Alain Thionnet

Damage accumulation processes and life prediction in unidirectional composites

A parallel has been made between the design and the failure of some composite structures, such as pressure vessels, and that of a unidirectional carbon fibre-reinforced epoxy resin. This type of composite has many characteristics that make it attractive, however, long-term damage accumulation in it, when under load, is not understood in any quantitative manner. It is therefore necessary to identify the processes involved in the degradation of the composite when subjected to long-term loading. Acoustic emission (AE) has been used to detect damage in the composite material and to relate the microstructural damage processes to the activity recorded from composite specimens through the development of a detailed analytical model of the damage processes involved. A multiscale model beginning at the scale of individual fibres and the surrounding matrix has been developed by taking into account the accumulated effects of individual fibre breaks on the behaviour of the whole composite and therefore explains the AE detected during the tests.

Life prediction for carbon fibre filament wound composite structures

The processes governing the failure of filament wound composite structures have been examined. It is shown that the fibres controlling the failure of such a structure, when it is internally pressurised, can be considered to be subjected only to tensile loads. A multi-scale model has been developed which considers the effects on the scale of the elastic fibres and includes the effects of the viscoelastic matrix as well as debonding around fibre failures. The intact fibres neighbouring fibre-breaks are subjected to an increase in stress and a higher probability of failure than elsewhere in the composite. During a monotonic failure test, the initially random fibre failures are seen to begin to coalesce in a way governed by the stochastic nature of the fibre-breaks and this eventually leads to failure. Under prolonged loading, relaxation of the matrix around fibre-breaks causes overloads in the neighbouring fibres to evolve and induce delayed fibre-breaks, which eventually lead to instability in the structure. The model allows these processes to be considered in calculating the behaviour of the whole composite structure.

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.

Stochastic factors controlling the failure of carbon/epoxy composites

The intrinsic scatter in tensile properties of unidirectional (UD) carbon/epoxy composites is due to several factors, including variability in fibre strength and fibre volume fraction at the local microscopic level. A model included in a multiscale finite element process, previously developed to simulate fibre failure in composite laminates but having little variation in fibre strength and local fibre volume fraction, has been extended to cover the effects of such material variations. The present study investigates the effects of the variability in material properties which can occur in real UD composite materials subjected to monotonic increasing and sustained loadings. This latter case is one of the originalities of this study. In the interval of variation studied for the Weibull parameters of fibre strength and fibre volume fraction, the mean and standard deviation of the failure stress are never strongly affected. Concerning the time-to-failure, its mean and its standard deviation increase strongly if the mean of fibre volume fraction increases and if the standard deviation of the fibre strength decreases. The standard deviation of local fibre volume fraction was found to have only a secondary effect on failure stress and time-to-failure. Another original and important result concerns the scatter in the time-to-failure of composites due to the level of applied sustained loading.

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.

Modelling of failure of woven composites. Part 1 : Nomenclature defining the interzone concept

AbstractThe failure of woven composites has been examined. This study is presented in two parts:nModelling of failure of woven composites. Part 1: nomenclature defining the interzone concept;Modelling of failure of woven composites. Part 2: experimental and numerical justification of the interzone concept.n In the first part, the concepts of the interzone and the geometry of an interzone have been defined in a general way for a large panel of woven composites. In the second part, it has been shown that the failure of woven composites is well described by using the interzone concept. The load transfer between intact interzones and broken interzones has been evaluated for two types of loadings (tensile loading and loading in bending). The analysis of these load transfers explains why in the case of a tensile loading the failure is of a sudden-death type whereas in the case of bending loading the failure is progressive. The concept of failure of an interzone has been also defined.

Modelling of Failure of Woven Composites. Part 2: Experimental and Numerical Justification of the Interzone Concept

AbstractThe failure of woven composites has been examined. This study is presented in two parts: nModelling of failure of woven composites. Part 1: nomenclature defining the interzone concept;Modelling of failure of woven composites. Part 2: experimental and numerical justification of the interzone concept.n In the first part, the concepts of the interzone and the geometry of an interzone have been defined in a general way for a large panel of woven composites. In the second part, it has been shown that the failure of woven composites is well described by using the interzone concept. The load transfer between intact interzones and broken interzones has been evaluated for two types of loadings (tensile loading and loading in bending). The analysis of these load transfers explains why in the case of a tensile loading the failure is of a sudden-death type whereas in the case of bending loading the failure is progressive. The concept of failure of an interzone has been also defined.

The control of the residual lifetimes of carbon fibre-reinforced composite pressure vessels

The understanding of the degradation of carbon fibre composites, with emphasis on the use of these composites in filament-wound pressure vessels, is explored. Earlier studies by many researchers have led to a general appreciation of the mechanisms involved; however, only recently have both computational power and experimental techniques become sufficiently developed to allow for the use of quantitative analyses. It is shown that damage is controlled by fibre failure, and that initially this occurs randomly within the structure. In monotonic loading, the development of clusters of fibre breaks causes rapid failure; however, under maintained loads the kinetics of damage evolution are markedly different, and final strength depends on the rate of loading. The results have direct implications for the use of composite pressure vessels, suggesting that their design and testing can be adapted to ensure long-term reliability. The ability to quantify damage accumulation in carbon fibre–composite pressure vessels allows for their intrinsic safety factors to be postulated.

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

7.22 Health Monitoring of High Performance Composite Pressure Vessels

The most important form of damage in carbon fiber reinforced composite pressure vessels is the failure of the fibers however the rate of fiber failure is controlled by the viscoelastic nature of the matrix, which determines overall in-service lifetimes. This type of damage is very different from that encountered with metal pressure vessels and requires a detailed understanding in order to ensure reliability. Innovative proof testing methods based on these processes are necessary. The damage processes and the means of quantifying them are discussed. Their reliability under pressure over periods of decades is analyzed. Intrinsic safety factors linked directly to the properties of the composite components are proposed.