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Hui-tian Lu College of Engineering, South Dakota State University, Brookings, SD 57007-0001 (Received 24 October 2016; accepted 14 December 2016)

The high-cycle fatigue (HCF) behavior is significantly affected by surface roughness, especially for high strength metal FV520B-I. However, with surface roughness effect, neither the fatigue property, nor the high-cycle fatigue life model about FV520B-I with surface roughness has been reported. In this paper, designed fatigue experiment using the specimen with different surface roughness is presented to study the effectiveness of the roughness to the fatigue. The observations of the fatigue crack initiation sites and the crack propagation. Then the high cycle fatigue behavior of FV520B-I affected by surface roughness is analyzed. The existing very-high-cycle fatigue life model is not well-fit for high-cycle fatigue model of FV520B-I. A NEW high-cycle fatigue life prediction model of FV520B-I, taking surface roughness as a main effective variable is proposed. The model is built up by a comprehensive use of experimental data and the traditional fatigue modeling theory. The new finding between the fatigue strength coefficient and stress amplitude, with surface roughness, is adopted, leading to a NEW modified life prediction model. Study on fatigue model of FV520B-I with surface roughness is a very beneficial effort in fatigue theory and fatigue engineering development.

I. INTRODUCTION

Surface roughness defect has been proved as a main factor1–4 causing the high-cycle fatigue failure that can’t be neglected in the real engineering practice, especially for the high strength metal FV520B-I. Roughness index value Ra has numerous advantages: it can reflect not only the surface micro geometry, but also the profile peak of the surface morphology, whose information make the data process easy. Therefore, the Ry can be regarded as the deepest groove depth at the minimum cross-section of the specimen, the probability of Ry appearing at the minimum cross-section of a specimen was small. And it has been widely accepted that Ra is more representative than the other estimated values, like Ry, so Ra is widely used as the quantitative expression of the surface roughness defect to establish the high-cycle fatigue life model of FV520B-I, involving Ra as a main variable. FV520B-I possesses numerous good mechanical properties: high strength, corrosion resistance, abrasive resistance, good welding characteristics, etc.5 These properties lead to FV520B-I’s wide adoptions in the manufacturing of all different kinds of large-scale mechanical equipment.3,4 In normal working conditions, fatigue failure is irreversible. Contributing Editor: Jürgen Eckert a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.513

Also, the failure happens instantaneously without ‘warning’ that can cause accident, personal injury, and unexpected economic loss.6–10 Many real failure cases show that the fatigue failure also occurs on mechanical parts with smooth surface without obvious sur