Limiting possibilities of detecting cracks in ferromagnetic structures through an anti-corrosion coating using a smartphone based eddy current flaw detector

Detection of cracks in ferromagnetic steel structures by the eddy current method is often limited by additional noise associated with magnetic and structural heterogeneity, especially when it is necessary to detect structural cracks through a layer of dielectric anti-corrosion coating. A promissing approach to problem solution is based on the use of selective eddy current probes, which include the eddy current probes of double differential type. One of the features of such eddy current is the possibility of detecting hidden defects even through a layer of dielectric anti-corrosion coating. In this paper, the limiting possibilities of crack detection using an improved low-frequency eddy current probe of double-differential type with a diameter of 15 mm and a smartphone based eddy current flaw detector were investigated. This probe provides a high depth of penetration necessary to detect defects under the dielectric coating. The greater depth of penetration of this probe is achieved by increasing the diameter of the windings, the distance between them and the number of turns. The research was carried out using a rectangular specimen, in which a large crack (or through wall structure fracture) was simulated by a joint of two identical polished rectangular parts. The assembled specimen was covered with dielectric plates of different thicknesses up to 25 mm thick to simulate anti-corrosion coating of different thicknesses. It is shown, in particular, the possibility of detecting large cracks or structural fractures through an anti-corrosion dielectric coating up to 25 mm thick. The principle of design of an eddy current flaw detector based on a smartphone using the EddySmart application, which can provide remote control of ferromagnetic steel structures with wireless transmission of inspection results via mobile communication channels and the use of autonomous automated scanners, is considered.