A New Approach for Investigation of Mode II Fracture Toughness in Orthotropic Materials

Abstract Estimation of mode II fracture toughness (KIIC) in composite materials is known as a troublous and crucial problem. Dissipated values of KIIC that are reported in different fracture mechanics references is the evidence of the mentioned claim. This problem can signify the necessity of modification on common test methods and fixtures. The present study focuses on the causes of shear test results scattering in composite materials and presents some solutions in the form of necessary corrections that should be performed on the common test fixtures. Mixed mode I/II fracture limit curves are employed to show that the scattering in test results have strong relation with the creation of a considerable Fracture Process Zone (FPZ). It is shown that common test fixtures are blind in confrontation with FPZ and are not able to active toughening mechanisms in pure mode II, correctly. Therefore, estimation of KIIC with available test fixtures has considerable standard deviation. After that, by employing some structural modifications on common fixtures, a new scheme of a shear fixture is proposed that in addition to include the FPZ effects, prepare suitable condition in order to activate the mode II toughening mechanisms. In this regard, it could be found that by applying these reforms, shear load concentration as well as the accuracy of empirical test and repeatability and reproducibility are enhanced. Furthermore, a 3D finite element method (FEM) was considered as the numerical method in which the Iosipesque and new fixture’s specimens were analyzed by ANSYS software. It was found that by applying major amendments in the new shear test fixture, a remarkable precision in results can be obtained in comparison with the previous Iosipesque one摘要：复合材料II型断裂韧性（KIIC）的估算一直是一项棘手且关键的研究课题，不同断裂力学文献中报道的KIIC数值存在显著离散，这便是该论断的佐证。这一问题凸显了对现有通用试验方法与夹具进行优化改进的必要性。本研究针对复合材料剪切试验结果离散的成因展开分析，并提出了针对通用试验夹具的必要修正方案以改善该问题。本研究采用I/II混合型断裂极限曲线，证实试验结果的离散性与显著断裂过程区（FPZ）的形成存在显著相关性。研究结果表明，通用试验夹具无法有效应对断裂过程区，亦无法正确激活纯II型加载下的材料增韧机制，因此采用现有夹具估算KIIC时，会产生较大的标准差。在此基础上，本研究通过对通用夹具进行结构性改进，提出了一种新型剪切夹具方案：该方案不仅可纳入断裂过程区的影响，还能为激活II型增韧机制提供适宜条件。研究发现，通过实施上述改进，剪切载荷集中现象得到缓解，试验结果的准确性、重复性与再现性均得到提升。此外，本研究采用三维有限元法（FEM）作为数值分析手段，借助ANSYS软件对Iosipesque夹具与新型夹具的试样进行了仿真分析。结果显示，相较于传统Iosipesque夹具，经重大改进后的新型剪切试验夹具可使试验结果达到显著更高的精度。