The Influence of Mean Stress on Fatigue Properties of ADI

The paper presents the results of research focused on assessment of the influence of loading cycle asymmetry on fatigue limit values. For tests two heats of unalloyed nodular cast iron were used. Test bars made of keel blocks were heat treated in salt bathes (austenitization at 900 °C during 1 hour, isothermal transformation at 380 and 400 °C) and loaded at symmetrical, repeating and pulsating loading cycles at room temperature. Evaluation of fatigue properties was based on the determination of S-N curves in high-cycle region including the fatigue limit assessment for 107 cycles to fracture. Fatigue and static tests were completed by metallographic and quantitative phase analysis. Most important result obtained from the presented study is that the dependence of stress amplitude on mean stress cannot be approached by the linear relation but by general power law with exponent lower than 1 (i.e. the Haigh diagram has convex shape).