复合材料中桥连问题的裂纹动力学模型

复合材料大多由纤维和基体组成,宏观是各向异性的。产生裂纹后,其纤维处形成“桥连”。至今人们研究大多是桥连的静力学问题[1]。本文建立一种桥连动力学模型,将桥连处用载荷代替,当裂纹扩展时,纤维也连续的开裂。文中首次求得了正交异性体中扩展裂纹受运动的集中力 作用问题的解析解,并利用这一解,通过迭加最终求得了该模型的解。     

基于比例型Paris公式和逆高斯过程的金属疲劳裂纹扩展随机模型

疲劳失效是金属构件的主要失效方式之一,该文针对金属疲劳裂纹扩展过程中的不确定性,以“首次达到给定裂纹长度a的时间t(a)”为随机描述量,采用比例型Paris公式描述裂纹的平均扩展路径,建立基于逆高斯过程的单样本疲劳裂纹扩展随机模型和考虑样本异质性的裂纹扩展随机效应模型,分别采用最大似然估计法(MLE)和最大期望算法(EM)推导了单样本模型和随机效应模型的参数估计公式。最后,利用提出的裂纹扩展随机模型拟合了68个铝合金板的疲劳裂纹数据,对结果进行了拟合优度分析。结果表明:该文提出的疲劳裂纹扩展随机模型能够有效地分析和解释金属疲劳裂纹扩展过程中的不确定性。     

随机荷载下疲劳裂纹扩展寿命的统计模型

根据断裂力学和随机过程理论,提出了一个随机荷载作用下疲劳裂纹扩展的统计模型。在基于应变能密度因子变程的确定性疲劳裂纹扩展速率公式中引入材料内在的分散性和外部荷载的随机性,将疲劳裂纹扩展近似为连续型马尔可夫过程。应用随机平均法导出了裂纹扩展过程转移概率满足的向后Fokker-Planck方程,并得出相应的边界条件。采用本征函数法进行求解,以收敛的无穷级数表示出疲劳裂纹扩展寿命的分布函数。作为一个算例,具体计算出疲劳裂纹扩展寿命的分布密度曲线。     

正交异性钢桥面板疲劳裂纹扩展机理及数值模拟研究

为追踪正交异性钢桥面板的疲劳裂纹扩展过程及通用的正交异性板钢桥抗疲劳设计与开裂加固提供理论指导,提出基于实桥有限元模型进行正交异性钢桥面板疲劳裂纹扩展模拟方法及流程。建立桥梁整体有限元模型进行恒活载作用下整体分析,结合实桥调查结果确定全桥疲劳关键部位;建立含焊接细节的疲劳关键部位精细化模型进行疲劳应力幅分析,并基于车桥耦合振动分析考虑冲击系数对应力幅影响,确定裂纹扩展方向、路径及寿命,进行疲劳裂纹扩展全过程分析;以既有大跨度斜拉桥正交异性桥面板疲劳裂纹扩展分析为例,验证该方法与计算流程的可行性、准确性,并为该桥运营期疲劳失效维修、加固提供理论依据。     

42CrMo钢疲劳裂纹扩展剩余寿命评估

以高频三点弯曲疲劳试验机为平台,进行42CrMo钢疲劳裂纹扩展试验研究,通过建立裂纹扩展剩余寿命评估模型,实现对存在裂纹的工程机械零部件剩余寿命的评估。采用显微成像测试系统实时采集并测量疲劳扩展裂纹,使用声发射系统监测整个疲劳裂纹扩展过程。结果表明:声发射幅值、能量等特征参数可以实时反应疲劳裂纹萌生、稳定扩展和失稳扩展等各个损伤阶段,并在疲劳断裂时产生急剧的突变;裂纹扩展速率的对数值与应力强度因子幅的对数值具有较高的线性相关性,建立了不同应力工况条件下裂纹扩展剩余寿命评估模型,以双排链轮轴为例进行裂纹扩展剩余寿命评估;随着疲劳应力的增加,裂纹扩展剩余寿命减小。     

任意分布参数的涡轮盘裂纹扩展寿命可靠性分析

讨论了某型涡轮盘随机参数服从任意分布时的可靠性问题.在基本随机参数前四阶矩已知的情况下,以Pairs-Erdogan裂纹扩展模型为基础,应用随机摄动理论和Edgeworth 级数技术,采用疲劳寿命模型对某型涡轮盘随机参数服从任意分布时的可靠性进行分析,建立了涡轮盘疲劳寿命可靠性分析模型,并求得了涡轮盘裂纹扩展寿命的可靠度.模型计算结果与Monte-Carlo仿真结果非常接近,文章提出的方法对涡轮盘可靠性设计具有一定的参考价值.     

