Simple and robust methodology of defect thermal characterization based on thermal quadrupoles and polynomial approximation

An approach is proposed to evaluate the depth and thickness of planar defects detected by thermal nondestructive testing (TNDT) methods. A 1D 3-layer thermal problem is solved by using the technique of thermal quadrupoles, and the solution of the corresponding inverse problem is presented in the polynomial form. The inverse solution involves a number of TNDT experimental parameters, in particular, differential temperature signals, dimensionless contrasts and their observation times. The accuracy of defect characterization is in the range of 2–15% in defect depth and from 10 to 40% in defect thickness. It is believed that the proposed defect characterization approach can be easily implemented in existing TNDT systems to provide approximate values of defect parameters.