Reinforced Plastic Analysis

For RPs, the emphasis and difficulty in the design analysis depends on the nature of the RPs. For a thermoplastic reinforced with short fibers, the viscoelastic nature of the matrix remains an important factor, and the discussion given above for unreinforced plastics is relevant. In addition, there may be a significant degree of anisotropy and/or inhomogeneity due to processing that could further complicate the analysis (Chapter 2). For thermosets reinforced with short fibers (for example, BMC; Chapter 1) there may be only a low level of visco-elasticity, anisotropy, and inhomogeneity, and metals-type design analysis may be a reasonable approximation. However, thermosets reinforced with long fibers can have a high degree of anisotropy (depends on lay up of reinforcement), and this must be taken into account in the design analysis. When thermoplastics are reinforced with long fibers there may be significant anisotropy and viscoelasticity, and this creates a potentially complex design analysis situation. In all cases, RPs failure characteristics may be specified in terms of a critical strain, and this requires the design analysis to be performed for stress and strain.

Long-fiber materials can often be tailored to the product requirements, and therefore materials design analysis and component design analysis interact strongly. If the component design analysis is statically determinate (stresses independent of materials properties) then this can be carried out first, and then the material can be designed to carry the stresses in the most efficient manner. However, if the analysis is not statically determinate, then the component stresses depend on material anisotropy, and material and product design have to be carried out and optimized at the same time. This is also the case if component shape is regarded as one of the variable design parameters.

In summary, it can be seen that plastics and RPs design analysis follows the same three steps (a) to (c) as that for metals, but there are some differences of emphasis and difficulty. In particular, step (a) is usually more substantial for the newer materials, partly because a full stress/strain/deformation analysis is required and partly because of the need to take account of viscoelasticity, inhomogeneity, and/or anisotropy. For long fiber materials, the component design analysis may need to contain the associated material design analysis.

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