Effect of the loading rate on ultimate strength of composites. Application: Pressure vessel slow burst test

The strength of unidirectional elastic fibre composites is shown to depend on the loading rate as the viscoelastic nature of the matrix results in a fall in breaking load as the rate is reduced. The simulation of the accumulation of fibre breaks leading to failure, takes into account all physical phenomena involved fibre failure, including the stochastic nature of fibre strength, stress transfer through the matrix between reinforcements, interfacial debonding and the viscoelastic nature of the matrix. The kinetics of composite failure are seen to involve the initial formation of random fibre breaks which at higher loads coalesce into clusters of broken fibres. The rate at which this occurs depends on the viscoelastic properties of the matrix. These results have been shown to closely mirror the behaviour of carbon fibre composite pressure vessels.