断裂力学基本上有两种分析裂缝稳定性的方法：应力强度因子法和能量法。在混凝土断裂性能的描述上两者应该是等效的。针对目前大多数混凝土断裂模型以应力强度因子型的断裂韧度作为判定准则，本文尝试从能量的角度出发，结合线弹性断裂力学和虚拟裂缝区上的黏聚力分布，以能量释放率G作为断裂性能判定参数，建立了混凝土结构裂缝扩展的双G准则。与双K断裂参数相对应，双G准则引入了两个重要的裂缝扩展判定参量：起裂断裂韧度GiniIc和失稳断裂韧度GunIc。其中，起裂断裂韧度Ginilc对应于起裂荷载Pini和初始裂缝长度a0；失稳断裂韧度GunIc对应于极值荷载Pmax和临界有效裂缝长度ac。根据线性渐进叠加假定，可以把（Pini，a0）和（Pmax，ac）代入线弹性断裂力学相对应的公式直接求得GiniIc 和GunIc。考虑到起裂荷载Pini的不易确定性，本文引入黏聚力分布引起的能量损耗GcIc，通过三者GiniIc 、GunIc和GcIc的关系，给出了起裂断裂韧度GiniIc和失稳断裂韧度GunIc的适用计算公式，并通过三点弯曲梁实验得到了验证。

本文基于作者及其合作者近年新提出的两端切口Ⅱ型断裂试件几何和相应的应力强度因子公式的基础上，建议一种无切口Ⅱ型断裂试件。然后对不同切口长度的Ⅱ型断裂试件和不同高度和宽度的无切口Ⅱ型断裂试件的断裂破坏过程采用以微面模型（Micro-Plane Model）为基础的三维非线性有限元分析商用程序进行数值模拟，发现无切口试件在韧带上由剪切应力集中引起剪切裂缝后，在韧带区形成纯Ⅱ型断裂和剪切裂缝扩展过程。然后较为系统地研究了试件宽度和试件高度对断裂破坏过程和断裂破坏形态的影响。

混凝土断裂能的尺寸效应一直是研究的热点问题之一。借助于断裂功的概念，目前采用不同的试件形式来测试混凝土断裂能GF。其中，楔入劈拉试件因其在实际操作过程中的诸多优点而有希望成为测试混凝土断裂能GF的标准。本文采用楔入劈拉法研究了试件韧带高度对GF的尺寸效应影响。在测试过程中为得到稳定的荷载位移下降段而采用了柔度测试法。根据RILEM推荐的公式计算混凝土断裂能GF时，对荷载一位移曲线的尾部进行了合理的拟合处理，并考虑了试验夹具自重的影响。结果发现混凝土断裂能GF数值随试件韧带高度的增加有上升的趋势，具有明显的尺寸效应。进一步证实了尺寸效应是传统意义上的混凝土断裂能的属性特征。

In this paper a mode It fracture testing method has been developed for wood from analytical, experimental and numedcaI investigations. Analytical resufls obtained by other researchers showed that the specimen geometry and loading type used for the proposed mode Ⅱ testing method results in only mode Ⅱ stress intensity and no mode Ⅰ stress intensity at the crack tip Experiments have been carded out to determine mode II fracture toughness A'iic and fracture energy GIIF from the test data collected from both spruce (picea afiies) and popIar (popuIus nigra) specimens. It was found that there existed a very good relation between fracture toughness Kiic and fracture energy GIlF when the influence of orthotropic stiffness EI in mode Ⅱ was taken into account. It verified that for this mode Ⅱ testing method the formula of LEFM can be employed for calculating mode Ⅱ fracture toughness even for highly orthotropic materiaIs like wood In the numerical studies for the tested spruce specimen, the crack propagation process, stress and strain fields in front of crack tips and the stress distributions along the ligament have been investigated in detail. It can be seen that the simulated crack propagating process along the ligament is a typical s hear cracking pattem and the development of cracks along the ligament is due to shear stress concentrations at the crack tips of the specimen. It has been shown that this mode Ⅱ fracture testing method is suitable for measuring mode Ⅱ fracture toughness/(ElC for highly orthotropic materials like wood.

The edoubIe-e notched no: ched specimen made of normal s: rengrh tin, crete has been stlbsected to compressive loads, kis shown that a shear crack develops at the notch tip. Visual observation and evaluation of displacement measurements have proven that this specimen allows pure mode I testing, It was possible to determine fracture toughness KIIc, Recommendations for the testing are made.