Abstract

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.

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