This paper presents results from a literature review of defect characteristics essential for non-destructive testing (NDT). Most of the major NDT methods are included in the study - ultrasonic testing (UT), radiography (RT) and eddy current testing (ET). The study was performed by means of searching in scientific databases, technical journals, conference proceedings etc. Mainly, the following databases were used: CHANS (Chalmers database), INSPEC, NTIS, Ei Compendex, World Surface Coating Abs, METADEX, JICST Eplus, Aerospace database, Pascal, Eng Materials, SciSearch and Weldasearch.
It is concluded that for eddy current testing, the defect geometry, the defect size and the defect orientation influence the outcome signal. A number of investigations address the relationships between the defect parameters like defect depth, length and width and the outcome signal parameters like amplitude, phase and signal shape. Also the phenomena of the electrical contacts between the defect surfaces (for a crack) was studied.