Issues in strength models for unidirectional fibre-reinforced composites related to Weibull distributions, fibre packings and boundary effects

Abstract

Three issues are investigated that may influence the accuracy of strength models for unidirectional composites. Firstly, most authors limit themselves to 25-100 single fibre tests to determine the Weibull distribution. This is insufficient to accurately determine this crucial input parameter, leading to significant errors in the predicted composite failure strains. Secondly, random, square and hexagonal fibre packings are shown to lead to a similar predicted failure strain and cluster development. This is the first strong indication that the type of fibre packing has little influence on modelling predictions. Finally, boundary fibres introduced at the model perimeter were shown to prevent preferential cluster formation near the model perimeter. This makes the models less sensitive to the number of fibres. Boundary fibres do not influence the early cluster development, but do increase the critical cluster size. The results provide guidelines for when such boundary fibres should be included.

Synchrotron radiation computed tomography for experimental validation of a tensile strength model for unidirectional fibre-reinforced composites

“…In situ loading allows damage progression to be analysed. The work presented here compares the experimental data with the fibre-bundle model of Swolfs et al [33][34][35][36]. A similar comparison has already been attempted in Scott et al [37].…”

“…This distribution is described by [42]: [42]. This data set closely follows the recommendations of Swolfs et al [36] for setting up an accurate and reliable Weibull distribution: testing a large number of fibres at very short gauge lengths. We therefore consider this data set to be the most reliable one for T700S carbon fibres.…”

“…Alternatively, the presence of the composite matrix might also reduce the in situ defect sensitivity of the fibres when they are surrounded by matrix. While the explanation of a reduced in situ defect sensitivity is speculative, inaccuracies in the Weibull distribution are commonly described in literature [23,36,42,[52][53][54][55][56][57][58].The literature has not yet settled on the most reliable measurement method for the distribution of fibre strength [42,[56][57][58].…”

“…Unfortunately, the literature suggests significant uncertainty exists regarding the reliability of fibre strength data [36,52,57,58]. An alternative approach would be to fit the fibre break density to the data in Fig.…”

“…The model is based on Rosen's chain-of-bundles approach [14] and has already been extensively described [33][34][35][36]. Each fibre was divided into fibre elements, which were assigned a Weibull strength according to their length.…”