镍基合金材料塑性对应力腐蚀裂纹尖端应力应变场影响的研究

为了解材料塑性对高温水环境下核电关键结构材料应力腐蚀裂纹扩展的影响,利用大型非线性有限元软件,对镍基合金中由氧化膜和基体金属构成的应力腐蚀裂纹尖端应力应变场进行了分析,得出了材料屈服应力对应力腐蚀裂纹尖端应力应变场的影响规律.结果表明:在同样的外载条件下,随着材料屈服应力的增大,裂尖基体金属的应力增大而塑性应变减小;而...     

基于材料低周疲劳的裂纹扩展预测模型

该文基于疲劳裂纹尖端循环应力-应变场, 定义了基于应变幅的平均单位循环损伤参量D, 并引入Miner累积损伤率, 从而从理论上建立起材料低周疲劳性能和疲劳裂纹扩展行为之间的联系。以裂尖扩展方向上的单调塑性区尺寸作为疲劳过程区大小, 并提出了基于弹塑性应变疲劳累积损伤的疲劳裂纹扩展预测模型。模型改进了前人提出的疲劳裂纹扩展预测模型, 考虑了单调塑性区内所有材料的弹塑性应变疲劳损伤贡献;模型中参数均有物理意义, 不需要人为调试。基于完成的Cr2Ni2MoV 材料的低周疲劳结果所建立的该文新模型对该材料裂纹扩展速率的预测结果与实验结果有良好一致性。并且, 借助手册数据, 在TC4钛合金材料上进一步得到了验证。     

A537钢在盐水中疲劳裂纹尖端塑性区尺寸减小与声发射

研究了Ａ５３７低强度结构钢在空气和３．５％ＮａＣｌ中性水溶液中疲劳裂纹扩展及其相应的声发射规律。结果表明腐蚀疲劳裂纹扩展过程中声发射活动下降。由于裂纹尖端塑性区内的位错运动是疲劳与腐蚀疲劳裂纹扩展的主要声发射源，所以认为声发射活动性下降是氢使尖塑性尺寸减小所致。用激光散斑干涉法裂纹尖端应变分布，证明氢的存在使裂尖塑性区尺寸减小，氢的存在同提高裂尖局部材料的流变应力。     

拉—拉疲劳下15MnMoVN多裂纹扩展研究

为分析单裂纹或多裂纹在裂纹面承受疲劳拉伸载荷作用下尖端应力强度因子变化规律和裂纹形貌变化以及疲劳寿命情况,以含不同初始长深比的半椭圆单裂纹或双裂纹的薄片试样为研究对象,对试样在应力比R=0.1的疲劳拉伸载荷下单裂纹或双裂纹情况进行了仿真分析。建立含裂纹试样的有限元模型,仿真分析了裂纹在扩展过程中尖端应力强度因子的分布情况,并将单裂纹扩展结果与双裂纹相互作用影响下的结果进行了对比研究;进行含裂纹试样的疲劳实验,分析了含单裂纹或双裂纹的试样的断裂面的形成原因,并验证仿真结果正确性。结果表明,裂纹面之间的相互作用会逐渐影响裂纹的扩展方向、扩展速率以及在扩展过程中尖端应力强度因子的变化趋势;而且初始形貌为半椭圆形的双裂纹在相互作用影响下会逐渐过渡到半圆形。     

内嵌弹性区和结合力模型在薄膜/基底界面断裂中的应用

薄膜/基底结构是微电子学和材料科学中广泛应用的典型结构。由于加工工艺中材料力学、热学性能失配等原因导致的薄膜中出现的残余应力,是界面裂纹的萌生和扩展的重要原因。采用三参数(Γ₀, /σ_y,t)的修正的断裂过程区结合力模型,讨论了在塑性氛围下裂尖解理断裂的过程,裂尖应力分布,裂尖形貌和表征裂纹尖端断裂过程区特征参数对断裂过程的影响,并应用到均质金属薄膜/陶瓷基底结构中残余应力导致界面裂纹起裂和扩展的全过程分析中。     

