Magnetic response of damaged carbon fibre reinforced plastics measured by a HTS-SQUID magnetometer

Controlled states of damage were imparted to carbon fibre reinforced plastic panels, simply supporting them on a circular ring and applying a quasi-statically increasing transverse load at the centre. The damage state was controlled by interrupting the loading stage at predetermined values of the deflection. After mechanical tests, the panels were non-destructively inspected by a highly sensitive superconductive magnetometer (HTS SQUID), from which the maps of the parallel component of the magnetic field above the sample surface were obtained. A destructive analysis of the specimens was also carried out by optical microscopy, to assess damage features. The existence of a defect in the material was clearly signalled by a distortion in the magnetic field, even when an elastic behaviour could be guessed from the load-displacement curve. Until the loading conditions only resulted in failures in the matrix, the slope of the SQUID response along a line-scan was linearly dependent on the maximum energy applied to the specimen during the mechanical tests. The dependence of the slope on the energy was strongly altered when fibre failures were induced in the laminate. The magnetometer response was also influenced by the coil excitation frequency. The results obtained at different frequencies indicate that the optimum frequency can vary, depending on the scope of the non-destructive inspection under concern.