Abstract: Based on the observation of micro scratches， the cross section of the scratches has the geometric characteristics of triangle. Two principles which are direction-independent and length-independent were proposed for determining the projection area of micro scratches. Accordingly， a new parameter area_Δ was defined as the maximum square root of triangle area of micro scratch section， to describe fatigue damage quantitatively caused by the micro scratches. A modified fatigue strength model with the consideration of micro scratches was established by using the Murakami model and the term of area_Δ.The model was validated by the fatigue strength experiment of high strength steel FV520B-I and the existing data of Ti-6Al-4V titanium alloy from literature. The results showed that the model had a certain advantage of prediction accuracy in fatigue strength prediction.

Key words: micro scratch, fatigue damage parameter, Murakami theory, projection area, fatigue strength prediction