Longitudinal and transverse strain sensitivity of embedded carbon fibre sensors

Piezoresitivity describes the change in electrical resistance of a conductor due to an applied strain. Previous studies showed that certain ex-PAN carbon fibres exhibit an excellent linear piezoresistive behaviour. Besides the desired strain sensitivity, k(l), in the longitudinal direction, carbon fibres show a strain sensitivity, k(t), transverse to the fibre direction. An experimental method was developed to empirically characterize these electromechanical material properties. The strain sensitivities k(l) and k(t) of the investigated ex-PAN fibre (Torayca T300B) were determined by varying the ratio between longitudinal strain, epsilon(l), and transverse strain, epsilon(t), in uniaxial tensile tests and four-point bending tests. The results show a longitudinal strain sensitivity k(l) in the range of 1.72-1.78 and a transverse strain sensitivity k(t) in the range of 0.37-0.41. This significant transverse strain sensitivity must be considered in the case of strain measurements with carbon fibre sensors. An approach for dealing with the high transverse strain sensitivity of carbon fibre sensors is proposed, which is appropriate for orthotropic as well as isotropic materials. The approach is based on transfer functions, which includes both strain sensitivities of the carbon fibre k(l) and k(t). The transfer function of a biaxial carbon fibre sensors element with orthogonally aligned carbon fibre sensors is given and experimentally checked for validity. Furthermore, the transfer function of a right-angled triangle carbon fibre sensor element is given. These triangle carbon fibre sensor element allows the determination of the complete state of strain of a structure including the shear strain.