动态断裂韧性实验中DIC技术应用研究

高速冲击动态断裂韧性的加载和测试技术一直是近年关注的热点,随着计算机和光学传感器的发展,采用数字图像相关方法测量材料的动态断裂韧性已成为重要选择。该文基于分离式Hopkinson压杆原理的加载技术,通过高速摄影机拍摄高速冲击下三点弯曲试样裂纹的起裂和扩展,运用数字图像相关技术分析裂尖场的散斑图像,计算得到相应的应变场变化,试样外表面处于平面应力状态,其裂尖应变场呈现"0"型,而非试样对称面所处于的平面应力状态下呈"8"型。结果表明,DIC技术可以应用于动态断裂韧性实验,也证明裂尖场粘贴的应变片测量试样起裂的有效性。     

单晶γ-TiAl合金中裂纹沿[111]晶向扩展的分子动力学研究

为了从微观角度探索γ-TiAl合金中特定晶向的裂纹扩展机理,研究了γ-TiAl合金中[111]晶向微裂纹扩展的过程及其断裂机理。首先在单晶γ-TiAl合金中预置[111]晶向的微裂纹,然后通过分子动力学方法模拟该裂纹的扩展过程,最终分析了裂尖原子组态变化、微裂纹扩展路径以及应力-应变情况。研究表明,该晶向的微裂纹不是沿直线扩展,而是启裂时裂尖发生偏转,表现出明显的取向效应;微裂纹以裂尖发射滑移位错以及裂尖上形成孪晶的方式进行扩展;受边界的影响,微裂纹扩展到一定阶段会在边界位错堆积处萌生子裂纹,且扩展机制与主裂纹类似;在两个裂纹尖端发射滑移位错的相互作用下,在主裂尖前端再次萌生子裂纹,最终主、子裂纹相连导致断裂;微裂纹扩展过程中的应力分布主要集中于裂尖和扩展过程中形成的孪晶面上,并且随着微裂纹的扩展,裂尖应力值随时间的增大而减小。     

压电材料中渗透性周期共线反平面裂纹问题

本文利用复变函数方法,借助于Riemann-Schwarz延拓技术和保形映照方法,研究了渗透性边界条件下周期共线反平面裂纹问题,获得了解的表达式,得到了力学和电学强度因子。结果表明在裂纹尖端应力和电位移的奇异性都与远场作用的应力载荷和裂纹长度有关,其中应力的奇异性与材料无关,电位移的奇异性则与材料有关,电载荷对裂尖的奇异性没有影响。最后,运用数值算例,给出周期裂纹间的干涉效应和裂纹的尺度效应。     

Fe—3%Si 单晶体裂纹尖端位错行为及氢效应的 TEM 动态观察~*

在透射电镜中对 Fe-3%Si 单晶体薄膜进行拉伸试验,观察了在受力状态下裂纹尖端的位错行为及固溶氢的影响。结果表明,在受力时裂尖会自动发射位错。位错一旦从裂尖发射,便很快离开裂纹尖端以反塞积群形式存在于塑性区中。在裂纹尖端与塑性区之间存在一无位错区(DFZ)。裂纹扩展包括裂尖发射位错的塑性过程以及 DFZ 解理的脆性过程。固溶的氢改变了裂纹尖端的位错分布,促进了位错从裂尖发射,致使 DFZ 的长度减小甚至消失。     

定向断裂双孔爆破含缺陷介质裂纹扩展的动焦散试验

利用爆炸加载数字激光动态焦散线试验系统,进行双孔爆破爆炸应力波作用下缺陷介质裂纹扩展试验。研究了含水平预制裂纹和竖直预制裂纹的介质裂纹扩展路径、速度、加速度和裂尖动态应力强度因子变化规律。试验结果表明:在爆炸应力波作用下,预制裂纹尖端起裂,并扩展。炸药爆炸后,主裂纹的扩展速度迅速达到峰值,之后开始振荡减小,其加速度呈现波浪起伏式的振荡变化。次裂纹起裂后速度增大至峰值,然后开始减小。主裂纹尖端的动态应力强度因子K_Ⅰ从峰值振荡减小,又振荡增加至第二个峰值,之后振荡减小。次裂纹尖端的动态应力强度因子K_Ⅰ达到最大时,次裂纹起裂,之后K_Ⅰ振荡减小。裂纹扩展的过程中K_Ⅱ基本都小于K_Ⅰ。     

