Investigation of complex failure modes in fibre bundles during dynamic mechanical testing using acoustic emission and Weibull statistics

Abstract

Mechanical strength studies have been carried out on fibre bundles used in composite manufacture. Logarithmic Weibull plots derived from dynamic fibre bundle tensile tests, involving acoustic emission (AE) techniques are not linear over the entire fibre bundle failure strain range. This makes it impossible to use the two-parameter Weibull strength distribution function to describe fully a fibre bundle response in dynamic situations. The plots exhibit portions of different slopes, with no sharp boundaries demarcating them. This is attributed to the overlapping of the various fibre failure modes occurring with increasing fibre bundle strain. AE event-strain (fibre failure) analysis showed that with increasing strain, the fibre failure mode changes from predominantly singlets (a single-fibre failure at a time) to doublets (simultaneous failure of two fibres), and then higher multiple fibre failure modes. The various failure modes overlap about the maximum fibre bundle stress, and each multiple fibre failure mode contributes towards the combined Weibull plot with a slope of the corresponding multiple of the slope due to the singlet fibre failure mode. In the light of these observations, we have modified the two-parameter Weibull function, which is valid only when singlets are dominant, to include contributions from higher order fibre failure modes for a better description of fibre bundles dynamic stress-strain responses. The fit between theory and experimental data appears to confirm the role played by the higher order fibre failure modes in changing the slope of the Weibull plots and in defining the shape of the fibre bundle stress-strain response, particularly about and beyond the maximum bundle stress position.