Detection of subsurface cracks using inductive thermography

Abstract

Inductive thermography is a well-established NDT method to detect surface cracks in metallic materials. The induced eddy current density decays exponentially below the surface, penetrating up to the so-called skin-depth. This depth depends on the excitation frequency and on material parameters, as the magnetic permeability. As a surface crack is an obstacle for the eddy current and for the heat flow, it becomes visible in the infrared images. It is investigated whether cracks ending below the surface, can be detected by inductive thermography. It is stated, that when the crack end is lying closer the surface than the half skin-depth, then it can be detected. This statement is investigated for ferro-magnetic and non-magnetic samples and for different excitation frequencies. The inspection is usually done in reflection mode, but for thin wall work-pieces the transmission mode provides a good detection possibility. Experimental results are compared with finite element simulation results.