Microbuckle initiation from a patch of large amplitude fibre waviness in acomposite under compression and bending

A finite element couple stress formulation is used to predict microbuckle i nitiation from a patch of fibre waviness in a unidirectional fibre composit e under remote compression and bending. Attention is focused on the knock-d own in strength due to large amplitude waviness, with the effects of the ph ysical size of the imperfection included by incorporating the fibre bending resistance within the formulation. The predicted strengths deviate signifi cantly from the simpler kinking theory which neglects the role of fibre ben ding. Initial imperfections in the form of an infinite band and a circular wavy patch are considered: when these imperfections are of large spatial ex tent and possess a large misalignment angle, the compressive strength appro ximates the steady state band broadening stress for an infinite band. The e ffect of an imposed spatial gradient of stress within the composite is expl ored by determining the compressive strength of beams of finite height B fo r the loading cases of pure bending and axial compression. It is found that the compressive strength is sensitive to the magnitude of the imposed stre ss gradient: the compressive strength of the outer fibres of the beam in be nding increases with diminishing height of the beam. This size dependence i s much reduced for the case of uniform compression. (C) 2001 Editions scien tifiques et medicales Elsevier SAS.