Compressive Failure of Carbon Fibre Composites due to Instability at Structural, Material and Constituent Level
Topic(s) : Special Sessions
Co-authors :
Michael R. WISNOM (FRANCE)Abstract :
Compressive failure is controlled by instability at three different levels. Firstly there is overall stability at the structural level. Secondly there is shear instability at the material level, and thirdly at the constitutent level there can be instability within the fibres themselves. This paper considers these three mechanisms and the way they may interact to control compressive behaviour of carbon fibre composites.
Structural stability affects all materials, but is particularly important in carbon fibre composites because the elastic modulus decreases with strain. Fitting stress-strain data for high strength carbon/epoxy shows a reduction in tangent modulus of 50% or more at high strains.
This non-linearity can cause a big reduction in buckling stresses at high strains. For example simple Euler buckling calculations for rectangular specimens 2 mm thick and 20 mm long built in at both ends gave a buckling stress of 1556 MPa when fibre-direction non-linearity was taken into account, compared with 2101 MPa when response was assumed to be linear elastic.
There can also be an interaction between the fibre direction material nonlinearity and the geometrical nonlinearity. Classical Euler buckling theory indicates a neutral stability when the critical buckling load is reached, with increasing lateral displacements at constant load. But as a result of the reduction in tangent modulus with strain, in carbon fibre composites even simple strut buckling can produce unstable behaviour as the load has to reduce with increasing displacement to maintain equilibrium.
Failure of carbon fibre composites typically occurs due to shear instability caused by fibre misalignment and the non-linear shear response of the composite. This is quite well understood, and there are many experimental and modelling studies explaining how the instability develops and leads to kink bands. But the interaction of this shear instability with structural instability is less appreciated. In many situations compressive failure may start with structural instability, but with failure then occurring due to material shear instability.
There is also potentially instability within the fibre itself, whereby the graphite planes buckle, leading to a kink band such as in Fig. 2. This mechanism is the origin of the fibre non-linearity, and may also lead to failure of the fibres. Failure of high modulus carbon composites may occur by fibre fracture rather than instability, and this is sometimes referred to as shear fracture, but is actually controlled by instability at the local level. This fibre instability may interact with the composite instability, causing differences in the compressive failure mechanism and strain in carbon fibre composites hybridised with other materials.
These three mechanisms and the way they interact will be presented, with examples showing how the compressive failure strain can vary considerably depending on the composite architecture and loading.
Structural stability affects all materials, but is particularly important in carbon fibre composites because the elastic modulus decreases with strain. Fitting stress-strain data for high strength carbon/epoxy shows a reduction in tangent modulus of 50% or more at high strains.
This non-linearity can cause a big reduction in buckling stresses at high strains. For example simple Euler buckling calculations for rectangular specimens 2 mm thick and 20 mm long built in at both ends gave a buckling stress of 1556 MPa when fibre-direction non-linearity was taken into account, compared with 2101 MPa when response was assumed to be linear elastic.
There can also be an interaction between the fibre direction material nonlinearity and the geometrical nonlinearity. Classical Euler buckling theory indicates a neutral stability when the critical buckling load is reached, with increasing lateral displacements at constant load. But as a result of the reduction in tangent modulus with strain, in carbon fibre composites even simple strut buckling can produce unstable behaviour as the load has to reduce with increasing displacement to maintain equilibrium.
Failure of carbon fibre composites typically occurs due to shear instability caused by fibre misalignment and the non-linear shear response of the composite. This is quite well understood, and there are many experimental and modelling studies explaining how the instability develops and leads to kink bands. But the interaction of this shear instability with structural instability is less appreciated. In many situations compressive failure may start with structural instability, but with failure then occurring due to material shear instability.
There is also potentially instability within the fibre itself, whereby the graphite planes buckle, leading to a kink band such as in Fig. 2. This mechanism is the origin of the fibre non-linearity, and may also lead to failure of the fibres. Failure of high modulus carbon composites may occur by fibre fracture rather than instability, and this is sometimes referred to as shear fracture, but is actually controlled by instability at the local level. This fibre instability may interact with the composite instability, causing differences in the compressive failure mechanism and strain in carbon fibre composites hybridised with other materials.
These three mechanisms and the way they interact will be presented, with examples showing how the compressive failure strain can vary considerably depending on the composite architecture and loading.