This paper shows how the double-K fracture parameters Kini Ic and Kun Ic can be determined for concrete using CT-specimens and wedge splitting specimens. The experimental results collected from the fracture tests on the very large size CT-specimens and small size wedge splitting specimens carried out by many researchers are utilized to investigate the characters of the obtained double-K fracture parameters Kini Ic and Kun Ic. It was found that the double-K fracture parameters Kini Ic and Kun Ic determined from fracture tests on the large size CT-specimens are size-independent. And the values of Kini Ic and Kun Ic determined from small size wedge splitting specimens with same dimensions are independent of the relative preformed notch length a0=D. However, when the dimensions of small size wedge splitting specimens change from 150

In this paper, an attempt is made to determine the double-K fracture parameters Kini Ic and Kun Ic using three-point bending notched beams. First, based on the knowledge from extensive investigations which showed that the nonlinearity of P-CMOD curve is mainly associated with crack propagation, a linear asymptotic superposition assumption is proposed. Then, the critical effective crack length ac is analytically evaluated by inserting the secant compliance cs into the formula of LEFM. Furthermore, an analytical result of a fictitious crack with cohesive force in an infinite strip model was obtained. The double-K fracture parameters Kini Ic and Kun Ic as well the critical crack tip opening displacement CTODc were analytically determined. The experimental evidence showed that the double-K fracture parameters Kini Ic and Kun Ic are size-independent and can be considered as the fracture parameters to describe cracking initiation and unstable fracture in concrete structures. The testing method required to determine Kini Ic and Kun Ic is quite simple, without unloading and reloading procedures. So, for performing this test, a closed-loop testing system is not necessary.

采用标准三点弯曲梁砼试件，利用试验中测得的最大荷载Pmax及对应的裂缝口张开位移CMODc计算了砼裂缝亚临界扩展量Dac。提出了由虚拟裂缝闭合力s（x）产生的应力强度因子cKI的实用解析方法，进而计算了砼双K断裂参数ini KIC及un KIC。采用这一方法，只需在试验中测得最大荷载Pmax及对应的裂缝口张开位移CMODc、砼弹性模量及抗拉强度即可确定双K断裂参数并且无需对cKI进行数值积分。大量的计算分析表明，这一实用解析方法不但可使计算过程进一步简化，而且具有很高的精度。

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.

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。