The problem of assessment of main carrying elements (belts) of clamp-forming machine boom made of the angle 45×45×5 mm (Steel 3) in the process of diagnostics is considered. The fragment of the belt is modelled by means of a plate, hawing the crack with typical sizes. Analytical relations are given, stresses intensity factors (SIF) K1 and rates of their change dK1/da near the apex of a crack are investigated. Six cases of potentially possible crack-like defects initiation are considered, criteria lengths of cracks are determined for them, proceeding from the obtained experimental data.