Stiffness of materials with reference to GRP

The stiffness of common plastics materials is very low compared with steel (flexural modulus of most common thermoplastics 2 GN m 2, modulus of steel 200 GM m 2). Glass reinforcement greatly increases the stiffness of plastics, both thermosetting and thermoplastic - see Philips glass-coupled polypropylene washing machine tank. Chapter 9. For example, the flexural modulus of cast unsaturated polyester resin 3.5 GN m 2, flexural modulus of GRP (glass reinforced polyester resin) SGM m 2. 

There are various other ways of further increasing stiffness of panels, e.g. sandwich construction and use of ribbing (e.g. on motor-car bonnets), and use of curvature (e.g. motor-car wheel arches). Perhaps the simplest way to increase stiffness is to increase the thickness of the part since the stiffness of a rectangular cross-section is roughly proportional to the cube of the thickness (Table 15.1). See also Table 15.2 for thickness requirements of various materials to achieve the same stiffness. 

Three approaches can be used, viz. comparative design, theoretical design, and empirical design. 

If a competitive material has been used for the same, or similar, application it can be used as a basis for determining the wall thickness in, say, GRP. A table is prepared giving thickness of materials having equivalent panel stiffnesses (Table 15.3). The table is used as follows  

a part in aluminium is 1.27 mm thick. For equal stiffness how thick must a hand lay-up part be  

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