三轴应力状态下复合型裂纹尖端前缘的塑性区分布

利用Ｍｉｓｅｓ屈服准则从理论上分析了Ⅰ－Ⅱ复合型裂纹尖端前缘的塑性区分布。推导出了由三轴应力约束参数Ｔｚ参与表征的裂纹尖端前缘塑性区尺寸ｒｐ的表达式，并绘制出了Ⅰ－Ⅱ复合型裂纹在单轴、双轴载荷作用下裂纹尖端塑性区的分布图。     

Characterisation of crack tip stresses in elastic-perfectly plastic material under mode-I loading

Asymptotic crack tip stress fields are developed for a stationary plane strain crack in incompressible elastic-perfectly plastic material under mode-I loading. Detailed investigations have revealed that in between the two extreme conditions of crack tip constraint, that is, between the fully plastic Prandtl [1] field and the uniform stress field the most general elastic-plastic crack tip fields can be completely described by the 5-sector stress solution proposed in this article. The 3-sector stress field proposed by Li and Hancock [2] and the 4-sector field proposed by Zhu and Chao [3] are subsets of the general elastic-plastic field proposed in this work. This study has revealed that cases arise where the severe loss of crack tip constraint can lead to compressive yielding of crack flank. This particular situation leads to 5-sector stress field. Detailed studies have revealed that, in the most general case of elastic-plastic crack tip fields, the T_π parameter proposed by Zhu and Chao [3] cannot be used as a constraint parameter to represent a unique state of stress at the crack tip. A new constraint-indexing parameter T_CS-2 is proposed, which along with T_p is capable of representing the entire elastic-plastic crack tip stress fields over all angles around a crack tip. Excellent agreement is obtained between the proposed asymptotic crack tip stress field and the full-field finite element results for constraint levels ranging from high to low. It is demonstrated that the proposed constraint parameters are adequate to represent the crack tip constraint arising due to specimen geometry and loading conditions as well as the additional constraint that arises due to weld strength mismatch.     

小范围屈服条件下复合材料裂纹尖端塑性区分析

基于复合材料力学,推导Tsai-Hill强度准则在平面应力和平面应变条件下的一般表达式,得到了小范围屈服条件下,含中心裂纹无限大板Ⅰ型裂纹、Ⅱ型裂纹和Ⅰ/Ⅱ复合型裂纹尖端塑性区的解析解。针对不同裂纹倾角及泊松比 和,对裂尖塑性区进行了计算和分析。结果表明平面应变条件下塑性区范围小于平面应力条件下塑性区范围,参数、和 对复合材料裂尖塑性区范围和形状有明显的影响,不同的参数值得到的塑性区结果差别很大。另外,该解既适用于各向异性复合材料,也适用于各向同性材料。     

Analysis of a conducting crack in an electrostrictive ceramic under combined electric and mechanical loading

A conducting crack in an electrostrictive ceramic under combined electric and mechanical loading is investigated. Analysis based on linear dielectric model predicts that the surfaces of the crack are not open completely but they are contact near the crack tip. The complete solution for the crack with a contact zone in a linear electrostrictive ceramic under combined electric and mechanical loading is obtained by using the complex variable formula. The asymptotic problems for a semi-infinite crack with a partial opening zone as well as for a fully open semi-infinite crack in a nonlinear electrostrictive ceramic are analyzed in order to investigate the effect of the electrical nonlinearity on the stress intensity factor under small scale nonlinear conditions. Particular attention is devoted to a finite crack in the nonlinear electrostrictive ceramic subjected to combined electric and mechanical loading. The stress intensity factor for the finite crack under small scale nonlinear conditions is obtained from the asymptotic analysis.     

Numerical analysis for prediction of fatigue crack opening level

Finite element analysis (FEA) is the most popular numerical method to simulate plasticity-induced fatigue crack closure and can predict fatigue crack closure behavior. Finite element analysis under plane stress state using 4-node isoparametric elements is performed to investigate the detailed closure behavior of fatigue cracks and the numerical results are compared with experimental results. The mesh of constant size elements on the crack surface can not correctly predict the opening level for fatigue crack as shown in the previous works. The crack opening behavior for the size mesh with a linear change shows almost flat stress level after a crack tip has passed by the monotonic plastic zone. The prediction of crack opening level presents a good agreement with published experimental data regardless of stress ratios, which are using the mesh of the elements that are in proportion to the reversed plastic zone size considering the opening stress intensity factors. Numerical interpolation results of finite element analysis can precisely predict the crack opening level. This method shows a good agreement with the experimental data regardless of the stress ratios and kinds of materials.     

