Mean Compressive Stresses - Experimental and Theoretical Investigations Into Their Influence on the Fatigue Strength of Welded Structures

Abstract

To indicate the effect of mean compressive stresses on the structural durability, fatigue tests have been carried out under constant-amplitude load. As a particularly suitable design detail, the longitudinal stiffener with non-load-carrying fillet welds was selected. This detail represents the permissible limit of notch condition in highly stressed ship structures. The fillet weld was made with two layers, so that a fully developed residual stress field could be expected.In order to gain more insight into the technical background under consideration of the weldinduced residual stresses and their redistribution during cyclic loading, finite element analyses as well as measurements were carried out. On uncracked specimens the weld-induced residual stresses were measured using the X-ray method. The redistribution of the residual stresses during crack propagation was measured using the neutron diffraction method.Different models were used for the numerical simulation using the finite element (FE) method. In the first phase the FE calculation could be limited on a linear analysis, because the semielliptical crack propagating through the plate thickness remained open over the full cycles, which is due to the high residual stresses. After the crack became a through-thickness crack, closure effects became significant, requiring a non-linear crack propagation analysis with a shell model. Therefore the weld-induced residual stresses must be reproduced with a solid model and transmitted to the shell model.Taking into account the complete crack length during the simulation, it was possible to estimate the fatigue life under constant amplitude loads in a satisfactory way. Downloaded from Fig. 7 Analysis of the weld-induced residual stresses in the longitudinal direction 638 J Rörup

Corrosion Fatigue of Welded Joints in Marine and Naval Structures, A Review

“…It was also reported that, when the stress concentration is moderate, the yield strength of the base material can affect the fatigue resistance [5]. Rörup et al (2004), investigated the influence of mean compressive stresses on the fatigue resistance of a S355 J2 G3 steel welds, fabricated by longitudinal stiffener employing fillet welds. The specimens were made from structural steel S355 J2 G3.…”

“…The specimens were made from structural steel S355 J2 G3. It was concluded that, the magnitude of residual stresses influenced the fatigue life resistance of the material, significantly [6].…”

Recent developments and future challenges in fatigue strength assessment of welded joints

“…Several experimental and numerical investigations showed high residual stresses at this location (Figure 13) which were considered responsible for fatigue cracks appearing even if the cyclic stresses were fully compressive. 36 The longitudinal shrinkage of the weld between stiffener and plate is mainly responsible for the tensile residual stresses.

Figure 13.Computed residual stresses in longitudinal direction in a plate with a longitudinal stiffener (quarter model).

…”

Distributions of residual stresses in stiffened plates with one and two stiffeners

“…Previous studies have shown how the residual stresses in welded steel members can vary depending on the level of heat input used in the welding process (Gao et al 1997;Wimpory et al 2003;James et al 2006); the type of weld, such as fillet and butt welds (Wimpory et al 2003;Rörup 2005), and weld bead on plate (Paradowska et al 2005;Price et al 2006); and variations in weld filler material (James et al 2006). However, these studies were undertaken on specimens not representative of ship hulls.…”

“…For example, sectioning methods can only measure the strain components at the surface of the plate and hole-drilling methods are limited to near-surface strains (Paik 2009). The X-ray diffraction method is able to measure strain components several microns deep by removing material using electropolishing, but it is believed that the electro-polishing alters the true residual stresses and their distributions (Gao et al 1997;Rörup 2005).…”