The influence of void content on the structural flexural performance of unidirectional glass fibre reinforced polypropylene composites

Voids are often present in composite materials and have, as may be expected, negative effects on the mechanical properties. As a consequence, it is regarded to be important to optimise manufacturing techniques of composite materials towards minimum void content. However, it is not obvious that voids also have negative effects on the properties of a structure since those are not just determined by the properties of the material, but also by the dimensions. For a given mass of material, void will increase the dimensions and as a result, for example, the moment of inertia. Thus in principle, it is possible that void may actually have positive effects on the properties of a structure. In this paper this hypothesis is evaluated by studying the influence of void content on the flexural properties of beams manufactured by compression moulding multiple unidirectional commingled glass/polypropylene fibre tows. By varying the time under moulding pressure, beams with void contents between 1 and 14% could be manufactured (with the mass, width and length fixed, and thus only the height free). As expected, voids were found to have a negative effect on the flexural modulus and strength, which both decreased by about 1.5% for each 1% of voids. However, what is more interesting is the fact that voids actually had a clear positive effect on the beam stiffness EI ; which increased by about 2% for each 1% of voids. For example, beams which contained 14% porosity exhibited about 28% higher EI than beams with less than 1% porosity. Moreover, the flexural failure load did not decrease up to a void content of 14%. On the contrary, a least square fit suggested a weak increase with void content. It can therefore be concluded that, in this case, voids actually have positive effects on the structural flexural performance.