Evaluation of the fatigue limit of welded joints, taking into account the total plastic deformation of the material in the state of plane stress

Evaluation method of welded structure damage under the influence of short-term static limit loads has been developed. The role of loads on the magnitude of the limiting amplitudes for ferrite-pearlite materials has been estimated. The critical lengths of cracks under conditions of preliminary plastic deformation in the state of plane stress have been determined.

The obtained simple calculated dependences allow to determine the limit amplitudes of the stresses for various parameters of the external load cycle for materials with cracks under conditions of
a plane-stressed state (with maximum stresses exceeding the yield strength). It has been established that with an increase in the average stress of the cycle, the minimum length of the “cracking” of
a crack during plane deformation and the plane-stressed state strongly changes to downward. The threshold effective stress intensity factor has a predominant effect in reducing the crack length “cracking”, when the average stresses approach the yield point. However, then this influence is reducing, and eventually a sharp yield strength increase has a leading value.

The proposed calculation method is based on the well-known model of fracture with a cleavage and the criteria underlying of the destruction mechanism. The dependences obtained and the estimation method is valid for ferrite-pearlite steels with a yield strength of up to 700 MPa.