Effects of fibre volume fraction on the stress transfer in fibre pull-out tests

The elastic stress transfer taking place across the fibre/matrix interface is analysed for the fibre pull-out test by means of both micromechanics and finite element (FE) analyses. A special focus has been placed on how fibre volume fraction, V_f, affects the interface shear stress fields in the model composites containing both single and multiple fibres. In a so-called ‘three-cylinder model’, where a fibre, a matrix and a composite medium are coaxially located, the constraint imposed on the central fibre due to the surrounding fibres is properly evaluated. It is shown in the FE analysis that the differences in stress distributions between the composite models containing single and multiple fibres become increasingly prominent with increasing V_f. The principal effect of the presence of surrounding fibres in the multiple-fibre composite model is to suppress effectively the development of stress concentration near the embedded fibre end and thus eliminate the possibility of debond initiation from this region for all V_f considered. This is in sharp contrast to the single-fibre composite model, in which the interfacial debond can propagate from the embedded end if V_f is larger than a critical value. These findings are essentially consistent with the results from micromechanics analysis on the same specimen geometry. The implications of the results for the practical fibre pull-out test as a means of measuring the interface properties are discussed.