Issue 38

E. Shams et alii, Frattura ed Integrità Strutturale, 38 (2016) 114-120; DOI: 10.3221/IGF-ESIS.38.15 115 consideration of geometrical and statistical size effects. In the first step of this method, the effective stress distribution over an area to be investigated according to Neuber [7] is to be determined. Next, the highly stressed surface based on the effective stress distribution is calculated using the weakest link model according to Weibull [8]. Many research activities have been initiated during the last years for the investigation of fatigue design of multiaxially loaded welded joints [9-11]. Vormwald [12] explored in the event of variable amplitude loading conditions a list of challenges have to be considered. Nevertheless, the case of weld ends under such loading is not sufficiently explored yet. However, the application of the method NuMeSiS opens up the possibility to standardise the S-N curves belonging to various weld shapes and loading conditions. In the present paper, the fatigue behaviour of weld ends under combined in- and out-of-phase multiaxial loading in thin sheet structures, which is of special interest in the automotive industry, is addressed. In the experimental part of this research, cycles to rupture at different stress amplitudes were derived from fatigue testing. Due to the complex geometry of weld ends, the notch stress concept was used in order to assess the multiaxial stress-states based on an idealised weld end model. E XPERIMENTAL INVESTIGATION Specimens and Testing atigue tests on MAG-welded tube-tube joints from fine-grained and engineering steels (S340+N and E355+N) under constant amplitude loading in the range of 10 4 to 5·10 6 cycles to rupture were conducted. Any effect of residual welding stresses is excluded because all the specimens were stress-relieved by heat treatment prior to testing. The 490 mm long test specimen consists of two tubes with an overlap length of 60 mm. Two seam welds at opposing quadrants joined the two tubes, see Fig. 1. The sheet thicknesses of the inner and outer tubes are t 1 = 2.0 mm and t 2 = 2.5 mm. Figure 1 : Overlapped tube-tube specimen. For the experimental investigations eight different loading types, shown in Fig. 2, were considered. The specimens were subjected to both alternating and pulsating pure axial force, pure torsional moment and proportional as well as non- proportional combinations of both loadings. In the latter case the phase shift was set at 90°. The selected ratio of the torsional moment to the axial force stress amplitudes of the combined loading was M T,a / F ٣ ,a = 28 Nm/kN. This is the ratio between the torsional moment and axial force amplitudes both led to 1·10 6 cycles to rupture in uniaxial tests. The experiments have been conducted using a servo-hydraulic multiaxial test rig with testing frequencies of 8-10 Hz for uniaxial and 1-2 Hz for multiaxial loading conditions. During testing, the fatigue cracks were monitored by taking photographs of the four existing weld start and end points at predefined numbers of cycles. Prior to testing, the specimens were sprayed using a scan spray, in order to ease the optical detectability of both formation and growth of the fatigue cracks after the test. The experimental set-up is depicted in Fig. 3 on the left hand side. Results Typical failures of specimens under pulsating uni- and multiaxial loading are shown in Fig. 3 on the right hand side. In all of the tested specimens fatigue cracks initiated at the transition area between weld toe and root, either at the weld start or at the weld end position. In the case of specimens under pure axial loading, the fatigue cracks were initiated in both the weld start and end locations. The crack fronts propagated to each other at the weld toe on the outer tube side during F