It is shown that the strength of fibrous composite laminates has not been well represented by the innumerable abstract mathematical theories for anisotropic elasticity. Only a fibre, a matrix, or the interface can fail; not the ‘lamina’ which is a useful concoction for establishing the elastic constants only. The classical maximum-shear-stress failure criterion for ductile metals is extended to include orthotropic materials like carbon fibres. The greatest differences with respect to past theories are found for biaxial loads, both when the load is of the same sign as well as of the opposite. A comparison with available test data is included.
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