影响三维编织CMC断裂性能的孔洞问题

在气相沉积三维编织陶瓷基复合材料(CMC)的编织结构中，孔洞的存在在所难免。研究孔洞对断裂性能的影响必须面对孔洞带来的问题。过去研究孔洞问题的工作主要集中在等效模量和强度上，并取得了一些理论成果。然而，除了微裂纹、纤维和偏转增韧等的研究外，很少有关于孔洞对断裂性能的文献报道。因此这里探讨一下孔洞对断裂的影响问题。通常研究者认为孔洞能够通过两种途径增韧材料的断裂韧性：即裂尖的钝化和材料柔性增加。前者只能够定性地认识和分析裂尖形貌的增韧效果(本文不涉及此内容)；后者可以做一些定量的分析。基于此，首先研究了孔洞的形态与分布，并给出了孔洞模型。其次用两次等效的工程方法均匀化材料的有效弹性模量，并且和实验相比较，其结果令人满意。接着又分析了应力强度因子K及K控制区随孔洞体积分量的变化规律。最后引用能量释放率G用图的形式表述了孔洞的增韧效果。显然图中的结论表明，孔洞的增韧效果并不理想。

Holes Problem Influencing on Fracture Characteristic of 3D Braided CMC

For 3D braided CMC deposited ceramic matrix, holes maybe always exist in the braided structure. The problem of holes has to be faced in studying its influence on fracture characteristics. In the past, researches about holes problem focused on the effective moduli and strengths, and had made some theoretic achievement, but there were little reports about the influence of holes on fracture properties except for the toughness enlargement of micro-cracks, fiber and deflexion et al. So the holes influence on fracture is discussed in this paper. Usually, the influence of holes on fracture property is regarded as a positive effect by some researchers for two ways of toughness enlargement, namely, crack tip halt for decreasing singularity and ductile enlargement of materials. The first one can only be discussed in qualitative analysis, and the second one can be researched in quantitative analysis. Based on the above cognition, we first investigate shapes and layout of holes in the braided structure, and make out a model of hole. Secondly, for the complicated shape of holes, the material effective moduli can only be drawn up by the twice homogenization of engineering methods, and comparing with the test results, its results are satisfactory. Then the increment of stress intensity factor K and its controlling section rp are analyzed with the volume content of holes in 3D braided CMC. At last, energy release rate G is quoted to describe the fracture toughness increment of holes to 3D braided CMC in a figure. Obviously, there is a conclusion that toughness increment of holes is less efficient.