Quantifiable analysis of the failure of advanced carbon fibre composite structures leading to improved safety factors

The increasing use of advanced composite materials means that they must now be considered as major materials for a wide variety of structures some of which will be in service for decades. There is therefore an important need to be able to quantify damage accumulation leading to failure in these materials particularly as they are often used in extreme situations for which failure must be avoided. This review shows how damage accumulation in many major composite structures is dominated by fibre failure but that the viscoelastic nature of the matrix induces time effects including delayed failure. It is shown that damage accumulation can be quantitatively modelled using a multi-scale approach linking damage at the fibre level to the overall reliability of the structure. The structures of particular interest are composite pressure vessels. At the microscopic level it is necessary to identify a representative volume element which includes all the processes which govern composite behaviour and failure. In this way the macroscopic loading of the composite structure can be evaluated at every point and overall behaviour qualitatively assessed. This allows the effects of loading to failure to be quantitatively calculated as well as the effects of loading at a constant level for long periods. It is shown that failure occurs when clusters of fibre breaks reach a critical level as has been observed experimentally. The effects of speed of loading and long term failure have also been observed experimentally as predicted by the sim-ulations of structures including filament wound carbon fibre composite pressure vessels. This leads to the quantitative analysis of intrinsic safety factors for advanced composite structure based on a knowledge of the physical damage processes involved.