Fracture Mechanics Study for Toughening Fiber Reinforced Composites

本研究首先建立了基于断裂力学的随机不连续和定向连续纤维增强伪应变硬化复合材料的细观力学模型，并在此基础上提出了复合材料强度和韧性的优化方法。以下任务构成本研究的主要课题。我们建立了一个微观力学模型，该模型提供了复合材料性能（强度和韧性）与复合材料参数（纤维参数：尺寸、体积分数和力学性能；基体参数：韧性和弹性模量；界面抗剪强度；缓压系数）之间的联系。该模型适用于纤维拔出和纤维断裂两种情况。任务(2)纤维增强复合材料的制备与评价采用火花等离子烧结方法，制备了连续碳纤维增强SiC复合材料和304型不锈钢短纤维增强LAS/硅灰石复合材料。通过三点弯曲试验表明，微力学模型具有预测复合材料力学性能的能力。此外，LAS/硅灰石复合材料显示出伪应变硬化行为。基于细观力学模型，设计了包含伪应变硬化条件、纤维均匀分布条件和纤维断裂条件的设计图表。最后，提出了一种通过参数化研究来优化高韧性复合材料纤维参数的方法。

The objective of this research is firstly to develop a fracture mechanics based micromechanical model for the design of pseudo strain hardening composites reinforced with random discontinuous and aligned continuous fibers, and then to propose a method for optimizing the strength and the toughness of composites on the basis of themicromechanical model. The following tasks form major subjects of this research.Task (1) Development of Design ModelWe developed a micromechanical model which provides a link between the composite properties (strength and toughness) and the composite parameters (fiber parameters : dimensions, volume fraction and mechanical properties ; matrix parameters : toughness and elastic moduli ; interface shear strength ; snubbing coefficient). The model is applicable to both the cases of fiber pull-out and fiber rupture.Task (2) Preparation and Evaluation of Fiber Reinforced CompositesUsing a spark plasma sintering method, we prepared continuos carbon fiber reinforced SiC composites and LAS/wolastonite composites reinforced with short random fibers of a 304 type stainless steel. It was shown by conducting three point bend tests that the micromechanical model has a capability of predicting the mechanical properties of the prepared composites. Furthermore, the LAS/wolastonite composites were shown to produce pseudo strain hardening behavior.Task (3) Optimization of Composite ParametersBased on the micromechanical model, we devised a design chart which embodied the conditions for pseudo strain hardening, uniform fiber distribution, and fiber rupture. Finally, we proposed a methodology of optimizing the fiber parameters for high toughness composites by conducting parametric studies.