Fatigue strength of steel and aluminium welded joints based on generalised stress intensity factors and local strain energy values

Abstract

Weld bead geometry cannot, by its nature, be precisely defined. Parameters such as bead shape and toe radius vary from joint to joint even in well-controlled manufacturing operations. In the present paper the weld toe region is modelled as a sharp, zero radius, V-shaped notch and the intensity of asymptotic stress distributions obeying Williams' solution are quantified by means of the Notch Stress Intensity Factors (NSIFs). When the constancy of the angle included between weld flanks and main plates is assured and the angle is large enough to make mode II contribution non-singular, mode I NSIF can be directly used to summarise the fatigue strength of welded joints having very different geometry. By using a large amount of experimental data taken from the literature and related to a V-notch angle of 135 degrees, two NSIF-based bands are reported for steel and aluminium welded joints under a nominal load ratio about equal to zero. A third band is reported for steel welded joints with failures originated from the weld roots, where the lack of penetration zone is treated as a crack-like notch and units for NSIFs are the same as conventional SIF used in LEFM. Afterwards, in order to overcome the problem related to the variability of the V-notch opening angle, the synthesis is made by simply using a scalar quantity, i.e. the mean value of the strain energy averaged in the structural volume surrounding the notch tips. This energy is given in closed form on the basis of the relevant NSIFs for modes I and II and the radius R-c of the averaging zone is carefully identified with reference to conventional arc welding processes. Rc for welded joints made of steel and aluminium considered here is 0.28mm and 0.12mm, respectively Different values of Rc might characterise welded joints obtained from high-power processes, in particular from automated laser beam welding. The local-energy based criterion is applied to steel welded joints under prevailing mode I (with failures both at the weld root and toe) and to aluminium welded joints under mode I and mixed load modes (with mode II contribution prevailing on that ascribable to mode I). Surprising, the mean value of Delta W related to the two groups of welded materials was found practically coincident at 2 million cycles. More than 750 fatigue data have been considered in the analyses reported herein

Crack initiation life in notched Ti-6Al-4V titanium bars under uniaxial and multiaxial fatigue: synthesis based on the averaged strain energy density approach

“…he strain energy density (SED) averaged over a control volume, the radius of which is thought of as a material property according to Lazzarin and Zambardi [14], proved to efficiently account for notch effects both in static [14,21] and fatigue [14,22,23] structural strength problems. The idea is reminiscent of the stress averaging to perform along a material-dependent microstructural length, according to the approach proposed by Neuber.…”

A Review of the Fatigue Strength of Load Carrying Shear Welded Joints

“…Each volume has a radius 0.28 mm and depth equal to 0.28 mm. The radius of the volume of the control has been chosen to be equal to the radius of the critical volume for the application of the EDI approach of steel, reported Livieri and Lazzarin (2005). The depth of each volume of control has been chosen to be equal to the radius (Lazzarin et al, 2008b).…”

“…The connection between the life of the fatigue of Joints welded to the local tensions to finger from no welding or root, the main issue is the definition of geometric parameters of the weld bead, such as the radius of the finger of the foot and the form of the heel (Livieri and Lazzarin, 2005;Lazzarin et al, 2003). These parameters may vary not only for the different faces of the same design, but also within the same interdiction; moreover the radius of the tep may be considered to be equal to zero or tends toward zero (Atzori, 1985;Lazzarin and Tovo, 1998).…”

“…determined NSIFs pad (or, in the case of bigger tasks, spring plasticdetermined by stress the notch or factors of intensity strain) may describe the voltages of the region of toe of weld, quantifying the intensity of the distributions of stress on the basis of conditions of orders of the Williams solution (1952). NSIF approach was originally proposed to estimate the stage of initiation of the cracks (Boukharouba et al, 1995;Verreman and Nie, 1996), but only later was demonstrated later that NSIFs parameters are able to predict the service life of the fatigue of the welded connections (Livieri and Lazzarin, 2005;Lazzarin and Tovo, 1998;Atzori et al, 1999). In fact, a large quantity of life to fatigue for welded structural is spent at the depth of the crack in the short term, within the region governed by the singularities V-notch: Therefore, nucleation and increase the short cracks are dependent on the distributions of stress present in the component Nesfari-stopper.…”

“…The radius r0 is a parameter of the characteristic material and shall be assumed to be independent of the load mode. For welded joints, the values of the radius r0 proved to be 0.28 mm for steel and 0.12 mm for aluminum (Livieri and Lazzarin, 2005;Lazzarin et al, 2003). A summary based on 900 experimental data obtained for the stainless steel construction has been accomplished, into a ∆W-N strip of the Scattering.…”