Fatigue investigation of complex weldments by the means of the local strain energy density approach

Luigi Mario Viespoli,Filippo Berto,Bård Nyhus,Antonio Alvaro,G Hénaff

doi:10.1051/matecconf/201816522003

Luigi Mario Viespoli, Filippo Berto + Show 3 more

Open Access

https://doi.org/10.1051/matecconf/201816522003

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Abstract

The paper investigates the use of an energetic approach based on the strain energy density failure criteria to predict the fatigue life of welded joints in aluminum alloy. The cited criterion has already been proved valid to assess the failure of components in presence of sharp and blunt notches, and several results are present in the literature for different materials. The geometry tested in the present work is a double V-grooved full penetration butt weld, subsequently heat treated, and loaded orthogonally to the welding direction with load ratio R=0. This configuration makes the weld toe a notch of great opening angle. The aim of the paper is to verify the soundness of the energetic criteria for this class of welded joints, comparing the tests results with the numerical predictions. In the computational part, the energy in a given volume can be computed directly from the nodal displacements, thus not needing the stress values. Differently from the stress intensity factor approach, this property allows to a fast computation of the strain energy density by the means of a coarse mesh. The results of different configurations of geometry and meshing are compared to find the simplest modeling scheme capable of providing an accurate estimate of the fatigue life of the joint.

Highlights

The fatigue life assessment of welded components can be executed with different techniques [1] of which, due to their great simplicity, commonly used are the Nominal Stress and the Hot Spot Stress approaches [2]
The local energetic criterion states that the failure occurs when the mean SED, W, averaged over a control volume of precise radius R, equals a critical value Wc, not dependent on the geometrical characteristics of the notch, but only on the material properties, as it is the radius R
If the opening angle is greater than 103o, the Mode II stress field is not singular, so its contribution to the strain energy around the notch tip is negligible compared to the contribution of the Mode I, allowing to express the mean SED as: W W

Summary

Introduction

The fatigue life assessment of welded components can be executed with different techniques [1] of which, due to their great simplicity, commonly used are the Nominal Stress and the Hot Spot Stress approaches [2]. In the presence of a sharp notch, that is a weld toe or root, the static and fatigue resistance can be evaluated using the Notch Stress Intensity Factors [3] Such procedure presents two drawbacks: the necessity of an accurate evaluation of the stress field [4], so a very refined discretization and a non-constant critical value, but function of the opening angle through the William’s eigenvalues [5]. If the opening angle is greater than 103o, the Mode II stress field is not singular, so its contribution to the strain energy around the notch tip is negligible compared to the contribution of the Mode I, allowing to express the mean SED as:. In the case of aluminum welded joints, the radius found with a number of experimental tests is R=0.12 mm [12]

Specimens

FEM modeling

Thickness effect

Conclusions