The majority of composite structures are designed with fibre dominated mechanical properties. As such the stress/strain relationship of most structural fibres are linear elastic to failure. Thus the fibre-dominated properties of structural composites also behaves in a linear elastic manner to failure. However, some structural configurations can behave in a plastic manner at high stress levels. A series of 'U' channel beams were manufactured from glass fibre composite prepreg fabric. The beams were then loaded in three point bending. Several different laminate lay-up configurations were prepared to examine the laminate configuration effects. The resulting load/deflection curves for all beam laminate configurations experienced non-linear behaviour to failure. Beams tested to 75% of the failure load were removed from the test rig and showed permanent deformation changes. During loading of an open section the load out-of-plane deformation of the flange cause the section to open out. This contributes significantly to the non-linear load/deflection characteristics. The stresses induced into the structure result in a high level of shear stresses. The shear properties of most composite structures are dominated by the matrix material properties, which are non-linear and can result in permanent deformation.
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