变幅载荷下纤维金属层板的疲劳与寿命预测

文章建立了纤维金属层板等幅疲劳载荷下的疲劳裂纹扩展速率与寿命预测模型。在此基础上对玻璃纤维-铝合金层板（GLARE）的疲劳裂纹扩展与分层扩展行为进行了试验研究,探讨了层板过载疲劳行为的机理,提出了纤维金属层板变幅载荷下疲劳寿命预测的等效裂纹闭合模型,并在GLARE层板上得到了验证。     

疲劳载荷下工业纯铁表面与内部裂纹长度的研究初探

疲劳裂纹扩展试验中,采用表面裂纹长度作为疲劳裂纹长度进行疲劳裂纹扩展计算和疲劳裂纹扩展模型建立的过程中,会导致计算的最终结果和模型产生一定的误差。对工业纯铁板材试样进行疲劳裂纹扩展试验,分析疲劳裂纹扩展过程中裂纹长度与裂纹扩展速率的关系,以及对疲劳裂纹扩展断口形貌特征进行观察。结果表明,上述2种方法均可来确定表面裂纹长度和内部裂纹长度之间的关系。     

镍基合金超声疲劳裂纹扩展寿命预测研究

针对发动机结构材料承受高频循环载荷的特点,应用超声疲劳试验技术开展了镍基合金材料的疲劳裂纹扩展试验研究。考虑高频载荷下疲劳裂纹扩展过程中的温升效应,测试了超声疲劳裂纹扩展过程中的温度变化,基于温度变化对材料弹性模量的影响和热膨胀效应,数值计算了疲劳裂纹扩展应力强度因子。研究了温度变化对超声疲劳裂纹扩展的影响机制,并在现有模型基础上,建立了考虑温度影响的超声疲劳裂纹扩展模型,完善疲劳裂纹扩展寿命预测方法。     

金属疲劳裂纹扩展规律及其微观机制(中)

二、疲劳裂纹扩展的力学方程 远在本世纪三十年代中期,人们已认识到疲劳裂纹是工程设计中的一个严重问题。Deforest等为了确定疲劳裂纹出现后金属的剩余寿命,首次开展了一项探索裂纹扩展的研究工作。此后,人们对疲劳裂纹扩展过程进行了大量的试验与理论分析,建立一些半经验式和解析式力学方程,有的已应用于零件与结构的实际寿命的估算中。 1.早期的裂纹扩展公式 表3列出了一些早期的疲劳裂纹扩展的半经验公式。这些公式在一定试验条件下是     

Stochastic analysis of fatigue crack growth for metallic structures

In order to estimate the statistical variability of fatigue crack growth in metallic structures, a stochastic model is proposed by combining stochastic theory with experimental results. A stochastic differential equation is derived from the stochastic model for fatigue crack growth. By using the solution of the stochastic differential equation, some distribution functions related to fatigue crack growth were derived. Sample functions of fatigue crack growth time histories have been simulated as random processes.     

A study of stochastic fatigue crack growth modeling through experimental data

To capture the statistical nature of fatigue crack growth, many stochastic models have been proposed in the literature. These models may have been verified by only one data set, and therefore not appreciated by other fellow researchers. Part of the reason is the difficulty and time-consuming in obtaining the statistically meaningful fatigue crack growth data. In the present study, experimental work is carried out to obtain the fatigue crack growth data of a batch of 2024-T351 aluminum alloy specimens. A rather universal stochastic fatigue crack growth model proposed by Yang and Manning is employed to analyze the data. The solution of the stochastic differential equation associated with the stochastic model gives us the crack exceedance probability as well as the probability of random time to reach a specified crack size. Through comparison between the analytical and experimental results, it is found the model with a minor modification can fit the experimental data rather well. Once the appropriate stochastic model is established, it can be used for the fatigue reliability prediction of structures made of the tested material. In the present study, in particular, it can be used for the reliability assessment of aging aircraft made of 2024-T351 aluminum alloy.     

