金属材料疲劳裂纹扩展速率Paris模型中材料常数的相关性

金属材料疲劳裂纹扩展速率是工程应用中进行损伤容限设计、疲劳寿命评估的一项重要力学性能指标。Paris模型是疲劳裂纹扩展速率最常用的表达形式,该模型认为疲劳裂纹扩展速率随裂纹前缘应力强度因子范围的变化呈幂函数关系,涉及到两个材料常数C和m。本研究借助部分文献中的试验数据,分析合金钢、铜合金、钛合金、铝合金等不同金属材料疲劳裂纹扩展速率Paris模型中材料常数C和m的关系。结果表明:Paris模型中材料常数C和m的关系可用m=alnC+b线性模型来描述,且不受试样取样方向、焊缝位置、试验环境等因素影响;但不同应力比对该模型的线性度有一定影响,尤其是当应力比为负值时,影响较为显著;不同金属材料常数C和m线性模型的斜率并不相等,其影响因素有待于进一步系统研究。分析结果可为金属材料在工程中的疲劳设计与应用提供参考。     

金属材料疲劳裂纹扩展曲线的拟合方法研究

提出了一种金属材料疲劳裂纹扩展曲线(a～N曲线)的拟合方法,即修正的双曲函数拟合方法。为检验这种曲线拟合方法的适用性,利用2024-T42铝合金CCT试样的疲劳裂纹扩展试验数据,对该铝合金的疲劳裂纹扩展曲线进行了拟合分析,并给出了疲劳裂纹扩展速率和扩展寿命。通过疲劳裂纹扩展寿命计算结果和试验结果的对比分析,证明这种拟合方法具有较高的拟合精度。     

一种悬臂弯曲加载表面裂纹扩展试样及其应用

介绍了一种带有上下对称圆弧的板状试样,试样上圆弧几何中心预制有半椭圆表面裂纹,这种设计能够较好地模拟压力容器具有表面裂纹时的特点.利用这种试样,可以完成悬臂弯曲加载恒总应变控制下的低周疲劳表面裂纹扩展速率试验,获得表面裂纹扩展长度和深度的关系,最终建立表面和深度裂纹扩展速率与总应变的函数关系.据此,可为含有表面裂纹体的压力容器低周疲劳剩余使用寿命评估提供依据.     

金属材料疲劳裂纹扩展速率不同模型应用分析

金属材料疲劳裂纹扩展速率是对金属材料及其零构件含有裂纹体后安全服役评价及疲劳寿命评估的一项重要力学性能指标。基于金属材料疲劳裂纹扩展过程的不同阶段、闭合效应、应力比对疲劳裂纹扩展速率的影响等多个方面,对几种常见金属材料疲劳裂纹扩展速率模型的应用特点进行了对比分析,为安全设计及寿命评估时不同模型的选择应用提供参考。     

金属材料疲劳小裂纹扩展速率试验方法编制说明

概述了疲劳小裂纹试验方法航标的编制背景,论述了航标中几个主要技术问题,如,小裂纹试样和小裂纹监测方法的选择,三维应力强度因子K的确定,小裂纹数据有效性的判据及物理小裂纹门槛值,并且与ASTME647－9a方法中附录X3进行了比较。     

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。     

疲劳裂纹测试的裂纹扩展片技术

提出了一种间接直流电位技术——裂纹扩展片技术,用于测量疲劳裂纹长度。该技术具有测量精度高、操作简单、可实现数据自动采集处理和测试设备成本较低等优点,并可用于绝缘材料裂纹和表面裂纹测量,是一种较有前途的裂纹测试技术。     

无网格法模拟复合型疲劳裂纹的扩展

本文提出了用无网格Galerkin法模拟构件在复合变形作用下疲劳裂纹扩展路径并预估其疲劳寿命的方法。该法能够自然模拟疲劳裂纹的扩展,不需要网格重构,避免了裂纹扩展过程中的精度受损。应用无网格数值结果计算了J积分和应力强度因子K_I和K_Ⅱ;按照最大周向应力理论获得了裂纹扩展偏斜角。基于最小应变能密度因子理论,确定了裂纹扩展量Δa,并能获得疲劳载荷的循环周数ΔN。文末对数值模拟结果和实验拟合结果进行了对照。     

裂纹闭合与疲劳裂纹扩展力的测试方法

介绍了一种在近十年里发展的裂纹扩展力测量方法，这种方法将裂纹扩展的门槛值行为与直流电位法结合起来，可以测量某一载荷下裂纹开始扩展的应力强度因子Ｋｐｒ，并由此计算有效应力强度因子范围ΔＫｅｆｆ＝Ｋｍａｘ－Ｋｐｒ。     

Automatic 3-D mode I crack propagation calculations with finite elements

An algorithm is presented which allows for fully automatic linear elastic low cycle fatigue (LCF) crack propagation calculations of mode I plane cracks in large structures by means of the finite element technique. The bulk of the algorithm consists of an automatic procedure to introduce the geometry of a plane crack with an arbitrary crack front in an existing three-dimensional (3-D) mesh. Once the K-distribution for the initial crack has been calculated, the use of the superelement technique reduces the computing time for the subsequent cycles by a factor of up to 40 or more. Two industrial examples illustrate the accuracy and effectiveness of the method. © 1998 John Wiley & Sons, Ltd.     

