Issue 41

M. Vormwald et alii, Frattura ed Integrità Strutturale, 41 (2017) 114-122; DOI: 10.3221/IGF-ESIS.41.16 115 These models base on the real weld geometry obtained by means of high-precision 3D-scanner. In connection with these models, recommendations for determining notch stresses by applying the finite element (FE) method were developed. Nowadays, different S - N curves are assigned to weld toe and weld root failure scenarios induced by normal and shear stresses. For the case of failure at weld root, a fatigue assessment concept covering both geometrical and statistical size effects was presented in [7]. Many research activities have been initiated during the last years for the investigation of fatigue design of multiaxially loaded welded joints [8-10]. Nevertheless, the case of weld ends under such loading is not sufficiently explored yet. 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 failure 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. The critical plane oriented criteria according to Findley [11] has been applied, in order to determine interaction lines for proportional and non-proportional loading cases. E XPERIMENTAL INVESTIGATION Specimens and Testing atigue tests were conducted on welded tube-tube joints from fine-grained and engineering steels (outer tube: S340+N and inner tube: E355+N) under constant amplitude loading in the range of 10 4 to 5·10 6 cycles to failure. The 490 mm-long test specimen consists of two tubes with an overlap length of 60 mm. The external diameters of the inner and outer tubes are d a,1 = 40 mm and d a,2 = 45 mm, respectively. The inner tube has a sheet thickness of t 1 = 2.0 mm; the sheet thickness of the outer tube is t 2 = 2.5 mm. Two seam welds at opposing quadrants joined the two tubes, see Fig. 1. The tube-tube joints were manufactured using gas shielded metal arc welding. The welding was carried out in a twin-robot system, where two robots work simultaneously. Figure 1 : Overlapped tube-tube specimen. The fatigue test program shown in Tab. 1 was carried out. The specimens were subjected to alternating 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°. In the case of combined loading two ratios were considered for the torsional moment to the axial force amplitudes M T,a / F ٣ ,a . This ratio was set to 28 Nm / kN on the one hand and 17.9 Nm / kN on the other hand, see series 01-06 in Tab. 1. Herein, dimensionless ratios expressed by nominal stress amplitudes  a /  a are also given. In series 01-06 any effect on residual welding stresses is excluded because all the specimens were stress-relieved by heat treatment (600°C for 6 hours and followed by slow cooling) prior to testing. Furthermore, four test series (series 07- F