The influence of stress-controlled tensile fatigue loading on the stress–strain characteristics of AISI 1045 steel

• Residual tensile curves of AISI 1045 steel has been determined after pre-fatigue.
• At low number of fatigue cycles hardening effect of tensile curve is observed.
• Close to fatigue strength at high cycle stress material softening may be found.
• Material softening is in coincidence with high ratcheting and plastic strain.

This study analyses the influence of fatigue loading on the residual tensile properties of AISI 1045 steel. The fatigue tests were carried out under stress-controlled tensile loadings at a stress ratio equal to 0. The maximum applied stresses were within the range from 550 MPa to 790 MPa. An analysis of ratcheting strain and plastic strain amplitude evolution due to fatigue loading was performed on the experimental data. In the next stage of this study, the initial fatigue loadings were introduced. Two maximum stresses, 550 MPa and 750 MPa, and three cycle lengths, 25%, 50% and 75% of the total number of cycles required to fracture the material at a given stress, were used. The pre-fatigued specimens were subjected to tensile testing at strain rates from 10−4 to 100 s−1. A large number of fatigue cycles, equal to 75% of the fatigue life, induces material softening as well as a drop in elongation and a reduction of area. Pre-fatigue at maximum stress equal to 550 MPa results in the increase of the elastic limit and offset yield point as well. Both parameters reach almost constant value after number of cycles equal to 25 % of the fatigue life. The further increase in the number of cycles does not affect elastic limit and offset yield point in a clearly visible way. The increase of maximum stress of the initial fatigue loadings up to 750 MPa induces similar but stronger effect i.e. increase and stabilization of elastic limit and offset yield point values, however decrease of both parameters value is observed at large number of pre-fatigue cycles corresponding to 75% of the fatigue life.