In order to be able to make reliable measurements of the depth of damage in a rail, the inspection system must be calibrated. This was done by carefully grinding the surface of a rail with gauge corner defects at a specified low material removal rate. After each grinding operation, eddy-current measurements and cross-sectional measure-ment were performed to determine exactly the amount of material removed. The result is a calibration curve that allows one to assign a crack length on the basis of the recorded eddy-current signal. In order to estimate the damage depth (Fig. 10) it is necessary to know the angle at which the crack is propagating through the rail material. If reliable conclusions are to be made, further investigations of material properties and fracture mech-anics will be required but for most of the present
applications a middle angle of about 238 is assumed. The error in the defect depth determination for a 10 mm long head check is approximately ^ 1 mm.