First passage failure of SDOF nonlinear oscillator with lightly fractional derivative damping under real noise excitations

The first passage failure of single-degree-of-freedom (SDOF) nonlinear oscillator with lightly fractional derivative damping under real noise excitations is investigated in this paper. First, the system state is approximately represented by one-dimensional time-homogeneous diffusive Markov process of amplitude through stochastic averaging. Then, the backward Kolmogorov equation governing the conditional reliability function and the Pontryagin equation governing the conditional mean of first passage time are established from the averaged Itô equation for Hamiltonian. The conditional reliability function, the conditional probability density and mean of the first passage time are obtained by solving these equations together with suitable initial condition and boundary conditions. Finally, two examples are worked out in detail and the analytical solutions are checked by those from the Monte Carlo simulation of original systems.     

A semi-Markovian model for low-cycle elastic-plastic fatigue crack growth

Elastic-plastic fatigue crack growth is described by a continuous time semi-Markovian stochastic process. The load on the member is considered to be deterministic and the resistance is assumed to be random. Crack growth resistance is described by an equation employing the J-integral. Quantities of interest that can be obtained from this model include emergency statistics and probability distributions on the time to reach a critical crack length. The model is tested with fatigue crack propagation data for A533B pressure vessel steel. The results indicate that the model underpredicts the mean first-passage time at the start of a test but predicts it well in the mid-ranges and towards the end of the test; the variance of the process is better predicted for specimens that undergo large plasticity than those with low plasticity.     

Random hydrogen-assisted fatigue crack growth in steel plates

A stochastic analysis of hydrogen-assisted fatigue crack growth in steel plates is presented. First, a simplified deterministic model of the process is proposed. It captures the basic empirical property that the influence of hydrogen diminishes, as the crack growth rate increases. However, it only applies to cases, when diffusion is rate limiting. Next, the model parameters are randomized to reflect the uncertainty inherent in the physical situation. On the basis of the obtained stochastic equation, probabilistic moments of the time, in which the crack reaches a critical length, are computed. Theoretical results are illustrated by a numerical example.     

FORMULATION OF A STOCHASTIC MODEL OF FATIGUE CRACK GROWTH

Based on a global/local energy balance a deterministic model of fatigue crack growth under constant amplitude loading is derived. The energy terms resulting from the continuous plasticity and localized fracture around the crack tip are determined for small scale yielding leading to the fatigue crack growth equation involving the stress intensity factor and its amplitude. Four material parameters which should be identified from experimental data have a physical interpretation; these are eventually assumed to be random variables and model the statistical scatter of the crack growth versus N curves observed in experiments. A Gaussian white noise random field is additionally assumed to describe the stochastic material non-homogeneity within a specimen. Its effect on the crack growth is derived and results in a positive non-stationary random function depending on the crack length. Statistical parameters of the random fields are identified. Verification of the model by comparison with experimental results is undertaken in a subsequent paper.     

THE RANDOMNESS OF FATIGUE CRACK GROWTH UNDER CONSTANT-AMPLITUDE LOADS

An analysis of fatigue crack growth from an statistical point of view has been carried out. Eighteen pre-cracked specimens obtained from the same sheet of aluminium alloy were subjected to identical load and environmental conditions. Tests were conducted under constant-amplitude loads in order to isolate the effect of material randomness. The experimental results exhibited two different types of dispersion: one showed as a change in the mean growth rate between tests and the other as nonuniform growth in the same specimen. At a subsequent stage, the statistical distribution of the experimental results was studied and a theoretical model was developed to account for the growth pattern observed. The proposed model uses a growth law comprising random parameters to account for the low-frequency component (slow changes). The comparison of the performance of two different laws, viz. the Paris-Erdogan law and the cubic law, is presented. Both were tested on the assumption of randomness in two of their fitting parameters. In addition to the above-mentioned law, the model uses a stochastic log-normal process to model the high-frequency component (rapid changes). The parameters for this process were determined by time series analysis of fatigue crack growth rate data.     

Advanced beam model for fiber-bridging in unidirectional composite double-cantilever beam specimens

This work investigates the interlaminar fracture and fiber-bridging in double-cantilever beam specimens from the theoretical and experimental points of view. Crack initiation and propagation tests were performed on unidirectional E-glass/polyester double-cantilever beam specimens. The well-known classical beam theory-based solution agrees excellently with the experimental results in the case of crack initiation tests. In contrast, the classical model seems to be inadequate for the evaluation of the propagation test data. The apparent contradiction was attributed to the fiber-bridging phenomenon. Thus, a novel beam model was developed, which accounts for the effect of fiber-bridging. Based on the solution of beam theory, the number of the bridging fibers and the bridging force can be approximated. The former reaches a peak value and decreases notably, while the force tends to a plateau value as the crack grows.