On the tension-compression switch hypothesis in arterial mechanics

The tension-compression switch hypothesis for soft tissue proposes that when collagen fibres are compressed they do not contribute to the mechanical response which is then assumed to be the isotropic response of the extra-cellular matrix in which they are embedded. Such an assumption would seem reasonable. However, three difficulties with its use are discussed here. First it is shown that the admittedly scant experimental evidence available suggests that it does not have a completely sound physical basis. The possible paradoxical nature of the tension-compression switch has been previously identified by other authors. If the fibres are predicted to be in compression by an anisotropic model, then the resulting isotropic stretch of the fibres predicted by the tension-compression switch can be tensile. Thus the fibres can have a duality of being simultaneously tensile and compressive if the tension-compression switch is employed. This is illustrated here with a particular emphasis on the nature of the isotropic stretching of the fibres when compressed and the essential role of the linear theory when predicting the mechanical response for non-linear deformations of arterial tissue. Finally it is shown that use of the tension-compression switch can result in the failure of some models to predict the mechanical response in the material characterisation tests that were used to determine the values of their material constants. Thus some anisotropic models can have better predictive capability without the tension-compression switch.