An improved fragmentation model to assist the selection of fibres in hybrid composites

High performance composites used in lightweight structures make use of carbon fibres and a polymer matrix (most often thermosets such as epoxy). Many different efforts have been made to enhance their fracture toughness, at the constituent or at the ply level. Interlaminar toughness has been increased by modifying the matrix with thermoplastic particles. However, intraply fracture of a unidirectional composite involves small deformations and it is essentially brittle. Fibre hybridization, that is, making use of fibres of different properties, has raised the prospect to escape from brittleness and to achieve a pseudo-ductile behaviour. There have been attempts at different scales: at the laminate scale by mixing plies of different reinforcements, at the ply level by developing fabrics with different fibres, or at the tow/bundle level. This last possibility is particularly appealing because tow hybridization avoids the detrimental stress concentrations found at the other scales, and because the differences in the mechanical properties of the fibres is expected to lead to the best synergetic effect. There are, however, remarkable technological challenges to achieve hybrid tows/bundles. Currently, however, the decision of which fibres should be mixed, and at which ratio, to achieve a desired mechanical effect is an objective and informed decision. Attempts to base this choice on qualitative arguments has not lead to the best results and the explanation of the positive synergetic effects of hybridization remains a matter of debate. Computational micromechanics will certainly bring light to this topic, but it is a complex and time consuming approach. In this communication we revise an advanced analytical fragmentation model, and illustrate the capability to estimate the tensile response of a unidirectional hybrid composite.