相变增韧陶瓷Ⅲ型裂纹增韧分析

本文采用压力敏感相变准则和权函数法对相变增韧陶瓷Ⅲ型裂纹的增韧效应进行了理论预测,分别给出了静止裂纹和定常扩展裂纹相变塑性屏蔽的理论表达式。结果表明:相变体积膨胀对增韧无贡献,裂尖屏蔽来自于晶格剪切和裂纹扩展尾区的贡献。     

Ⅰ—Ⅲ混合型裂纹相变增韧分析

本文采用压力敏感准则和权函数法对相变增韧陶瓷Ⅰ－Ⅲ混合型裂纹的增韧效应进行了理论预测。分别给出了静止裂纹和定长扩展裂纹相变塑性屏蔽的理论表达式，结果表明：相变对静止裂纹有负屏蔽效应，并随ＫⅢ／ＫⅠ比值的增大而趋于零，对扩展裂纹的增韧结果除与材料的弹性模量，相变尾区高度和相变体积分数有关外，还与ＫⅢ／ＫⅠ的比值有关。     

Ⅱ-Ⅲ混合型裂纹相变增韧分析

本文采用压力敏感准则和松函数法对相变增韧陶瓷Ⅱ-Ⅲ混合型裂纹的增韧效应进行了理论预测。分别给出了静止裂纹和定常扩展裂纹相变塑性屏蔽的理论表达式。结果表明：相变对静止裂纹有甚微的负屏蔽效应，并随KⅢ/KⅡ的比值而波动；相变对定常扩展裂纹的增韧结果除与材料弹性模量、相变尾区高度和相变体积分数有关外，还与KⅢ/KⅡ的比值有关。     

陶瓷相变对Ⅰ—Ⅱ混合型裂纹增韧的理论分析

采用压力敏感准则和权函数法对相变增韧陶瓷I－II混合型裂纹的增韧效应进行了理论预测,分别给出了静止裂纹和定常扩展裂纹相变生屏蔽的理论表达式,并通过计算机进行了数值计算,结果表明：相变对静止裂纹有负屏蔽效应,并随KII/KI的比值增大而增大,对定常扩展裂纹的增韧结果除与材料弹性模量,相变尾区高度和相变体积分数有关外,还随KII／KI的比值增大而增大,说明相变对II型裂纹的增韧作用比对I型裂纹的增韧作用更显著。     

含弧形微裂纹相变复合陶瓷增韧分析

弧形微裂纹是复合陶瓷内的常见现象,含少量弧形微裂纹复合陶瓷比无裂纹复合陶瓷的韧性更好,而粒子相变能够提高陶瓷的断裂韧性已经得到普遍承认.本文考虑粒子相变与弧形微裂纹的联合效应,建立含弧形微裂纹相变复合陶瓷的混合型裂纹增韧模型.首先应用压力敏感准则和应变能释放率断裂准则,对含弧形微裂纹复合陶瓷的增韧效应进行了理论预测;然后采用权函数法推导出混合型裂纹的增韧结果,分别给出了静止裂纹和稳态扩展裂纹相变塑性屏蔽的理论表达式;最后根据相变区域与弧形微裂纹相之间的关系,分析增韧结果的尺度效应.结果表明:相变对静止裂纹无增韧作用;相变对稳态扩展裂纹的增韧结果除与材料弹性模量、相变尾区高度和相变粒子的体积分数有关外,还与复合陶瓷的颗粒直径和弧形微裂纹的体积分数有关.     

结构增韧材料在裂纹扩展中的韧度增值

本文分析了由裂纹扩展过程中形成的耗能尾区和桥联段所产生的增韧效应。根据对扩展裂纹的能量积分的贡献,结构增韧材料在裂纹扩展中的韧度增值机制可分解为:尾区体膨胀塑性增韧;尾区剪切屈服带增韧;裂纹面夹杂第二相桥联;未贯穿裂纹的基体桥联。本文结合结构高分子材料和短纤维复合材料等材料体系进行了具体的增韧分析计算。     

