Fracture Mechanics Study for Toughening Fiber Reinforced Composites

本研究的目的是首先建立一个基于断裂力学的细观力学模型，用于设计随机不连续和排列连续纤维增强的伪应变硬化复合材料，然后提出一种基于细观力学模型的复合材料强度和韧性优化方法。本研究的主要任务如下：任务（1）设计模型的开发我们开发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，体积分数和机械性能;基体参数：韧性和弹性模量;界面剪切强度;阻尼系数）之间的联系。该模型适用于纤维拔出和纤维断裂的情况。任务（2）纤维增强复合材料的制备与评价采用放电等离子烧结法，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。三点弯曲试验结果表明，该模型具有预测复合材料力学性能的能力。（3）复合材料参数的优化基于细观力学模型，设计了一个复合材料参数设计图，该图体现了纤维分布均匀、纤维断裂和纤维伪应变硬化的条件。最后，我们提出了一种方法，优化纤维参数的高韧性复合材料进行参数研究。

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.