弹塑性断裂的内参量热力学分析——Ⅱ.弹塑性裂纹体的裂纹扩展

采用内参量热力学方法讨论弹性—理想塑性体的裂纹扩展,主要在下两方面有更进一步的理论分析。一是对弹塑性断裂力学中的各种能量关系作出了统一的整理,讨论了弹塑性裂纹体的裂纹扩展和塑性变形的驱动力与阻尼力,并给出了相应的过程准则表达式以及能用于弹塑性柔度标定的计算公式。二是以中心裂纹体为例,分析了裂纹扩展的热力学稳定性条件,明确弹塑性裂纹扩展的热力学稳定性的全局性条件是不能满足的,然后转而结合具体的极限势函数方程来讨论恒位移状态下满足稳定性的局域性条件的各有关情况,区分出裂纹失稳扩展—止裂、明显稳定扩展(撕裂)和缓慢稳定扩展三种情况,并举例给出了应变—裂纹长大和应力—应变的计算曲线。     

复杂飞机结构裂纹萌生寿命确定的工程方法

采用静拉伸应变测量试验的方法,确定了典型飞机复杂结构在载荷作用下的应变分布,并准确判断出其疲劳危险部位。对典型飞机复杂结构模拟件进行了等幅加载疲劳试验,并在疲劳试验过程中采用着色探伤方法测得了疲劳裂纹扩展数据。在此基础上,提出并建立了一种适用于确定复杂结构疲劳裂纹萌生寿命的工程方法,包括疲劳裂纹萌生寿命反推法和含有不确定寿命样本的统计推断方法。根据该方法对试验样本疲劳数据进行处理得出了裂纹萌生中值寿命及其在95%置信水平下的单侧置信下限。     

混凝土坝裂纹扩展算法(英文)

混凝土建筑物中的裂纹扩展追踪计算是确定结构安全度的重要手段。本文提出用虚拟裂纹单元及断裂力学准则(应变强度因子)追踪和模拟裂缝初始断裂、扩展并最终导致破坏的过程,给出一条逐段扩展的裂纹线,本方法计算结果与某些混凝土坝实测资料及模型试验结果吻合较好。     

弹塑性断裂的内参量热力学分析——Ⅰ.线弹性裂纹体的裂纹扩展

本文在内参量热力学的理论框架内详细讨论了线弹性裂纹体的裂纹扩展问题。对线弹性断裂力学中所涉及到的各种能量关系作了统一的整理,其中包括Irwin的弹性能释放率——裂纹扩展力,Griffith的脆性断裂准则,以及Irwin-Kies的柔度公式和Paris的位移公式。本文以中心裂纹体为例,详细计算了裂纹扩展的稳定性,并区分恒载荷和恒位移两种情况,对材料断裂韧性测试中所遇到的失稳扩展,稳定扩展以及进发现象提供了理论说明。     

含裂纹工作辊热轧过程有限元分析

利用大型有限元软件ANSYS/LS-DYNA的模拟功能,对轧辊的裂纹尖端开裂模式进行了模拟分析。在将裂纹尖端区域细化的基础上,得出含表面裂纹的热轧工作辊两个裂纹尖端的开裂模式是不同的,并且随着轧制深入以及裂纹扩展规律的影响,出现了裂纹尖端局部塑性区以及材料局部的卸载情况,使应力强度因子K以及J积分之间不再满足在线弹性过程中的换算关系形式。     

考虑焊趾形貌的T形焊接接头三点弯疲劳试验

针对弯曲载荷作用下焊趾疲劳裂纹的萌生与扩展问题，考虑焊趾形貌的影响，开展了T形焊接接头的三点弯疲劳试验。试验前，观察和测量试件双侧焊趾的形貌，得到系列形貌不规则的特征点，通过在部分特征点设置FBG应变传感器测量焊趾局部的应变时间变化。施加了2级载荷谱块的循环载荷，在疲劳断口获得了清晰的海滩条带，结合断口形貌和焊趾形貌对焊趾裂纹的萌生和扩展行为进行了分析。结果表明，受弯焊接接头更易在焊趾形貌更恶劣的一侧萌生裂纹，且复杂的焊趾形貌容易导致多裂纹的产生，多裂纹的萌生位置与焊趾形貌特征点有较强的对应关系。     

