复合材料中基体的现场强度研究

A world-wide challenge in the composite community is to calculate the ultimate strength of a composite under any load only upon its constituent material properties measured independently. Although the Bridging Model established by the writer can accurately calculate the internal stresses in the constituent fiber and matrix of a composite, determination of its ultimate load carrying capacity depends on, among others, in-situ strengths of the matrix. Experiments have revealed that a transverse strength of a UD (unidirectional) composite is much smaller than an original strength of its matrix, indicating that the introduction of fibers has significantly reduced the strength of a monolithic matrix material. It is the main task of this project to determine the stress concentration factor due to fiber occurrence corrsponding to transverse loading. Then, the matrix in-situ strengths are obtained by its original counterparts divided by this factor. Experiments will be carried out to explore relationship of an in-situ matrix strength with a processing technique for the composite fabrication, and to correct the definition on the stress concentration factor of the matrix. The influences of several typical imperfect interfaces between fibers and matrix in composites on the matrix in-situ strengths will be investigated in the project. The Bridging Model only applicable to two-phase, i.e. fiber and matrix, composite materials at the present will be developed to a more general theory for the analysis of a three-phase (fiber-interface-matrix) composite. With these works done, the determination of an ultimate load sustaining ability of a composite subjected to any arbitrary loads can be reasonably achieved only based on the independently measured properties of the constituents as well as geometrical parameters of the composite.

如何仅仅根据组份材料的原始性能预报受任意载荷作用的复合材料强度是一个世界性难题。虽然申请者创建的桥联理论可准确计算纤维和基体中的内应力，但复合材料极限承载能力的确定还取决于基体的现场强度。实验发现单向复合材料横向强度远低于基体的原始强度，表明添加纤维后导致基体现场强度大幅降低。本项目将研究确定在基体中添加纤维后引起的（横向）应力集中系数，基体的现场强度则由其原始强度除以该应力集中系数后得到。通过对不同成型工艺所制复合材料试样的实验研究，揭示基体现场强度与成型工艺的关系并完善应力集中系数的定义；研究纤维和基体之间一些典型非理想界面对基体现场强度的影响；在目前仅限于纤维和基体两相材料的桥联模型基础上，发展出应用于三相（纤维-界面-基体）复合材料分析的更一般理论，为实现由原始纤维和基体性能及复合材料几何参数计算复合材料极限承载能力的目标做出贡献。

如何仅仅根据组份材料的原始性能预报受任意载荷作用下的复合材料强度是一个世界性难题。虽然申请者创建的桥联理论可准确计算纤维和基体中的内应力，但复合材料极限承载能力的确定还取决于基体的现场强度。实验发现单向复合材料横向强度远低于基体的原始强度，表明添加纤维后导致基体现场强度大幅降低。通过本项目的研究，我们成功建立起基体各现场强度的解析理论，等于基体的原始强度除以相应方向的基体应力集中系数。但是，添加纤维后的基体应力集中系数不可再按经典方式定义，否则，界面开裂处的基体应力集中系数将为无穷大。我们准确地将基体应力集中系数定义为沿破坏面外法线方向的平均应力除以基体相应方向的均值应力。不仅导出了理想界面下基体的各应力集中系数，而且还得到了界面开裂后的基体横向拉伸应力集中系数公式。采用三届“破坏分析奥运会”组织者所提供的组分材料数据，基于基体原始强度和现场强度的复合材料强度预报结果与实验对比误差相差5倍，说明基体的现场强度对复合材料的破坏分析和强度预报至关重要。本项目的研究成果对复合材料力学学科发展以及复合材料的有效工程应用都具有重要意义。