Crack initiation and endurance limit of a hard steel under multiaxial cyclic loads

Abstract

A B S T R A C T The endurance limit and the mechanisms of fatigue crack initiation in the high-cycle regime were investigated using round specimens of the bearing steel SAE 52100 in a bainitic condition under longitudinal forces, torsional moments and combinations of these loads. Three specimen types were examined: smooth specimens and specimens with circumferential notches with radii of 1.0 and 0.2 mm. The surfaces of the specimens including the notches were ground resulting in compressive residual stresses in the nearsurface region. The influence of mean and multiaxial stresses on the endurance limit can be understood by consideration of crack initiation mechanisms and micromechanics. Crack initiation occurred at oxides, carbonitrides and at the surface. The oxides had little adhesion to the bainitic matrix and acted like pores. The carbonitrides were well bonded to the matrix and caused stress concentrations due to their higher elastic modulus when compared to that of the matrix. The mechanisms of crack initiation could be related to the load type: loads with rotating principal stresses cause more damage for nitrides than for oxides. Increasing maximum stresses are more dangerous for nitrides than for oxides, and damage the surface more than the nitrides. Normal stresses produce more damage for oxides than shear stresses. The endurance limits were calculated by means of an extended weakest-link model which combines volume and surface crack initiation with individual fatigue criteria. For volume crack initiation, the criterion of Dang Van was used. For the correct description of the surface crack initiation, a criterion proposed by Bomas, Mayr and Linkewitz was applied. With this concept, a prediction of the endurance limit is possible. The influence of the notch geometry on the endurance limit is well characterized. A 0 = reference surface d = net diameter of the specimens E = elastic modulus f = load frequency F = longitudinal load, distribution function K t,σ = concentration factor for normal stress K t,τ = concentration factor for shear stress m = exponent of the Weibull distributionCorrespondence: H. Bomas.

Validation data set for testing the criteria for multiaxial fatigue strength estimation

“…The BKL test set is based on experiments on DIN 100Cr6 presented by Bomas et al The authors tested the specimens on several load levels for each load case, and they evaluated the fatigue endurance as the median of the 2‐parametric Weibull distribution. The number of specimens used for defining the fatigue endurance is generally high—the lowest value here is 13.…”

“…The ArcSin√P method then fits a sinusoidal distribution to these results by applying a least squares routine to the fracture probability data. A slightly different method is used for one of the data sets presented here later on, where the authors used 2‐parametric Weibull distribution instead.…”

Influence of inclusion type and size on the fatigue strength of high strength steels

“…Cracks are formed either at the surface or in the interior of the specimen which propagate and lead finally to the fracture of the specimen. In case of internal failures inclusions frequently act as crack initiating sites, where oxides, nitrides and sulphides are the most frequently mentioned inclusion types [2][3][4][5]. It is well known that with increasing inclusion size the fatigue strength of steel specimens is reduced [6].…”

A fatigue design concept for metal injection molded components of 100Cr6

“…It is well known that considering conventional high‐strength steels, fatigue cracks are initiated at material discontinuities such as non‐metallic inclusions or grinding marks at the surface . As metal injection molding is a powder metallurgical (PM) process, additional defects in the form of pores are expected.…”