The article outlines the basics of numerical-experimental method for the interpretation of technical diagnostics results of long-term operated drill pipes containing internal semi-elliptic crack-like defects. Estimation of the stress state in the vicinity of a semi-elliptic crack located on the inner surface of a hollow cylinder under axial load is performed by using the finite element method. The stress state in the vicinity of the vertex of the semi-elliptic crack is determined and on this basis the characteristics of the fracture mechanics ($J$-integral and stress intensity factor) are calculated. The conditions of destruction of low- and medium-carbon low-alloying steels with ferrite-perlite structure of drill pipes of strength groups ‘K’ and ‘G-105’, which have been used for a long time, are experimentally determined, i.e. critical crack resistance is determined and the value of critical stress intensity factor is calculated. Diagrams of fracture risk assessment in the coordinates ‘crack-like defect—drilling depth—stress intensity factor’ are constructed. They allow estimating the safe depth of internal semi-elliptic crack-like defects detected by means of technical diagnostics taking into account the results of experimental research. As established, during tripping operations there are no conditions for the failure of drill pipes made of low- and medium-carbon low-alloying steels with ferrite-perlite structure, with a transverse semi-elliptic crack-like defect which depth does not exceed 50% of the pipe wall thickness.