EFFECT OF HYDROGENATION AND PLASTIC PREDEFORMATION ON THE CRACK GROWTH RESISTANCE OF STRUCTURAL STEEL 1 D. Yu. Petryna, 2 A. D. Markov, 1 Ye. I. Kryzhanivskyi, 2 H. M. Nykyforchyn 1 Ivano-Frankivsk National Technical University of Gas and Oil, Ukraine 2 Karpenko Physico-Mechanical Institute of the NAS of Ukraine ABSTRACT: The synergistic effect of hydrogenation and plastic predeformation (PPD) on the fracture toughness and fatigue crack growth resistance in the tempered 0.4-1Cr-1Ni steel was studied. In the no hydro- genated state, PPD of steel slightly decreases its fracture toughness and in- creases considerably the fatigue crack growth resistance in the near thresh- old region of da/dN - ∆K diagram. Crack closure is responsible for the latter effect. Hydrogenation of the material drastically decreases the fracture toughness and intensifies the fatigue crack growth, especially in the middle region of da/dN - ∆K diagram. The synergistic effect of PPD and hydro- genation of the investigated steel causes the maximum decrease of crack growth resistance, both under static and cyclic loading. The possible conse- quences of such effect from the point of view of structural strength of drill pipe string are analyzed. INTRODUCTION Fracture of structural elements, which are supposed to cyclic loading in aggressive environments, is caused often by fatigue crack growth, in the first turn in place of stress concentration. Although structures in such cases are usually calculated by yield stress, one should not neglect the possibility of single overloads that cause plastic deformation of a material in the zone of constructional stress concentrators. As a result, cracks can appear in this zone. Under cyclic loads, they grow and lower the load-carrying capacity and life time. This fact demonstrates the importance of considering the ef- fect of PPD on the static and cyclic crack resistance of such structural mate- rials. On the other hand, it is also necessary to consider the possible hydro- genation of material in working environments, primary, at the places of stress concentration, which can, in addition, negatively effect on the fatigue