Development of a probabilistic model for the prediction of fatigue life in the very high cycle fatigue (VHCF) range based on inclusion population

Abstract

Abstract. The VHCF behaviour of metallic materials containing microstructural defects such as non-metallic inclusions is determined by the size and distribution of the damage dominating defects. In the present paper, the size and location of about 60.000 inclusions measured on the longitudinal and transversal cross sections of AISI 304 sheet form a database for the probabilistic determination of failure-relevant inclusion distribution in fatigue specimens and their corresponding fatigue lifes. By applying the method of Murakami et al. the biggest measured inclusions were used in order to predict the size of failure-relevant inclusions in the fatigue specimens. The location of the crack initiating inclusions was defined based on the modeled inclusion population and the stress distribution in the fatigue specimen, using the probabilistic Monte Carlo framework. Reasonable agreement was obtained between modeling and experimental results.In the VHCF range the cyclic strength of metallic materials containing nonmetallic inclusions is predominantly determined by the size and location of the inclusions [1] They arise during manufacturing processes and result in a localized distribution of plastic strain in isolated microstructural regions at VHCF relevant load amplitudes, and their presence and impact are hardly predictable analytically. The aim of the present work is to develop a statistical approach for the correlation between the quality of metallic materials with respect to the size and arrangement of inclusions and fatigue life in the VHCF regime by using the example of an austenitic stainless steel AISI 304 [2]. The steel was subjected to solution annealing treatment and was subsequently prestrained in order to induce a volume fraction of 60% of deformation-induced martensite. The texture effect on fatigue life was assumed to be negligible. However, due to the elongated form of inclusions resulting from the hot rolling process, the fatigue life of the specimens stressed parallel or transversal to the rolling direction (RD and TD respectively) is different (Fig. 1). The analysis of fracture surfaces shows that about 50% of the failure-relevant inclusions are situated at the specimen surface and are related to a shorter fatigue life compared to crack initiation at interior inclusions.The data base for statistical inferences about the fatigue behavior of AISI 304 was created on basis of metallographic observations on 80 plane cross sections each with a size of 28 mm 2 . The