INVESTIGATION OF COMPLEX FAILURE MODES IN FIBER-BUNDLES DURING DYNAMIC-MECHANICAL TESTING USING ACOUSTIC-EMISSION AND WEIBULL STATISTICS

Mechanical strength studies have been carried out on fibre bundles use d in composite manufacture. Logarithmic Weibull plots derived from dyn amic fibre bundle tensile tests, involving acoustic emission (AE) tech niques are not linear over the entire fibre bundle failure strain rang e. This makes it impossible to use the two-parameter Weibull strength distribution function to describe fully a fibre bundle response in dyn amic situations. The plots exhibit portions of different slopes, with no sharp boundaries demarcating them. This is attributed to the overla pping of the various fibre failure modes occurring with increasing fib re bundle strain. AE event-strain (fibre failure) analysis showed that with increasing strain, the fibre failure mode changes from predomina ntly singlets (a single-fibre failure at a time) to doublets (simultan eous failure of two fibres), and then higher multiple fibre failure mo des. The various failure modes overlap about the maximum fibre bundle stress, and each multiple fibre failure mode contributes towards the c ombined Weibull plot with a slope of the corresponding multiple of the slope due to the singlet fibre failure mode. In the light of these ob servations, we have modified the two-parameter Weibull function, which is valid only when singlets are dominant, to include contributions fr om higher order fibre failure modes for a better description of fibre bundles dynamic stress-strain responses. The fit between theory and ex perimental 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, partic ularly about and beyond the maximum bundle stress position.