氢对裂纹尖端塑性变形及裂纹扩展影响的TEM观察

利用透射电镜原位动态拉伸方法观察、分析氢对裂纹尖端塑性变形及裂纹扩展过程的影响。结果表明，氢促进裂纹尖端的局部塑性变形，改变裂纹扩展方式，并使裂纹以“Ｚ”字型的途径扩展。     

对拉压超载后裂纹扩展迟滞模型的改进

对已有的裂纹扩展迟滞模型进行改进,合理地考虑压缩载荷对裂纹扩展的影响,特别是拉伸超载后紧随的压缩载荷对有效塑性区的影响,以及拉伸超载后紧随多个压缩载荷的处理方法.此外,改进门槛值随应力比R变化的计算公式,使计算结果与实际情况更接近,提高模型参数—门槛值的精度.最后,将改进后模型的预测寿命与试验结果进行比较.从对比结果可...     

高频谐振载荷作用下Ⅰ型疲劳裂纹尖端力学参数变化规律

为研究高频谐振式疲劳裂纹扩展试验中带有Ⅰ型预制裂纹的紧凑拉伸(CT)试件裂纹尖端力学参数的变化规律,利用动态有限元方法,采用ANSYS和MATLAB软件编写程序,计算了CT试件在高频恒幅正弦交变载荷作用下,在一个应力循环及裂纹扩展到不同长度时裂纹尖端区域的位移、应变场及裂纹尖端的应力强度因子,并分析了其变化规律。在计算裂纹尖端应力强度因子时,首先采用静态有限元方法和理论公式验证了有限元建模和计算的正确性,然后采用动态有限元方法研究了裂纹扩展过程中裂纹尖端应力强度因子的变化规律。最后进行了高频谐振式疲劳裂纹扩展试验,采用动态高精度应变仪测量了裂纹扩展到不同阶段时裂纹尖端点的应变,并对有限元计算结果进行了验证。研究结果表明：在稳态裂纹扩展阶段,高频谐振载荷作用下Ⅰ型疲劳裂纹尖端位移、应变及应力强度因子均为与载荷同一形式的交变量;随着裂纹的扩展,Ⅰ型疲劳裂纹尖端的位移、应变及应力强度因子幅不断增大;静态应力强度因子有限元计算值和理论值的误差为2.51%,裂纹尖端点应变有限元计算结果和试验结果最大误差为2.93% 。     

The effect of deformation anisotropy on the plastic zone at the tip of a crack under biaxial loading

A theoretical analysis for investigating the form and dimension of the plastic zone in the tip of a crack in a plate made from a material with deformation anisotropy is presented. The anisotropy is explained by the hardening process during plastic deformation caused by loading along a straight-line trajectory until a crack is formed in the plate plane. The idea of this method is to take into account the second term in the Williams’ series representation in eigenfunctions for stress components for the 2D case. The contribution of the second term in the Cartesian coordinate system is independent of the distance from the crack tip for the K ₁ − T concept. It is shown that for the case of anisotropy, the dimension of the plastic zone decreases. The stress along the crack’s axis changes the plastic zone significantly.     

基于新裂纹扩展驱动力参量的焊接接头疲劳裂纹扩展的研究

传统上疲劳裂纹扩展速率以一个参量——应力强度因子幅(PARIS模型)或有效应力强度因子幅(ELBER模型)来表达。PARIS模型不能统计应力比效应和变幅加载历史。ELBER裂纹闭合模型虽被广泛应用,但确定其开闭口载荷的测量方法很多,且测量结果均存在主观性。最近研究表明,疲劳裂纹扩展不仅依赖于应力强度因子幅,还与最大应力强度因子有关。并且KUJAWSKI提出了两参量模型,该模型避开了有争议的裂纹闭合效应。基于一个载荷循环中柔度变化与裂纹尖端开闭口与弹塑性行为的关系,提出一个新的具有物理意义的两参量驱动力模型。针对Q345钢焊接接头各区域进行两种应力比R=0.1和0.5的疲劳裂纹扩展试验。使用该模型针对Q345钢焊接接头各区域的疲劳裂纹扩展数据进行验证。结果表明,提出的新模型在预测应力比对裂纹扩展速率的影响时比上述三个模型更有效。     

