Inverse reconstruction of fibre orientation in multilayer CFRP using forward FEM and eddy current pulsed thermography

Eddy current pulsed thermography (ECPT) is an effective non-destructive testing technique for evaluating the integrity and safe operation of conductive composite components such as carbon fibre reinforced polymer (CFRP) in aerospace applications. However, since any thermography technique measures a surface thermal distribution, ECPT does not directly provide comprehensive information at a layer level, e.g., about fibre orientation, which instead is important for indicating possible failures, e.g., fibre misalignment, debonding, etc. In this work, it is shown how ECPT data can be used to both reconstruct the layers' orientation and to estimate the thermal and electrical conductivity of multilayer CFRP samples. This is achieved by implementing an iterative inversion procedure that processes experimental measurements together with finite element method simulations of the ECPT data. The procedure is applied to two multilayer CFRP samples with known ply orientations. Firstly, the electrical and thermal conductivities are estimated by the early experimental transient response of the first layer having a visible fibre's orientation. Then, an iterative inverse procedure minimizes the discrepancy between measured and simulated data to reconstruct orientations of each layer using the estimated conductivity. Further, the results of this procedure are validated by exploiting a feature-based approach for orientations reconstructions, i.e. the Radon transform. It is also found that the error increases as the layer depth increases due to the diffusive nature of both electromagnetic and thermal waves.