H.Y. Chou

Fibre Break Failure Processes in Unidirectional Composites. Part 1: Failure and Critical Damage State Induced by Increasing Tensile Loading

The purpose of these three papers is not to just revisit the modelling of unidirectional composites. It is to provide a robust framework based on physical processes that can be used to optimise the design and long term reliability of internally pressurised filament wound structures. The model presented in Part 1 for the case of monotonically loaded unidirectional composites is further developed to consider the effects of the viscoelastic nature of the matrix in determining the kinetics of fibre breaks under slow or sustained loading. It is shown that the relaxation of the matrix around fibre breaks leads to locally increasing loads on neighbouring fibres and in some cases their delayed failure. Although ultimate failure is similar to the elastic case in that clusters of fibre breaks ultimately control composite failure the kinetics of their development varies significantly from the elastic case. Failure loads have been shown to reduce when loading rates are lowered.

Intrinsic Safety Factors for Glass & Carbon Fibre Composite Filament Wound Structures

The determination of intrinsic safety factors for glass and carbon fibre unidirectional composites and filament wound internally pressurised structures, is described. In such structures the fibres are placed on geodesic paths and the pressure induces tensile forces in them. The fibres ensure the strength of the composite and must break for it to fail. Failure is seen in such structures, to depend mainly on the accumulation of fibre breaks. These are initially randomly distributed but become critical when clusters of breaks develop. Long term behaviour of carbon fibre composites is controlled by the viscoelastic relaxation of the matrix around breaks, which can lead to further delayed fibre breaks. Failure in glass fibre structures can additionally be induced by stress corrosion of the glass fibres. This process does not seem to occur with carbon fibres and as the latter are increasingly used in critical structures emphasis is given to them. Until the development of clusters of fibre breaks, in a filament wound structure, no macroscopic changes in the composite behaviour are evident so that failure occurs in a sudden death manner. Multi-scale simulation, taking into account the characteristics of the composite components and scaling up their behaviour under load, accurately describes the overall behaviour of the composite structure. This approach not only allows the behaviour to be described, as a function of time, but also calculates the scatter which will occur in the behaviour of the structure. This allows the intrinsic safety factors of the composite structure to be quantified.

Effect of the Loading Rate on Failure of Composite Pressure Vessel

A multi-scale model has been successfully applied to the simulation of the effects of pressurisation rate on damage accumulation in carbon fibre/epoxy plates and composite pressure vessels. The results of the simulations agree with experimental results and reveal that the point at which the structures become unstable in a monotonic pressurisation test depends on the speed of loading. The faster the loading rate the higher the applied stress at which the composite structure becomes unstable. The mechanism which governs this behaviour is seen to be the viscoelastic nature of the matrix material through which stresses are transferred from broken to neighbouring intact fibres. At loading rates that allow greater relaxation of the resin around fibre breaks neighbouring fibres are subjected to increased loads over a significantly greater length, leading to further earlier breaks.Copyright

Intrinsic Mechanisms Limiting the Use of Carbon Fiber Composite Pressure Vessels

The viscoelastic properties of the resins used in carbon fiber composite pressure vessels introduce time effects which allow damage processes to develop during use under load. A detailed understanding of these processes has been achieved through both experimental and theoretical studies on flat unidirectional specimens and with comparisons with the behavior of pressure vessels. Under steady pressures, the relaxation of the resin in the vicinity of earlier fiber breaks gradually increases the sustained stress in neighboring intact fibers and some eventually break. The rate of fiber failure has been modeled based only on physical criteria and shown to accurately predict fiber failure leading to composite failure, as seen in earlier studies. Under monotonic loading, failure is seen to be initiated when the earlier random nature of breaks changes so as to produce clusters of fiber breaks. Under steady loading, at loads less than that producing monotonic failure, greater damage can be sustained without immediately inducing composite failure. However, if the load level is high enough failure does eventually occur. It has been shown, however, that below a certain load level the probability of failure reduces asymptotically to zero. This allows a minimum safety factor to be quantitatively determined taking into account the intrinsic nature of the composite although other factors such as accidental damage or manufacturing variations need to be assessed before such a factor can be proposed as standards for pressure vessels.

Visual End-of-Life Criterion for Composite High Pressure Vessels for Hydrogen Storage

The storage of hydrogen at pressures clearly presents a challenge to designers of pressure vessels to ensure their utmost reliability. At present there is a paucity of information on the reliability of carbon fibre composite pressure vessels which are intended for use over periods of decades and those testing procedures which are mentioned in standards have been shown to be largely unsuitable as they are based on the failure of metal structures. This study deals with failure and failure prediction in advanced fibre reinforced composite pressure vessels. The aim of this study has been to evaluate the possibility of providing a simple, cost effective and unambiguous visual means of detecting approaching failure at a stage when the pressure vessel can be safely removed from service. It is proposed to benefit from the wide family of carbon fibres which is produced so as to add a layer of carbon fibre composite material, the fibres of which have the same stiffness as the carbon fibres in the bulk of the structure but which possess lower strength. The failure of this layer would provide advanced warning of progressive and unacceptable damage to the pressure vessel and would avoid any ambiguous interpretation.Copyright