Plastic intensity factors for cracked plates

An elastic-plastic analysis is performed for two problems relevant to fracture mechanics: a semiinfinite body with an edge crack in a far out-of-plane shearing field and an infinite plate under plane stress conditions containing a finite line crack in a remote tensile field. Amplitudes of the dominant singularity in the plastic region at the crack tip, the plastic stress and strain intensity factors, are calculated for applied stress levels approaching the yield stress. A technique is developed for using the dominant singular solution in conjunction with the finite element method to make accurate calculations for the near-tip fields. Additionally, a comparative study of deformation theory with flow theory is performed for cracks in an anti-plane shear field. Elastic fracture mechanics is extended to high levels of applied stress for which the plastic zone is no longer small compared to the crack length by relating the critical stress for fracture initiation to the plastic intensity factors.     

Experimental and finite element studies on mode I and mixed mode (I and II) stable crack growth—II. finite element analysis

Finite element studies are presented on both mode I and mixed mode stable crack growth under static loadings through an aluminium (D16AT) alloy. A COD based criterion has been used to predict the load-displacement diagram from initiation to instability. The theoretical predictions are compared with experimental results presented in Part I. Results on computed crack profiles, stress-strain distribution ahead of the crack tip, J integrals, J resistance curves, plastic zones, etc., are included. The study indicates that the load-displacement diagram associated with a mixed mode stable crack growth in a compact tension type of specimen geometry can be predicted reasonably accurately using the criterion of a fixed crack opening displacement at a finite distance behind the crack tip provided the crack is allowed to grow in the direction of initial growth in the finite element analysis. The crack assumes a more blunted profile in a mixed mode than in the mode I at all the stages of stable extension. The distributions of normal stress and strain in the direction perpendicular to the crack extension line, ahead of the current crack tip, have similarities between the mode I and mixed mode, irrespective of loading angle. Both the stress and strain levels increase as the crack extension proceeds. In a mixed mode, the J integral at the onset of crack extension is the lowest compared with the values at the later stages of the extension. Further, the tearing modulus associated with initial kinking is very small; it becomes close to the mode I values at the later stages. The tearing modulus remained approximately constant during the whole mode I stable growth and it had a similar trend subsequent to kinking in a mixed mode. The specific work of crack extension is zero as Δa → 0 and it increases gradually with Δa irrespective of the mode of loading; the actual variation depends on the loading angle. The plastic zone size grows as the stable extension progresses; the growth is approximately the maximum along the crack extension line.     

Investigation by scanning electron microscope of the effect of geometry and loading mode on blunting of copper sheets

Ductile blunting and fracture under conditions of plane stress in metals are described by two main phases: static crack growth, and slow crack propagation. In the first phase the crack tip is deformed in an elastic-plastic mode, the main characteristics of which are the formation of the plastic enclave around the crack tip, and the evolution of blunting. In the second phase the crack propagates through expansion and coalescence of microvoids and microdefects developed at the vicinity of and in front of the crack tip. Ductile blunting under plane-stress conditions in metallic plates was studied, and the influence of the geometry and the mode of loading specimens was defined. This was achieved by interrelating the amount of crack opening displacement at its tip, and its comparable effect of the crack tip advance displacement. The experimental study was executed in a scanning electron microscope with thin specimens under dominating plane-stress conditions. The mechanism of development of ductile blunting up to the point of initiation of slow crack propagation was interrelated with these characteristic quantities.     

Dynamic crack growth along an elastoplastic bimaterial interface

Summary The near-tip stress and deformation rate fields of a crack dynamically propagating along an interface between dissimilar elastic-plastic bimaterials are presented in this paper. The elastic-plastic materials are characterised by theJ ₂-flow theory with linear plastic hardening. The solutions are assumed to be of variable-separable form with a power-law singularity in the radial direction. Two distinct solutions corresponding to the tensile and shear solutions exist with slightly different singularity strengths and very different mixities at the crack tip. The phenomenon of discrete and determinate mixities at the interfacial crack tip is confirmed in dynamic crack growth. This is not an artifact of the variable-separable solution assumption, arising from the linear-hardening material model. The dynamic crack analysis shows that the mixity of the near-tip field is mainly determined by the given material parameters and affected slightly by the crack propagation velocity. A significant variation of the mixity is observed near to the coalescing point of the tensile and shear solutions. The strength of the singularity is almost determined by the smaller strain-hardening alone, and dynamic inertia decreases the stress intensity. The asymptotic solutions reveal that the crack propagation velocity changes only the stress field of the tensile mode significantly. With increasing the crack propagation velocity, the stress singularity of the tensile solutions decreases obviously and the stress triaxiality at the tip (=0) falls considerably at the unity effective stress. These observations imply that the fracture toughness of the interface crack under tensile mode may be significantly higher than that under quasi-static conditions.     

Model of fatigue crack propagation by damage accumulation at the crack tip

Modelling fatigue crack propagation by damage accumulation at the crack tip originally proposed by McClintock is reworked out using metallurgical considerations. On a physical basis it is shown that the validity of such models is actually confined to the low crack growth rate range corresponding to stage I growth such as along crystallographic planes. At higher crack growth rates there is a transition to plastic stretching mechanisms which could be described by crack opening displacement models. An evaluation of two models has been carried out on a ³³Co Ni alloy where extensive information was available: both are based on the strain singularity as computed by Tracey from a finite element analysis of plane strain small scale yielding and as adapted to cyclic loading under Rice's hypothesis and taking the grain as the critical element below which size continuum mechanics do no longer apply. It is pointed out that the use of this strain singularity which is not able to account for crack closure, renders models unable to predict experimental crack growth rates curves but only intrinsic curves relating the growth rate to the effective stress intensity amplitude as defined by Elber. The first model which assumes fatal cracking of a grain with an average uniform cyclic equivalent strain, underestimates the crack growth rates and in addition it yields results very sensitive to the shape and size of the grain. The second model which assumes progressive cracking in a grain with a cyclic equivalent strain gradient is able to predict the intrinsic crack growth rate curve in the ³³Co Ni alloy and to yield predictions consistent with other experimental data.     

THE SIGNIFICANCE OF CRACK TIP DEFORMATION FOR SHORT AND LONG FATIGUE CRACKS

Abstract— The behaviour of physical short mode I cracks under constant amplitude cyclic loading was investigated both numerically and experimentally. A dynamic two-dimensional elastic-plastic finite element technique was utilised to simulate cyclic crack tip plastic deformation. Different idealisations were investigated. Both stationary and artificially advanced long and short cracks were analysed. A parameter which characterises the plastically deformed crack tip zone, the strain field generated within that zone and the opening and closure of the crack tip were considered. The growth of physically short mode I cracks under constant amplitude fully reversed fatigue loading was investigated experimentally using conventional cast steel EN-9 specimens. Based on a numerical analysis, a crack tip deformation parameter was devised to correlate fatigue crack propagation rates.     

拉-压循环加载下铝合金疲劳裂纹扩展的压载荷效应研究

应用弹塑性有限元方法与增量塑性损伤理论指出疲劳裂纹扩展的压载荷效应是裂纹尖端塑性损伤的结果, 建立了在拉-压循环加载下铝合金疲劳裂纹扩展速率的双参数预报模型, 对LY12-M 高强铝合金MT 试件在应力比R=0、-0.5、-1、-2 进行了疲劳裂纹扩展实验。结果表明:当最大应力强度因子K_max相同时, 恒幅拉压加载(应力比R<0)的疲劳裂纹扩展速率明显高于恒幅拉拉加载(应力比R=0)的情况, 拉-压循环载荷的压载荷部分对疲劳裂纹扩展速率具有促进作用。该文得出的LY12-M 铝合金在拉-压循环加载下的疲劳裂纹扩展速率预报模型与实验结果符合较好。     

Fem prediction of the influence of warm prestressing on fracture toughness of heat-resistant steel

We present a procedure of prediction of the influence of warm prestressing combined with cycling on the brittle strength of steel 15Kh2MFA. Using a finite-element method, the effect of the combined warm prestressing on the stress-strain state at a fatigue crack tip is studied in an elastic-plastic statement. Electron microscopic observations of fracture surfaces have revealed that fracture is initiated at some distance from the fatigue crack front. Based on the pattern of influence of the plastic prestrain level on the cleavage stress of steel 15Kh2MFA and the experimental CID value, a method is put forward for finite-element modeling of the stress-strain state at a crack tip during the specimen fracture. Using the results of the finite-element modeling, the relevant curves have been plotted and an approximating formula has been proposed to represent the influence of the combined warm prestress level on the fracture toughness of steel 15Kh2MFA.