相变增韧陶瓷平面应变Ⅰ型定常扩展裂纹的渐近分析（Ⅰ）

采用Ｂｕｄｉａｎｓｋｙ等人提出的相变增韧陶瓷材料的体膨胀本构关系，利用渐近解法得到该材料平面应变Ⅰ型定常扩展裂纹尖端附近的应力、速率沿角度的分布情况，给出相变区高度和相变造成的韧性增值的计算公式。     

增韧环氧树脂的动态裂纹扩展研究

本文主要进行了环氧及增韧环氧树脂的断裂韧性及裂纹快速扩展的试验研究。试验过程中采用了ＧＬＣ－１型高速裂纹扩展测试仪来测试裂纹的扩展速度,得到在裂纹扩展过程中裂纹扩展速度曲线。本文结合不同的计算公式及有限元分析方法,讨论了各个确定断裂韧性公式的准确程度,发现传统的静态断裂韧性的分析方法所得到的结果偏大,有一定的危险性,建议使用试验与数值计算相结合的方法;同时还发现增韧不仅可以提高材料的静态和动态断裂性能,而且在裂纹扩展过程中可以起到减缓裂纹扩展的作用     

相变滞后对裂纹屏蔽效应的数值分析

本文针对奥氏体-马氏体双相材料,研究裂纹尖端区弥散分布的奥氏体颗粒在应变诱发时发生的相变对裂纹的屏蔽效应。鉴于实验中已发现的不同相变滞后对裂纹屏蔽效应的不同影响,本研究通过在裂纹尖端区不同位置嵌入相变颗粒,考虑到裂纹尖端区应力应变场的奇异分布及其诱发的相变,将裂纹尖端区相变滞后问题转化为相变颗粒在裂纹尖端区的位置问题。计及奥氏体-马氏体相变的体积膨胀效应进行了平面应力裂纹问题数值模拟,得到了单个相变夹杂对裂纹屏蔽效应的影响规律。结果表明：裂纹尖端区相变夹杂的位置对裂纹的屏蔽效应在距裂尖2倍夹杂直径以内影响极大,且以裂尖86度方向为界。其影响规律与McMeeking 和 Evans理论预言的60度方向不同。     

一种镍基双晶材料疲劳裂纹扩展效应

对晶界平行裂纹和晶界垂直裂纹的双晶体进行三点弯曲疲劳实验，研究了双晶材料的疲劳裂纹扩展规律，测定了双晶的疲劳扩展速率，揭示了晶界对晶粒疲劳裂纹扩展的屏蔽效应：当裂纹距晶界某一特定长度时，裂纹扩展速率最快；而裂纹顶端交于晶界时，裂纹扩展速率最侵．进一步的晶体滑移有限元数值分析揭示了这种屏蔽效应的机理：晶界附近不协调的塑性变形，导致了裂纹尖端应力场的重新分布．     

Toughening analysis of mode III crack in transformation toughened ceramics

The fracture toughness enhancement of mode III crack in transformation toughened ceramics such as Ce-TZP and Mg-PSZ is predicted by using a pressure sensitive transformation criterion and the weight function method. The theoretical analyses of the toughening effect for both stationary and steady-state growing crack are given, respectively. The results show that the toughening is only associated with crack growth and there is no toughening effect for stationary crack.The project was supported by the National Science Foundation of China     

Crack growth resistance of shape memory alloys by means of a cohesive zone model

Crack growth resistance of shape memory alloys (SMAs) is dominated by the transformation zone in the vicinity of the crack tip. In this study, the transformation toughening behavior of a slowly propagating crack in an SMA under plane strain conditions and mode I deformation is numerically investigated. A small-scale transformation zone is assumed. A cohesive zone model is implemented to simulate crack growth within a finite element scheme. Resistance curves are obtained for a range of parameters that specify the cohesive traction-separation constitutive law. It is found that the choice of the cohesive strength t₀ has a great influence on the toughening behavior of the material. Moreover, the reversibility of the transformation can significantly reduce the toughening of the alloy. The shape of the initial transformation zone, as well as that of a growing crack is determined. The effect of the Young's moduli ratio of the martensite and austenite phases is examined.     

