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 10(5) to 2 x10(7) cycles. A change of crack initiation mechanism could be observed. Different specimen sizes D-4=4 mm, D-7.5=7.5 mm were tested at a stress ratio of R=-1 and T = 20 degrees C. D-7.5 were additionally tested at T = 350 degrees C. D-4 results showed a pronounced TA with defects mainly consisting of non-metallic inclusions (Al2O3, MgO and CaO). The average defect size (root area, D-4) was measured to be 28.98 pm. D-7.5 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 (root area, D-7.5) was measured to be 10.98 pm. Murakami's root area approach showed a negligible dependance of the fatigue strength according to the different defect sizes combined with a large scatter. Copyright (C) 2017 The Authors. Published by Elsevier B.V.