Effects of Inclusions on the Very High Cycle Fatigue Properties of a High Strength Martensitic Steel within the Transition Area

In this paper fatigue tests of a high strength steel are presented. Focus was put on the transition area (TA) from 105 to 2 x 107 cycles. A change of crack initiation mechanism could be observed. Different specimen sizes D4=4mm, D7.5=7.5 mm were tested at a stress ratio of R = -1 and T = 20°C. D7.5 were additionally tested at T = 350°C. D4 results showed a pronounced TA with defects mainly consisting of non-metallic inclusions (Al2O3, MgO and CaO). The average defect size (area, D4) was measured to be 28.98 µm. D7.5 specimens indicated a less pronounced gap between surface and subsurface crack origins. Same type of non-metallic inclusions could be found by SEM analyses. The average defect size (area, D7.5) was measured to be 10.98 µm. Murakami's area approach showed a negligible dependance of the fatigue strength according to the different defect sizes combined with a large scatter.