On the transformation toughening of a crack along an interface between a shape memory alloy and an isotropic medium

In this study, a bilinear cohesive zone model is employed to describe the transformation toughening behavior of a slowly propagating crack along an interface between a shape memory alloy and a linear elastic or elasto-plastic isotropic material. Small scale transformation zones and plane strain conditions are assumed. The crack growth is numerically simulated within a finite element scheme and its transformation toughening is obtained by means of resistance curves. It is found that the choice of the cohesive strength t₀ and the stress intensity factor phase angle φ greatly influence the toughening behavior of the bimaterial. The presented methodology is generalized for the case of an interface crack between a fiber reinforced shape memory alloy composite and a linear elastic, isotropic material. The effect of the cohesive strength t₀, as well as the fiber volume fraction are examined.     

The application of the Eshelby equivalent inclusion method for unifying modulus and transformation toughening

When a crack is lodged in an inclusion, both difference between the modulus of the inclusion and matrix material and stress-free transformation strain of the inclusion will cause the near-tip stress intensity factor to be greater (amplification effect) or less (shielding or toughening effect) than that prevailing in a homogeneous material. In this paper, the inclusion may represent a second phase particle in composites and a transformation or microcracked process zone in brittle materials, which may undergo a stress-free transformation strain induced by phase transformation, microcracking, thermal expansion mismatch and so forth. A close form of solution is derived for predicting the toughening (or amplification) effect. The derivation is based on Eshelby equivalent inclusion approach that provides rigorous theoretical basis to unify the modulus and transformation contributions to the near-tip field. As validated by numerical examples, the developed formula has excellent accuracy for different application cases.     

NiTi形状记忆合金裂纹尖端热力耦合行为研究

本文对NiTi形状记忆合金I型裂纹尖端热力耦合行为进行了数值仿真分析和实验验证。建立了包含相变和热力耦合的本构模型,通过有限元计算得到了裂纹尖端附近的纵向应变、马氏体体积分数和温度场分布,依据马氏体相变情况对裂纹尖端有效应力强度因子进行了修正,揭示了加载速率对形状记忆合金裂纹尖端有效应力强度影子的影响规律。参数研究表明,随着加载频率的增加,裂纹尖端附近温度逐渐升高,马氏体相变区域逐渐缩小,有效应力强度因子呈下降趋势,形状记忆合金表现出增韧效应,有助于减缓裂纹扩展。本研究结果对于揭示热力耦合作用下超弹性形状记忆合金疲劳裂纹扩展规律具有重要参考意义。     

Toughening by Phase Boundary Propagation

Transformation toughening has enhanced the fracture toughness of certain Zirconia-Toughened Ceramics (ZTC) by factors of 2–4. The primary explanation of toughening, by McMeeking and Evans [1] and Budiansky et al. [2], suggests that the main source of toughening is the energy stored by the transformed inclusions in the wake of a propagating crack. In the case of supercritical ZTC where the boundary of the transformed zone is a phase boundary (surface of strain discontinuity), this paper suggests an additional source of toughening – that due to propagation of the phase boundary. By extending the J-integral to cracked bodies containing surfaces of strain discontinuity, the transformation toughening for a steady Mode I crack is evaluated.     

BRIDGE-TOUGHENING ANALYSIS OF THE FIBER REINFORCED COMPOSITE CONTAINING INTERPHASE EFFECT

A detailed fracture mechanics analysis of bridge-toughening in a fiberreinforced composite is presented in this paper.The integral equation governing bridge-toughening as well as crack opening displacement (COD) for the composite withinterfacial layer is derived from the Castigliano's theorem and interface shear-lagmodel.A numerical result of the COD equation is obtained using the iteration solutionof the second Fredholm integral equation.In order to investigate the effect of variousparameters on the toughening,an approximate analytical solution of the equation ispresent and its error analysis is performed,which demonstrates the approximatesolution to be appropriate.A parametric study of the influence of the crack length,interracial shear modules,thickness of the interphase,fiber radius,fiber volumefraction and properties of materials on composite toughening is therefore carried out.The results are useful for experimental demonstration and toughening design includingthe fabrication process of the composite.