather than at the notch surface. The numerical results have also shown that the time to crack initiation depends strongly on the void location. The coupled diffusion-deformation studies have revealed that environmental effects reduce the time to crack initiation due to the oxidation-induced material softening in the vicinity of the notch and void. The applicability of a failure assessment approach, based on the linear elastic stress intensity factor, K, to predict the crack initiation time under creep loading is examined and a probabilistic framework for prediction of component lifetime is proposed.     

Prediction of fatigue life improvement in natural rubber using configurational stress

To some extent, continua can no longer be considered as free of defects. Experimental observations on natural rubber revealed the existence of distributed microscopic defects which grow upon cyclic loading. However, these observations are not incorporated in the classical fatigue life predictors for rubber, i.e. the maximum principal stretch, the maximum principal stress and the strain energy. Recently, Verron et al. [Verron, E., Le Cam, J.B., Gornet, L., 2006. A multiaxial criterion for crack nucleation in rubber. Mech. Res. Commun. 33, 493–498] considered the configurational stress tensor to propose a fatigue life predictor for rubber which takes into account the presence of microscopic defects by considering that macroscopic crack nucleation can be seen as the result of the propagation of microscopic defects. For elastic materials, it predicts privileged regions of rubber parts in which macroscopic fatigue crack might appear. Here, we will address our interest to a broader context. Rubber is assumed to exhibit inelastic behavior, characterized by hysteresis, under fatigue loading conditions. The configurational mechanics-based predictor is modified to incorporate inelastic constitutive equations. Afterwards, it is used to predict fatigue life. The emphasis of the present work is laid on the prediction of the well-known fatigue life improvement in natural rubber under tension–tension cyclic loading.     

镍基单晶合金多轴非比例加载低周疲劳研究

基于正交设计, 分别在680℃和850℃下进行DD3镍基单晶合金薄壁圆管试样([001]取向)拉/扭非比例加载低周疲劳试验, 研究等效应变范围、应变路径角、拉/扭载荷相位角、循环特性和温度诸因素对镍基单晶合金多轴低周疲劳寿命的影响作用. 疲劳试验数据的极差分析表明, 应变路径角、拉/扭载荷相位角和等效应变范围是影响疲劳寿命的主要因素. 将菱形应变加载路径区分为比例加载段和非比例加载段, 提出了表征非比例加载效应的等效应变参量, 并通过引入单晶应变三轴性因子反映拉/扭应变路径角对多轴疲劳寿命的影响. 用考虑非比例加载效应的等效应变范围和单晶应变三轴性因子构造循环塑性应变能损伤参量, 进行多元线性回归分析, 疲劳寿命回归模型与试验寿命具有很好的相关性, 所有试验数据都落在2.0倍的偏差分布带之内.      

A direct method for laboratory spectrum crack growth testing

In linear elastic fracture mechanics (LEFM), the fatigue crack growth rates are related to the elastic stress intensity factor range and the load ratio. However, this relationship is difficult to derive when the loading condition is of variable amplitude (spectrum loading). Attention is paid to the most encountered case of cracks growing in their own plane under mode I loading. A testing procedure is presented to directly evaluate the fatigue life of real cracks under spectrum loading. The direct method intrinsically has wide applicability. A separate communication will present an application of the method.     

岩石巴西圆盘动态劈裂的流形元法模拟

利用二阶流形元法,通过引入裂纹产生与扩展判据,对冲击载荷作用下岩石平台巴西圆盘的动态拉伸劈裂过程进行了数值模拟,再现拉伸波作用下圆盘被劈裂的过程。模拟现象与实验结果相符,动态平衡时的应力分布与有限元结果基本一致。从而验证了流形元在模拟冲击载荷作用下材料动态破坏过程的有效性和可行性。     

应力波载荷作用下线弹性断裂过程的动态分析方法研究

利用Hopkinson单压杆实验装置 ,对材料的线弹性动态断裂特性进行了研究 ,建立了应力波载荷作用下动态裂纹起裂及扩展过程的动态分析方法 ,采用该方法可同时测得材料的动态裂纹起裂时间、断裂韧性和裂纹扩展速度。40Cr钢三点弯曲试样的实验结果表明 :该钢的动态裂纹扩展过程主要是减速过程 ,在2 2 5TPam /s的加载速率下 ,起裂时间为 2 8 0 0 s,最大裂纹扩展速度为 478 91m/s ,动态断裂韧性为6 3 12MPam。     

一种估算谱载疲劳裂纹起始寿命的方法

提出了一种估算谱载疲劳裂纹起始寿命的等效载荷法，该法在变幅载荷的均方根算式中引入加权因子和修正系数来分别反映不同载荷变程和平均应力对变幅疲劳寿命的影响，并用相应获得的等幅载荷取代变幅载荷来估算谱载下的疲劳裂纹起始寿命．该法仅依赖于材料的等幅S-N曲线和单轴力学性能，不含任何待定参数，使用方便；两种材料3种谱载下15组变幅疲劳试验数据的评估结果显示，该法的平均寿命预测精度分别较同类型的Miner法则、修正Miner法则、均方根法提高了99．1％，24．6％和50．0％。     

航空导管弯曲试验的疲劳裂纹萌生寿命

飞行器液压导管受接头和卡箍等约束,在使用的振动环境中,会因弯曲应力而导致破裂,影响到飞行安全．本文对飞行器液压系统通用的不锈钢导管的裂纹萌生寿命进行了试验研究．首先在对8 mm、12 mm 无缺陷导管和含U 型缺口8 mm 导管的疲劳试验和有限元分析的基础上,得到了导管的最大拉应变-裂纹萌生寿命数据．然后采用基于强度极限和弹性模量估算法的Manson-Coffin 公式来预测导管裂纹萌生寿命．最后引入加载类型修正系数、表面质量修正系数、试样尺寸修正系数、应力集中敏感系数和有效应力集中系数,使修正后的公式对三种类型的导管均有较好的裂纹萌生寿命预测精度．     

