Issue 16

L. Susmel, Frattura ed Integrità Strutturale, 16 (2011) 5-17; DOI : 10.3221/IGF-ESIS.16.01 5 On the overall accuracy of the Modified Wöhler Curve Method in estimating high-cycle multiaxial fatigue strength Luca Susmel Department of Engineering, University of Ferrara, Via Saragat, 1 – 44100 Italy Department of Mechanical Engineering, Trinity College, Dublin 2, Ireland Department of Civil and Structural Engineering, University of Sheffield, Mappin Street, Sheffield, S1 3JD, UK ssl@unife.it A BSTRACT . The aim of the present paper is to systematically investigate the accuracy of the so-called Modified Wöhler Curve Method (MWCM) in estimating high-cycle fatigue strength of plain and notched engineering materials damaged by in-service multiaxial load histories. In more detail, the MWCM, which is a bi-parametrical critical plane approach, postulates that initiation and Stage I propagation of fatigue cracks occur on those material planes experiencing the maximum shear stress amplitude (this being assumed to be always true independently from the degree of multiaxiality of the applied loading path). Further, the fatigue damage extent is hypothesised to depend also on the maximum stress perpendicular to the critical plane, the mean normal stress being corrected through the so-called mean stress sensitivity index (i.e., a material constant capable of quantifying the sensitivity of the assessed material to the presence of superimposed static stresses). In the present investigation, the overall accuracy of the MWCM in estimating high-cycle fatigue strength was checked through 704 endurance limits taken from the literature and generated, under multiaxial fatigue loading, by testing both plain and notched samples made of 71 different materials. Such a massive validation exercise allowed us to prove that the MWCM is highly accurate, resulting in 95% of the estimates falling within an error interval equal to ±15%. K EYWORDS . Multiaxial fatigue; Critical plane approach; High-cycle fatigue strength; Notch. I NTRODUCTION ince the pioneering work done by Gough [1], over the last 60 years several researchers have made a big effort in order to devise sound criteria allowing high-cycle fatigue damage under multiaxial fatigue loading to be estimated accurately. As to the work that has been done so far, examination of the state of the art shows that the most important high-cycle fatigue criteria can be subdivided into the following three different groups: methods making use of the so-called microscopic approach [2, 3], fatigue assessment techniques based on the use of the stress invariants [4-6], and, finally, criteria taking full advantage of the classical critical plane concept [7-9]. Amongst the above different strategies, it is the author’s opinion that the most promising approaches are those based on the assumption that fatigue damage reaches its maximum value on that material plane (i.e., the so-called critical plane) experiencing the maximum shear stress amplitude [10]. According to the above firm belief, over the last decade we have made a big effort in order to systematically investigate the peculiarities of a bi-parametrical critical plane approach which is known as the Modified S