Effect of moisture on composite performance

In a Q°/9Q°/Q° laminate, Eqn (12.9) applies to the inner ply, but the thermal strain in the transverse direction of outer 0° plies will be slightly different and given by Eqn (12.10). The tensile thermal strain in the transverse (f) direction of the longitudinal (0 ply (e ) is given by 

The first failure of a composite, such as a simple 0°/90°/0° laminate usually occurs by transverse or matrix cracking of the 90° or transverse ply. Since the residual thermal stress in this direction is tensile, the first ply failure will occur at a lower applied strain [21]. 

To balance the tensile thermal stresses, the plies will go into compression in the longitudinal direction. Thus, the thermal strain in the longitudinal (/) direction of the longitudinal (J) plies ( ) is given by 

Therefore, the 0° plies will fail at a slightly higher strain because of the induction of a compressive stress into the fibres. 

Under multidirectional stresses such as an impact load, the fibres in the transverse plies will fail at a higher resolved strain. 

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