浅裂纹试样塑性位移与载荷关系的实验标定

本文用实验验证了对于两种中强度，低韧性的材料１８ＭｎＭｏＮｂ１０ＭｎＭｏＮｂｖｃＵ，Ｅｒｎｓｔ分离函数原理Ｐ＝σ０Ｂｂ＾２Ｗｇ（ａ／Ｗ）Ｈ（ΔＰ／Ｗ）仍适用，并由此用实验标定了两种材料的浅裂纹试样的ｇ（ａ／Ｗ）表达式，在此基础上，得到了浅裂纹试样塑性位移与载荷的函数关系式。     

直升机桨叶根部材料全范围裂纹扩展速率测定与数据处理

直升机结构设计的损伤容限分析,需要使用材料疲劳裂纹扩展速率曲线性能。在总结传统疲劳裂纹扩展曲线的基础上,本文讨论了四参数全范围da/dt曲线公式。根据断裂性能测试标准,分别实验测试了直升机桨叶根部材料断裂韧性K1C,3个应力比下的裂纹扩展门槛值ΔKth和裂纹扩展速率da/dt;然后,利用多元线性回归法对试验数据进行了曲线拟合,分别得到了断裂等寿命曲线和四参数全范围da/dt曲线公式的参数估计式;最后,将此公式运用于不同应力比下的裂纹扩展速率试验数据,拟合得到该材料四参数全范围da/dt曲线公式。     

等厚圆盘塑性区的本构关系

等厚圆盘在设计上有时允许出现局部塑性区,用全量理论导出塑性区的本构方程,力学概念较为明晰,在此基础上进一步导出了塑性状态下当量应变Ed和材料割线模量E′的计算显式,在进行塑性区内的应力应变换算时,将它们与本构方程联立,。可使试算逼近工作大大简化。     

一种Ni—Cr奥氏体钢蠕变裂纹扩展抗力的研究

本文通过对石油化工厂中常用于制造转化炉管的奥氏体耐热钢 HK40和 HP40合金,在1144K 温变下进行恒载蠕变裂纹扩展试验,发现蠕变裂纹扩展性能与 Ni元素的含量关系很大。为此,本文着重探讨了 Ni 对蠕变裂纹扩展抗力的影响。结果表明,Ni 量增加,蠕变裂纹扩展抗力增强,即 HP40合金的蠕变裂纹扩展性能好于HK40。     

裂纹岩体贯通机制的数值模拟研究

裂隙岩体在荷载作用下导致其内部裂纹的相互贯通,从而在裂纹尖端形成较大的塑性破坏区域,岩体工程的失稳大多是由于其内部节理、裂隙等缺陷发展导致的,应用断裂力学的方法,可以追踪岩体中节理裂隙的起裂、扩展至相互贯通使岩体局部破坏的过程,从而揭示岩体失稳的渐进破坏机制.本文采用有限元方法模拟了岩体中不同倾角的压剪裂纹的破坏问题,根据不同倾角裂纹的力学机理分析了其受力差异,依据最短塑性区距离判据分析了相近裂尖的扩展距离,得出了不同倾角的两共线裂纹相近裂尖破坏后岩桥之间的有效距离.     

Towards fully automatic modelling of the fracture process in quasi-brittle and ductile materials: a unified crack growth criterion

Fully automatic finite element （FE） modelling of the fracture process in quasi-brittle materials such as concrete and rocks and ductile materials such as metals and alloys, is of great significance in assessing structural integrity and presents tremendous challenges to the engineering community. One challenge lies in the adoption of an objective and effective crack propagation criterion. This paper proposes a crack propagation criterion based on the principle of energy conservation and the cohesive zone model （CZM）. The virtual crack extension technique is used to calculate the differential terms in the criterion. A fully-automatic discrete crack modelling methodology, integrating the developed criterion, the CZM to model the crack, a simple remeshing procedure to accommodate crack propagation, the J2 flow theory implemented within the incremental plasticity framework to model the ductile materials, and a local arc-length solver to the nonlinear equation system, is developed and implemented in an in-house program. Three examples, i.e., a plain concrete beam with a single shear crack, a reinforced concrete （RC） beam with multiple cracks and a compact-tension steel specimen, are simulated. Good agreement between numerical predictions and experimental data is found, which demonstrates the applicability of the criterion to both quasi-brittle and ductile materials.     

