Load and failure analyses of CFRP laminates by means of electrical resistivity measurements

Abstract The variation of the mechanical stiffness during fatigue loading has been shown to be an important qualitative and quantitative damage analogue. A correlation between stiffness reduction and the development of the various types of damage, such as transverse and longitudinal intraply cracking or delamination growth, has been made by various authors. However, the in-situ monitoring of fibre fracture occurring during static or fatigue loading in cfrp laminates has not yet been made except by indirect methods. The measurement of the variation of the electrical resistivity during loading promises to be a valuable technique for this purpose. In the case of a conventional metal sample, the conductivity is essentially the same for any direction of current flow through the sample. In cfrp samples, however, the conductivity is not isotropic and depends on the orientation and on the conductivity of the carbon fibres. Changes in the conductivity can therefore be related to fibre fracture. The resistivity also varies with temperature, and it is therefore necessary to correlate resistivity changes with the temperature changes observed during a fatigue test. The resistivity of cfrp laminates is further dependent on the applied load, the fibre volume fraction, the laminate stacking sequence and the fibre type. Knowing all of these factors, it is possible to correlate the change in the electrical resistivity with damage and failure of the load-bearing 0° fibres. However, the technique can also be used to monitor continuously the actual load situation in a composite component.