Low-endurance fatigue behavior under combined bending and torsion

Specimens made of thin-walled low-carbon steel tubes are subjected to combined bending and twisting moments of such magnitudes as to lead to fatigue failure within some one-thousand loading cycles. Tests are run, at room temperature, under conditions of controlled angle of twist and constant rate of loading. Experimental results show that the fatigue life is best expressed in terms of the amplitude of equivalent stress and ultimate strength of the material in potential form. For same maximum stress amplitude, test results show, within the scope of present investigations, that conditions of combined bending and torsion lead to lower fatigue life than those of reversed torsion.     

A Crack with an Electric Displacement Saturation Zone in an Electrostrictive Material

A crack with an electric displacement saturation zone in an electrostrictive material under purely electric loading is analyzed. A strip saturation model is here employed to investigate the effect of the electrical polarization saturation on electric fields and elastic fields. A closed form solution of electric fields and elastic fields for the crack with the strip saturation zone is obtained by using the complex function theory. It is found that the K _I -dominant region is very small compared to the strip saturation zone. The generalized Dugdale zone model is also employed in order to investigate the effect of the saturation zone shape on the stress intensity factor. Using the body force analogy, the stress intensity factor for the asymptotic problem of a crack with an elliptical saturation zone is evaluated numerically.     

干态下车轮材料表面疲劳裂纹萌生试验研究

利用WR-1轮轨滚动磨损试验机,结合安定极限理论研究了干态下影响车轮材料表面疲劳裂纹萌生与扩展的因素,探究了表面疲劳损伤形成机理和演变规律.结果表明:随垂向力、横向力和冲角增大,表面疲劳裂纹越容易萌生扩展;冲角对表面疲劳裂纹的萌生与扩展起着重要作用,大冲角下斜线状表面疲劳裂纹萌生扩展明显;只有横向力而不存在冲角时,试样表面不会出现斜线状表面疲劳损伤;车轮试样在周期性循环载荷作用下在表面先形成塑性流动,然后沿轮轨表面切向力方向扩展成斜线状的表面疲劳起皮剥落损伤;垂向力是影响表面裂纹萌生时间的重要因素之一.     

Modeling of fatigue crack growth from a notch

When a fatigue crack is nucleated and propagates into the vicinity of the notch, the crack growth rate is generally higher than that can be expected by using the stress intensity factor concept. The current study attempted to describe the crack growth at notches quantitatively with a detailed consideration of the cyclic plasticity of the material. An elastic–plastic finite element analysis was conducted to obtain the stress and strain histories of the notched component. A single multiaxial fatigue criterion was used to determine the crack initiation from the notch and the subsequent crack growth. Round compact specimens made of 1070 steel were subjected to Mode I cyclic loading with different R-ratios at room temperature. The approach developed was able to quantitatively capture the crack growth behavior near the notch. When the R-ratio was positive, the crack growth near a notch was mainly influenced by the plasticity created by the notch and the resulted fatigue damage during crack initiation. When the R-ratio was negative, the contact of the cracked surfaces during a part of a loading cycle reduced the cyclic plasticity of the material near the crack tip. The combined effect of notch plasticity and possible contact of cracked surface were responsible for the observed crack growth phenomenon near a notch.     

叠层橡胶隔震支座夹层受拉模型与分析：开裂机理与试验

为了研究叠层橡胶隔震支座的受拉性能及开裂破坏机理,本文基于叠层橡胶隔震支座橡胶夹层的受拉分析模型及橡胶夹层的应力与位移分布计算公式,分析不同形状系数的橡胶支座橡胶夹层的应力分布与变形规律（如橡胶层内外自由边的变形形状）,并基于理论分析结果,采用开裂能理论研究橡胶夹层的开裂能密度及梯度分布,据此分析橡胶夹层的开裂发生与发展规律,揭示其开裂破坏机理。最后通过叠层橡胶隔震支座的受拉性能试验进一步分析对比,试验结果表明了采用开裂能密度与梯度分析橡胶夹层的开裂破坏机理的可行性与合理性。     

基于非齐次复合Poisson过程的正交异性钢桥面板细节疲劳裂纹随机扩展研究

传统的正交异性钢桥面板疲劳损伤评估常采用确定性和可靠性分析方法,忽略了疲劳裂纹扩展的随机性影响,针对这一问题,提出钢桥面板细节疲劳随机扩展分析方法。本文以南溪长江大桥为工程背景,基于长期车辆荷载监测数据,建立了车辆荷载非齐次复合Poisson过程模型。建立钢桥面板有限元模型,采用瞬态分析方法将随机车辆荷载转化成细节疲劳应力,基于线弹性断裂力学理论推导U肋-顶板焊接细节疲劳裂纹扩展时变微分方程,实现宏观关系式疲劳应力幅次数-疲劳损伤至微观表达式应力时间序列-疲劳损伤转换,讨论了车载次序及超载对疲劳裂纹扩展的影响。研究结果表明,非齐次复合泊松过程模型能够较好描述随机车流运营状态,车辆荷载的次序对疲劳裂纹扩展速率的影响不可忽略,重车排序靠前时能够促使疲劳裂纹扩展增速,南溪长江大桥细节点的车辆超载迟滞效应修正系数取值0.804。