UBC Theses and Dissertations
Misalignment defects in unidirectional composite materials Stewart, Andrew Lawrence
The properties of reinforced composite materials are dominated by the orientation of their reinforcing fibres. This orientation is typically assumed, often implicitly, to be perfect leading to over-predictions of the final part’s strength. Traditional approaches for measuring misalignment are laborious which has limited their adoption. Further, as the desire for manufacturers to collect more and more data increases, both the data collection and reduction techniques must become automated in order to create value. In this thesis, several large data sets are created and ultimately analyzed using purpose-built automated scripts. One of these techniques analyzed over 2500 high resolution micrographs and returned information on over 200,000 fibres. Using the information from this analysis allowed the creation of an analytical model for the fibre bed. This phenomenological model uses the calculated excess length distribution to individually assign a unique excess length to each fibre in the system. It is shown that only with a distribution of excess lengths can the experimental unimodal misalignment distributions be properly modelled. Homogeneously dispersed variability was associated with each of the measured values which included in-plane and out-of-plane fibre alignment, cured and uncured ply thickness, and fibre volume fraction. Save the alignment distribution which lacks a standardized quality descriptor, the other metrics bounded the manufacturer’s data sheet values; however, these measurements showed that a non-trivial amount of variability should be expected in even high quality, aerospace grade, prepregs. A separate series of tests were developed which were able to impart small compressive strains into the compliant uncured prepreg. The localization of the uniformly distributed wrinkles was partially attributed to the homogeneous variability of the prepreg’s underlying architecture. These slow forming wrinkles were shown to have a consistent set of mechanics as fast tool-part debonding. A hypoelastic shear-lag relationship was developed which was able to predict the excess length introduced into the prepreg from the tool. This shear-lag approach predicts a zone of influence for the wrinkles which was experimentally determined using wrinkle initiators. Reducing tool-part interaction or rapid quenching were proposed as mitigation strategies for wrinkle management.
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