Threshold values for very high cycle fatigue failure of high‐strength steels

Abstract

Cracks leading to failure in very high cycle fatigue (VHCF) of high‐strength steels mostly initiate at subsurface inclusions at stress intensity factors (SIF) below the classical threshold value Kth for long cracks, especially for negative stress ratios. Here, a characteristic fine granular area (FGA) can be observed at the fracture surface in the vicinity of these initiating inclusions. According to various researchers, the FGA formation might be responsible for the late initiation of a propagable long crack. This study aims to clarify the threshold values of SIF for crack initiation for VHCF failure with FGA formation at nonmetallic inclusions. For this purpose, ultrasonic fatigue tests (R = −1) with the high‐strength steel 100Cr6 (SAE 52100) were carried out until an ultimate number of cycles of 109. In addition, very high cycle stress increase tests were performed. A serial grinding investigation of fatigued specimens acted as a technique to evaluate the distribution of unharmful inclusion in the tested volume. By the combination of these tests, a threshold value of the SIF for the VHCF by FGA formation at inclusions can be derived. This threshold value depends on the size of the crack initiating inclusion.

Influence of small defects and nonmetallic inclusions on the high and very high cycle fatigue strength of an ultrahigh‐strength steel

“…Nevertheless, assuming that the explanation given in the last section (i.e., that the fatigue limit is reduced by an environmental interaction and dissolved hydrogen, and Equation 5 generally serves well to predict the fatigue limit of the investigated steel in the presence of surface defects) is correct, there must be a reason for the observed reduction in fatigue strength in the case of internal fracture in the VHCF regime. Also, similar (nonconservative) results are reported for other ultrahigh‐strength steels, 52,53 although Equation 5 is in good agreement with experimentally determined fatigue strengths in the VHCF regime for high‐strength steels (with tensile strength below 2 GPa), 50,54–56 even in the case of failure from internal inclusions.…”

Influence of a Thermo-Mechanical Treatment on the Fatigue Lifetime and Crack Initiation Behavior of a Quenched and Tempered Steel

“…Particularly for failures after a high number of cycles, a fine granular area (FGA) may be observed in the center of the fisheye around the critical inclusion [6,7]. Many studies have shown that the formation of the FGA plays a significant role in crack initiation and eventual fracture [8][9][10][11][12]. However, the forming mechanism of the FGA is not fully understood [13].…”

The behavior of crack initiation and early growth in high-cycle and very-high-cycle fatigue regimes for a titanium alloy

“…With respect to crack initiation from the surface or from the interior of a specimen, there is a tendency for S-N curves to have a twofold or stepwise shape for high-strength steels [11,12]. Fatigue crack initiation at the interior of the specimen always originated from relevant inhomogeneity, usually an inclusion [13,14], due to localized plastic deformation constraints and stress concentrations [4,15]. For the internal crack initiation of VHCF, a rough region around the inclusion was observed and called the fine granular area (FGA) [12].…”