Characterization of fibre alignment in as-received aerospace grade unidirectional prepreg

Analysis of the fibre dominated properties of composite materials typically assumes, often implicitly, the perfect alignment of fibres. Increasingly, however, it is acknowledged that waviness (in-plane misalignment) and wrinkling (out-of-plane misalignment) play an important role in both manufacturing and structural response. In this paper, a previously developed experimental method to measure fibre misalignment has been improved and extended to also determine fibre excess length. Results are presented for two unidirectional aerospace grade prepregs (8552 and 8552-1 resins reinforced by AS4 fibre). The fibre alignment distributions are well fitted by an analytical model that allows for a distribution of excess lengths; the fibre excess length is not constant at the end of the prepregging process. It is shown that only with a distribution of excess lengths can the experimental unimodal misalignment distributions be analytically modelled. Misalignment distributions presented in this paper are consistent with those reported elsewhere, and furthermore it is shown that the fibres were significantly more aligned out-of-plane than in-plane. Finally, contrary to the best working hypothesis in the literature, samples prepared using a thermal cure process saw no correlation between fibre misalignment and initial location on a prepreg roll.