A novel engineering method based on the critical plane concept to estimate the lifetime of weldments subjected to variable amplitude multiaxial fatigue loading

Abstract

A B S T R A C T This paper summarizes an attempt at proposing a new engineering method suitable for estimating the fatigue lifetime of steel-and aluminium-welded connections subjected to variable amplitude multiaxial fatigue loading. In particular, the proposed approach is based on the use of the so-called Modified Wöhler Curve Method (MWCM), i.e. a bi-parametrical critical plane approach, whose accuracy has been checked so far solely in addressing the constant amplitude multiaxial fatigue problem. In order to extend the use of our criterion to variable amplitude situations, the critical plane is suggested here as being determined by taking full advantage of the maximum variance concept, that is, such a plane is assumed to be the one containing the direction along which the variance of the resolved shear stress reaches its maximum value. The main advantage of such a strategy is that the cycle counting can directly be performed by considering the shear stress resolved along the maximum variance direction: by so doing, the problem is greatly simplified, allowing those well-established cycle counting methods specifically devised to address the uniaxial variable amplitude problem to be extended to those situations involving multiaxial fatigue loading. The validity of the proposed methodology was checked by using two different datasets taken from the literature and generated by testing both steel and aluminium tube-to-plate welded connections subjected to in-phase and 90 • out-ofphase variable amplitude bending and torsion. This new fatigue life assessment technique was seen to be highly accurate allowing the estimates to fall within the calibration scatter bands not only when the constants in the governing equations were calculated by using the experimental uniaxial and torsional fully reversed fatigue curves, but also when they were determined by using the reference curves supplied, for the investigated geometry, by the available standard codes. These results seem to strongly support the idea that, thanks to its peculiar features, our method can be considered as an effective engineering approach capable of performing multiaxial fatigue assessment under variable amplitude loading which fully complies with the recommendations of the available standard codes.

Comparative study of multiaxial fatigue methods applied to welded joints in marine structures

“…This MVM is based on the experimental observation that fatigue damage shows proportionality with the variance of a random uniaxial load signal, both for Gaussian and non-Gaussian loadings [34]. Therefore, the critical plane corresponds to the material plane which coincides with the direction along which the variance of the resolved shear stress is maximum.…”

“…The MWCM presumes that fatigue damage is independent of the (nominal) load path as long as the stress ratio and shear stress range relative to the critical plane are the same. Therefore, a linear relationship is suggested for the construction of a modified load specific SN-curve [34]. Fig.…”

“…This SN-curve was defined earlier by [3]. For the MCSC and MWCM the pure Mode-I and pure Mode III curve were used [34]. …”

A modification of Smith–Watson–Topper damage parameter for fatigue life prediction under non‐proportional loading

“…Though it is very difficult to solve the multiaxial fatigue problem, fundamental understanding of this problem is essential for the reliability assessment under realistic service condition and is valuable for the design and maintenance against fatigue failures. 2,[4][5][6][7][8][9][10] This critical plane concept first used in stress-based criteria, was subsequently introduced to strain-based criteria and then to energy-based criteria. Although the life prediction technique has been made extensive progress in the past decades, yet there is no simple theory of fatigue that can be applied to wide class of materials and loading conditions, nor there exist a universal model of fatigue damage.…”

Multiaxial fatigue assessment of friction stir welded tubular joints of Al 6082-T6

“…In particular, accurate predictions can be made by applying the MWCM not only in terms of nominal [57][58][59][60] or hot-spot stresses [55,56,59,61], but also using the reference radius concept [59,62] or the Theory of Critical Distances [59,[63][64][65].…”