复合型裂纹小范围屈服下裂尖塑性区统一解

采用俞茂宏统一强度理论,推导Ⅰ、Ⅱ复合型裂纹在小范围屈服条件下裂尖塑性区尺寸的统一解析解.给出材料参数在不同拉压比α、泊松比v和中间主应力影响参数b下的一族裂尖塑性区形状与大小的轨迹.讨论以上参数对裂尖塑性区变化的影响,其中拉压比α对塑性区影响较大,α≠1导致塑性区在裂纹上下表面处不连续,b=0和b=1分别对应裂尖塑性区的上限、下限边界.同Tresca准则、Mises准则的解进行比较分析,已有解均是它的特例或线性逼近,该理论解具有理论的统一性和对不同材料的普适性.     

一种基于裂尖损伤区的裂纹扩展模型

提出了“裂尖损伤区”的概念,通过解析闭合模型对裂尖损伤区的应力应变历程进行了分析,应用累积损伤理论建立了一个用于分析变幅载荷下疲劳裂纹扩展寿命的模型。应用本模型计算了不同载荷谱下的裂纹扩展寿命,通过与试验结果的比较,表明本文的方法具有精度高、经济实用的特点,适合工程应用。     

Prediction of fatigue regimes in notched components

Fatigue crack growth from notches is described as a two-stage process. Cracks are assumed to initiate in, and their early growth controlled by, the plastic zone due to the notch. Later growth is controlled by the developing crack tip plastic zone which can be described by the elastic stress intensity factor. A simple model is proposed which qualitatively accounts for all observed notch phenomena including non-propagating cracks and size effects. The equation: is shown to quantitatively predict the experimental fatigue failure limit of sharply notched specimens.     

循环压缩应力作用下的疲劳裂纹扩展机制研究

裂纹在循环压缩载荷作用下的扩展已经被实验证实，但是由于难以进行观测因而无法准确描述，另外裂纹面在压缩载荷的作用下会出现闭合现象也增加了问题的复杂性。文中通过有限元方法，建立三种几何条件的裂纹模型，考虑裂纹面接触问题，进而对循环压缩载荷作用下的裂纹萌生扩展进行分析。结果表明，裂尖区域在循环压缩载荷作用下的残余拉伸应力是导致裂纹扩展的重要因素。同时，还对不同裂尖几何在裂纹描述的合理性方面提出一些看法。研究表明，裂纹在循环压缩载荷作用下的扩展能力是有限的、稳定的。     

用弹塑性有限元法对焊接接头裂尖场J积分的研究

采用平面应力弹塑性有限元法研究了中心裂纹板焊接接头裂尖场J积分参量及其应用的可行性，数值分析采用MARC软件的二维弹塑性分析模型，探讨了不同强度匹配（高，等，低匹配）的焊接接头试样在加载过程中裂尖场J积分的变化情况，计算结果表明，靠近焊接接头裂纹尖端的J积分回路明显的路径相关性，而远离裂尖的J积分回路表现出路径无关性，焊接接头强度匹配因子M对裂纹尖端的J积分值有很大的影响，对应于每一个载荷P／P0，J积分的值随M的增大而减小，特别是当P／P0＞1．0时这种情况更明显。     

Charpy裂纹体的冲击响应与动态断裂韧度

采用动态弹塑性有限元法分析了Ｃｈａｒｐｙ裂纹试样在动态加载下的冲击响应。分析结果表明：在一般冲击实验机的加载速度下，惯性力对裂纹尖端的Ｊ积分影响不大。实际中，只要对示波冲击测得的载荷－时间曲线进行数字滤波处理，动态Ｊ积分可按准静态加载下的Ｊ积分计算公式计算。另外，实验表明：裂纹尖端应变片法可很好地测得冲击加载下裂纹的启裂点     

Fracture of notched and perforated polymeric bars produced by longitudinal impact