基于D-M模型的研究与应用

在对D-M模型研究的基础上．给出了D-M模型塑性区尺寸的计算方法．并利用D-M模型对原裂纹尖端张开位移COD公式进行了推导。     

Finite element simulation of stress intensity factors in elastic-plastic crack growth

INTRODUCTION The finite element method (FEM) has been widely employed for solving linear elastic and elas-tic-plastic fracture problems. The evaluation of stress intensity factors in 2D geometries by FEM is a tech-nique widely used for non-standard crack configura-tions. Basically, there are two groups of estimation methods, those based on field extrapolation near the crack tip (Chan et al., 1970; Shih et al., 1976) and those using the energy release when the crack propagates. However…     

Digital Image Correlation Using Specific Shape Function for Stress Intensity Factor Measurement

The stress intensity factor (SIF) is a critical parameter associated with the fracture behaviour of materials. In this paper, we select the displacement function around a crack tip as the shape function of the digital image correlation (DIC), which makes it possible to directly calculate the SIF by the correlation scheme. Moreover, we use a non-rectangular subset, which can reduce the influence of plastic deformation and crack width on the DIC measurement accuracy. We measured the SIF of a mode I crack in a super-hard aluminium alloy specimen to verify the performance of the proposed method. Our experimental results show that a DIC with a specific shape function can be used to accurately and efficiently calculate the SIF. Furthermore, we also present a practical application of our proposed method for determining the SIF, crack propagation angle and crack tip displacement.     

蠕变条件下完整岩石的起裂和扩展准则

The mechanism and criterion of crack initiation and propagation of rocks were investigated by many researchers,And the creep behaviour of rocks was also theoretically and experimentally studied by some scientists and engineers.The characteristics of crack initation and propagation of rocks under creep condition.however,are very improtant for rock engineering and still not paid enough attention by researchers,In this paper,the criterion and mechanism of crack initiation and propagation under creep condition were investigated using specimens collected from sandstone rock formations outcropping in the Emei Mountain,the Sichuan Province of China.Cuboid specimens under three point bending were used in this investigation.All specimens were classified into four sorts and used for Mode-I fracutre of creep frcture tests.The experimental result shows that due to creep deformation.rock crack will inevitably initialt and propagate under a load of KI,which is less than fracture toughness KIC but not less than a constant(marked as KIC2),KIC2 indicates the ability of rock to resist crack initiation and propagation under creep conditions and is less than fracture tough ness KIC.defined as creep fracture toughness in this paper,KIC2 should be considered as an importnat parameter on design and computation of rock engineering.The microstructureal mechanism for crack initiation and propagation of rock materials under creep condition was introduced based on competitive model between softening effect and hardening effect,and the validity of test result was explained.The test result was also verified in rheological theory.When KI is more than KIC2 but less than KIC,rock crack will initiate and propagate after a time interval of sustained loading under creep condition.In order to find the relation between duration of sustained lading.which can lead to crack initiation and propagation,and the initial stress intensity factor KI,an unequal0interval time sequence forecasting and predicting model was introduced,and the relation was obtained for homogeneous and isotropic fine-grained red sandstone.Finally a modified fracture toughness formula was given,in which the influence of fracture process zone(FPZ) was fully considered.     

内压作用下容器和管道"破前漏"评定准则及应用

压力容器和管道"破前漏("LBB)准则是一种既安全又经济的评定方法。文章以弹塑性断裂力学为基础,提出了一种压力容器和管道的"破前漏"缺陷评定准则:若容器表面裂纹韧带的极限失稳压力Pu低于相应穿透裂纹的起裂压力Pc,则容器会泄漏失效;若容器表面裂纹韧带的极限失稳压力Pu等于或大于相应穿透裂纹的起裂压力Pc,则容器会爆破失效。同时,分别建立了轴向表面裂纹和周向表面裂纹韧带的极限失稳压力Pu和穿透裂纹的起裂压力Pc的表达式。进而用此方法分析了一些文献提供的实验数据,结果表明,预测的失效形式与实际情况完全相符。     

精冲过程的数值模拟及工艺参数优化

精冲过程是典型的局部塑形大变形过程，基于其成形特点建立有限元数值仿真模型，对精冲工艺进行弹塑形大变形有限元仿真．模型采用网格重划分方法解决网格畸变情况严重时造成计算终止以及计算结果不收敛的问题，同时引入McClintock韧性断裂准则来预测裂纹出现和扩展的时间及位置．在此基础上，给出了剪切区内应力、应变的分布以及发展趋势，同时针对精冲过程中的工艺参数，如凸凹模间隙、刃口圆角、压边力、反顶力等，对精冲成形面质量的影响加以了分析，给出了工艺参数的影响规律，模拟结果与实际生产情况基本符合，可用于指导工艺参数优化．