On the mechanism of fatigue crack propagation in ductile metallic materials

An overview of our research performed during the last 15 years is presented to improve the understanding of fatigue crack propagation mechanisms. The focus is devoted to ductile metals and the material separation process at low and intermedial crack propagation rates. The effect of environment, short cracks, small‐scale yielding as well as large‐scale yielding are considered. It will be shown that the dominant intrinsic propagation mechanism in ductile metallic materials is the formation of new surface due to blunting and the re‐sharpening during unloading. This process is affected by the environment, however, not by the length of the crack and it is independent of large‐ or small‐scale yielding.     

Modeling fatigue crack and spalling for rolling die under hot milling

In hot milling process, rolling die is subjected to nonsteady conditions which can rise the combinations of fatigue and spalling damage mechanism. An understanding about the failure mechanism of the rolling die is essential under hot rolling process. Fatigue crack growth and spalling process are governed by highly concentrated strain and stress in the crack tip region. Based on the theory of elastic‐plastic fracture mechanics, an analytical model are presented in this paper to determine the elliptical crack growth rate and spalling damage mechanism. The model includes new proposed constitutive equations for fatigue and spalling crack growth. To verify the models, finite element simulation and experimental data are considered. The results show good agreement with finite element simulation and experimental data.     

A new testing method to obtain mode II fatigue crack growth characteristics of hard materials

Flaking type failure in rolling‐contact processes is usually attributed to fatigue‐induced subsurface shearing stress caused by the contact loading. Assuming such crack growth is due to mode II loading and that mode I growth is suppressed due to the compressive stress field arising from the contact stress, we developed a new testing apparatus for mode II fatigue crack growth. Although the apparatus is, as a former apparatus was, based on the principle that the static K_I mode and the compressive stress parallel to the pre‐crack are superimposed on the mode II loading system, we employ direct loading in the new apparatus. Instead of the simple four‐point‐shear‐loading system used in the former apparatus, a new device for the application of a compressive stress parallel to the pre‐crack has been developed. Due to these alterations, mode II cyclic loading tests for hard steels have become possible for arbitrary stress ratios, including fully reversed loading (R=?1); which is the case of rolling‐contact fatigue. The test results obtained using the newly developed apparatus on specimens made from bearing steel SUJ2 and also a 0.75% carbon steel, are shown.     

Can pure mode III fatigue loading contribute to crack propagation in metallic materials?

The possibility of pure mode III crack growth is analysed on the background of theoretical and experimental results obtained in the last 20 years. Unlike for modes I and II, there is no plausible micromechanistic model explaining a pure mode III crack growth in ductile metals. In order to realize 'plain' mode III fracture surface, we propose the propagation of a series of pure mode II cracks along the crack front. Fractographical observations on crack initiation and propagation in a low alloy steel under cyclic torsion support such a model. The authors have not seen any clear indication of a pure mode III crack growth micromechanism in metals until now.     

疲劳裂纹扩展预测模型及其应用

在分析了灰色预测方法和支持向量机各自的优缺点基础上,提出了将二者相结合的一种新的预测模型———灰色支持向量机裂纹扩展预测模型.新模型发挥了灰色预测方法中"累加生成"的优点,弱化了原始序列中随机扰动因素的影响,增强了数据的规律性,同时避免了灰色预测方法及模型存在的理论缺陷.工程实例表明,文章所提出的裂纹扩展预测模型较传统的GM(1,1)模型、等维GM(1,1)模型精度都有所提高,为预测疲劳裂纹扩展提供了一种新的方法.     

Simulation of crack growth in composite material shell structures

This paper implements a domain integral energy method for modelling crack growth in composite material shell structures using the finite element method. Volume integral expressions to evaluate the dynamic energy release rate in a through‐thickness three‐dimensional crack are derived. Using the domain integral, the energy release rate computation is implemented in the DYNA3D explicit non‐linear dynamic finite element analysis program wherein crack propagation is modelled by releasing the constraints between initially constrained node pairs. The implementation enables the program to either determine the energy resistance response for the material (provided experimental data is available) or predict the rate of crack propagation in shell structures. The numerical implementation was verified by simulating mode I and mode III slow crack growth problems in semi‐infinite transversely isotropic media, for which analytic solutions are available. Oscillations of energy following the release of nodal constraints as the crack propagates in discrete increments were suppressed using light mass proportional damping and a moving averaging scheme. Copyright © 2004 John Wiley & Sons, Ltd.     

Simulation of dynamic crack curving and branching under biaxial loading by a time domain boundary integral equation method

Bifurcation and trifurcation of a fast running crack under various biaxial loading conditions is investigated numerically. The solution procedure for the 2D model in the framework of linear elastodynamics employs a time-domain boundary element method and allows for arbitrary curvilinear crack propagation. Branching events are controlled by the criterion of a critical mode I stress intensity factor while the propagation direction and growth rate of each branch are determined from the criterion of maximum circumferential stress. Numerical results are compared with experimental findings and are discussed with respect to macroscopic and microscopic aspects of dynamic fracture.     

Simulation of stage I-crack growth using a hybrid boundary element technique

Short fatigue crack propagation often determines the service life of cyclically loaded components and is highly influenced by microstructural features such as grain boundaries. A two-dimensional model to simulate the growth of these stage I-cracks is presented. Cracks are discretised by displacement discontinuity boundary elements and the direct boundary element method is used to mesh the grain boundaries. A superposition procedure couples these different boundary element methods to employ them in one model. Varying elastic properties of the grains are considered and their influence on short crack propagation is studied. A change in crack tip slide displacement determining short crack propagation is observed as well as an influence on the crack path.