An experimental investigation was executed to ascertain the stress intensity factors at the tip of stationary or moving cracks and the stress distribution at central holes in thin rectangular specimens of polymethyl methacrylate (PMMA) and polycarbonate of bisphenol (Lexan) as the result of longitudinal impact by 10 mm dia. steel spheres at velocities ranging from 15 to 130 m/s and by the hammers of standard Charpy impact testers. The initial and rebound velocities of the spheres as well as the strain histories in the specimens were measured. Crack kinematics, the stress intensity factor at the tip and the nominal stress at central holes were ascertained by means of a shadowgraphic technique using a 24-spark Cranz-Schardin camera.Stepwide propagation in the PMMA specimens, frequently found for low sphere-impact velocities, occurred at crack speeds of 250–340 m/s, while catastrophic failure in a single pass of the crack involved tip speeds from 625 to 700 m/s. For the Lexan samples, the velocities of all cracks whether moving intermittently or continuously, were observed to be between 435 and 510 m/s; thus, the average of the fast crack speeds corresponds to about 31% of the rod wave velocity for both materials. Dynamic stress intensity factors were found to be equal to or larger than corresponding static values of fracture toughness, and their variation with nominal stress in the bar followed different mechanical-optical paths during loading and unloading. Stress histories at holes in PMMA determined from strain gage data indicated a stress concentration when compared to those obtained from the shadowgraphs. Current analyses of the process based on static stress distributions in an infinite elastic plate under constant load require modification to provide an explanation of some of the phenomena observed. Such dynamic stress conditions in the vicinity of a crack or hole will also require an alteration of the photomechanical relations currently employed to ascertain crack tip intensity factors and stress conditions at circular holes determined from the geometry of shadow-graphic patterns.     

裂纹闭合临界状态磁特性表征模型

针对工程实际中低周疲劳裂纹闭合状态难以判定与表征的问题,研究了基于磁特性参数的裂纹闭合临界状态表征规律及模型。以Q235B钢为实验材料,进行分级加载、卸载实验,采用TSC-5M-32金属磁记忆检测仪采集磁特性参数,研究了裂尖裂尾不同塑性区对磁场的影响,通过对比柔度微分法,确定裂纹闭合临界载荷来分析裂纹闭合临界状态磁特性参数ΔH_p变化规律。结果表明：载荷小于裂纹闭合临界点P_op时裂纹闭合,ΔH_p出现极小值拐点;载荷等于P_op时处于裂纹闭合临界状态,ΔH_p向上剧烈跳变出现转折点;载荷大于P_op时,裂纹完全张开,ΔH_p维持在较高的水平上下波动。最后在实验的基础上,建立了基于合成磁特性参数ΔH_p的裂纹闭合状态表征模型,验证结果表明,模型加载阶段和卸载阶段的模型预测值与实测值的最大相对误差分别为9.398%和11.549%,为工程实际中判断并表征裂纹闭合状态提供新的思路。     

复合材料补片参数对裂纹尖端应力强度因子的影响

利用有限元法对复合材料补片修补前后的铝合金薄板的裂纹尖端应力强度因子KI进行研究,分析各类补片参数对裂纹尖端应力强度因子的影响.结果表明,在正确选择复合材料补片的参数后,修补后铝合金板裂纹尖端的应力强度因子有显著地下降.     

Prediction of crack tip plasticity induced due to variation in solidification rate of weld pool and its effect on fatigue crack propagation rate (FCPR)

Effect of solidification rate on crack tip plastic zone size at various crack lengths was calculated analytically and numerically by simplified Sih’s and Irwin’s models, respectively. Influence of plastic zone size is explained in terms of COD and elastic stress intensity factor within valid range of small scale yielding approximation. Up to plastic zone size range of 4–5 mm, a good agreement between numerical and analytical plastic zone size and elastic stress intensity factor for all weldments was observed. For high loads and greater crack lengths, experimentally obtained COD values were found 15–19 % more than simulation ones due to rapidly induced plasticity at high crack dimensions. Solidification rate showed a significant influence on FCPR, for solidification rates 13.75 °C/s, 6.97 °C/s and 4.32 °C/s the obtained fatigue strength was 35.29 MPa, 36.26 MPa and 41.32 MPa, respectively.