• Document: Strength Assessment of Injection Molded Short- Fiber-Reinforced Plastic Components
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Strength Assessment of Injection Molded Short- Fiber-Reinforced Plastic Components Wolfgang Korte, Marcus Stojek, Sascha Pazour PART Engineering GmbH, Germany Abstract: Components made of short-fiber-reinforced plastics (SFRP) are stressed highly both mechanically and thermally. Therefore an intelligent component design is required in order to fully exploit the potential of these materials. Hence, the design of such components must be based on a reliable strength assessment. For this purpose models for the description of the anisotropic and elasto-plastic failure behavior of SFRP are required. In contrast to the widespread use of SFRP, methods for a reliable strength assessment based on FE analyses for components made of these materials have not been sufficiently developed yet. This paper presents an approach for the strength assessment of SFRP components based on FE analyses. In the scope of this appropriate failure limits and failure criteria for these materials are presented. Keywords: Anisotropy, Failure Criterion, Injection Molding, Polymer, Plastics, Short-Fiber- Reinforced Composites, Tsai-Hill, elasto-plastic 1. Introduction The use of injection molded SFRP parts take place in many different industries. In particular, in the automotive industry SFRP are increasingly being used as the preferred engineering plastic. This is due to the excellent mechanical and thermal properties of these materials compared to non- reinforced grades. The use of these materials enable the manufacturer of automotive components to a significant weight reduction compared to metallic materials and lower manufacturing costs as well due to the fact that a plastic part is typically a finished part without any further processing necessary. The molding process is the cause for the formation of a specific microstructure within the molded part, which in turn is the root cause for the mechanical properties of the part. Especially for plastics this microstructure can be molecular orientations or in the case of SFRP fiber orientations, leading to an anisotropic material behavior. In this sense it can be said that the material is composed during the molding process. In figure 2 the effect of fiber orientation onto the mechanical behavior of a plastic is shown. 2015 SIMULIA Community Conference 1 www.3ds.com/simulia Figure 1: Influence of Fiber Orientation on Material Properties It is evident that due to the described strong impact of the injection molding process on the properties of the manufactured part itself the structural analyst is interested in considering the effect of anisotropy on part stiffness and strength. Such a consideration may result in a more accurate prediction of the mechanical behavior of the investigated component. Whereas the proper description of the stress-strain behavior of the part is a question of a suitable material model that in the case of SFRP is e.g. capable to consider anisotropic elasto-plastic behavior the proper assessment of the part failure is a question of a suitable failure limit and failure criterion. In this paper is the focus on the failure assessment, assuming that a suitable material model was already chosen in order to calculate the local stresses and strain in the part properly. 2. Local Component Properties In technical parts there is not a uniform component strength rather than a local strength distribution depending on manufacturing and design. Particularly for injection molded SFRP parts the material is generated during the molding, depending on the position in the part the material possess different properties dependent on the local fiber orientation. Additional factors that 2 2015 SIMULIA Community Conference www.3ds.com/simulia influence the local component strength are the degree of multiaxiality and to which amount the local stress on the component surface propagates into the cross section. In the following the influence of these different factors will be described. Influence of fiber orientation As already outlined SFRP have significant different mechanical properties parallel and transverse to the fiber direction. Opposed to unidirectional endless fiber reinforced composites where the part is composed of several unidirectional layers for injection molded SFRP parts there exist a fiber orientation distribution instead of ideally aligned fibers. The orientation state of these fibers at every discrete location in the part can be described more in a probabilistic rather than in a deterministic manner. It is evident that for nearly ideal aligned fibers (narrow fiber distribution curve) different mechanical properties result than for nearly randomly oriented fibers (broad fiber distribution curve). This has to be considered in the assessment of the local strength of the part (Figure 2).

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