Section 2 Concluding remarks

2.1 From the example, the highest stress factor occurs in the outer 225 g/m² chopped strand mat reinforcement (in tension) but this is significantly lower than the limiting stress fraction required by the Rules. The bending moment at the centre of the panel is smaller than that at the boundary and consequently, the stress factor will be correspondingly reduced. The design may be optimised by sequentially removing plies, changing reinforcement weights and/or by providing receiving strips under the base of the ‘top-hat’ stiffeners.

2.2 For the design of side shell laminates there are no shear and deflection criteria to be fulfilled. In this example a significant reserve exists between the actual and the ultimate stresses.

2.3 It is of paramount importance that the strain compatibility of the component materials is carefully considered.

2.4 Consider typical values of apparent strain, ∊_a' at failure for the following materials in laminate form:

2.5 The actual strain permissible is controlled by the material with the lowest apparent strain. The level of strain depends upon whether the reinforcements are in tension or compression and depends on their relative positions within the laminate. Consequently if, for example, a carbon fibre reinforcement is used in the outer plies of laminate then the strain must be constrained to a maximum of 0,33 x 0,9 per cent, i.e. 0,297 per cent. Therefore, the corresponding allowable stress in the other reinforcements must be related to the strain in the reinforcement relative to its position away from the neutral axis and that of the carbon fibre reinforcement, e.g.:

2.6 Where aramid reinforcements are being used, special consideration must be given to the compressive properties. For comparison purposes aramid reinforcements, at a fibre content of 0,45 (by weight), typically have the following properties: