SAE - CAE Design and Failure Analysis of Automotive Composites

Organization: SAE
Publication Date: 3 December 2014
Page Count: 144
scope:

Introduction

Composites are extensively used in applications that need outstanding mechanical properties combined with weight savings. Composite materials possess superior properties because of their unique microstructure. A composite is a material system that consists of two or more separate materials combined in a macroscopic structural unit. Unlike traditional materials (such as metals, ceramics, and polymers), whose microstructures are relatively fixed, composites are highly tunable in terms of microstructure and mechanical properties. As a result, composites are a desirable combination of the best properties of the constituent phases: they can be strong and lightweight at the same time. For example, carbon fiber composites can be more than 10 times stronger and 80% lighter than steels. With such extraordinary properties, composites have become the top choice for producing lightweight vehicles [1-1], [1-2], [1-3], [1-4], [1-5], [1-6], [1-7], [1-8]. The benefits of composites go far beyond weight savings. Polymer matrix composites have great potential for part integrations, which will result in lower manufacturing costs and faster time to market. The composite parts can have much smaller tooling costs than do metal ones. Composites also have much better corrosion resistance than metals and are more resistant to damage, such as dents and dings, than aluminums. Polymer composites possess superior viscoelastic damping and thus provide the vehicles with improved noise, vibration, and harshness (NVH) performance. Composites also have a high level of styling flexibility in terms of deep drawn panel, beyond what can be achieved with metal stampings. Finally, composite materials can possess multifunctional (mechanical, thermal, electrical, and magnetic) properties by integrating various functional components into the polymer matrices. The so-called multifunctional or smart composites provide significant benefits to the vehicles when compared with traditional materials, which only have monotonic properties.

Although the benefits of composites are well recognized, the use of composites in the automotive industry has faced some technical challenges. One major technical challenge has been the lack of knowledge in composites design. Traditionally, the automotive sector has designed structural components by using isotropic materials, such as steels, aluminums, and plastics. The basic material properties necessary to the design of a homogeneous structure are Young's modulus (E), Poisson's ratio (n), and failure strength (sf). These properties for common materials, such as steel and aluminum, are readily available in materials handbooks and online resources, making the overall design process of a structural component composed of an isotropic material relatively simple. In comparison, the design of structures involving anisotropic, composite materials is more challenging and complicated. A composite material is anisotropic in nature; that is, the properties at a point vary with direction of the reference axes and are associated with the scale. The basic material properties necessary to the design of a composite structure are the average properties of an individual lamina. Unlike conventional isotropic materials whose properties (E, n) are available in various data sources, the properties of the lamina for a composite system cannot be readily found. The primary reason is that those properties are dependent upon the fiber volume fractions. Even for the same composite system, such as the carbon fiber-epoxy composite, the basic lamina properties vary dramatically due to the amount of fibers used in the system. Therefore, it would be very difficult to establish a comprehensive composite material property database.

The other major technical challenge in using composite materials is the lack of effective design tools,(i.e., the computer-aided engineering [CAE] tools). Although the automotive sector has been routinely using CAE methods for various structural analysis (static, dynamic, durability, noise and vibration, etc.), the practices have mostly involved isotropic materials. For isotropic materials, there are many choices of CAE software, and the precision and accuracy of the computational models have significantly increased over time. However, for anisotropic, fiber composite materials, few CAE software exists that is capable of composite modeling. There is also a lack of sufficient, rigorous models to simulate the sophisticated failure process of composite structures.

This book focuses on the latest use of CAE methods in design and failure analysis of composite materials and structures. It begins with a brief introduction to the design and failure analysis of composite materials and then presents some recent, innovated CAE design examples of composite structures by engineers from major CAE developers and automobile original equipment manufacturers (OEMs) and suppliers.

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