Issue 37

L. Susmel et alii, Frattura ed Integrità Strutturale, 37 (2016) 207-214; DOI: 10.3221/IGF-ESIS.37.27 208 mid-90s to investigate the fatigue behaviour of aluminium FS welded joints subjected to uniaxial cyclic loading (see, for instance, Ref. [5] and references reported therein), no systematic research work has been carried out so far to formulate and validate specific methodologies suitable for performing the multiaxial fatigue assessment of this type of joints. In this context, this paper summarises a part of the outcomes from an International Network research project sponsored by the Leverhulme Trust (www.leverhulme.ac.uk) on multiaxial fatigue assessment of aluminium FS welded tubular connections. F UNDAMENTALS OF THE M ODIFIED W ÖHLER C URVE M ETHOD he MWCM is a critical plane approach which assesses fatigue damage under uniaxial/multiaxial fatigue loading via the maximum shear stress amplitude,  a , as well as via the mean value,  n,m , and the amplitude,  n,a , of the stress normal to that material plane experiencing the maximum shear stress amplitude [6]. The combined effect of the relevant stress components relative to the critical plane is quantified via the following stress index [7]: a a,n m,n eff m      (1) where m is the so-called mean stress sensitivity index [6, 7]. Index m is a material fatigue property that ranges from zero (no mean stress sensitivity) to unity (full mean stress sensitivity) [6]. According to the way it is defined, ratio  eff is sensitive not only to the presence of non-zero mean stresses [8], but also to the degree of multiaxiality and non- proportionality of the load history being assessed [6]. Figure 1 : Modified Wöhler diagram. The MWCM’s modus operandi is schematically explained through the modified Wöhler diagram of Figure 1. This log-log chart plots  a against the number of cycles to failure, N f . Much experimental evidence [6] suggests that, for a specific material, the modified Wöhler curves tend to move downward as  eff increases (Fig. 1). In other words, for a given value of  a , fatigue damage tends to increase as  eff increases. According to the diagram in Figure 1, the position and the negative inverse slope of any modified Wöhler curve can be defined through the following linear relationships [6]:      eff eff k (2)   b a eff eff fRe ,A    (3) T