Issue 41

M. Vormwald et alii, Frattura ed Integrità Strutturale, 41 (2017) 114-122; DOI: 10.3221/IGF-ESIS.41.16 116 10) were scheduled for specimens not exposed to any heat treatment in an as-welded condition in order to investigate the influence of residual stresses on fatigue resistance. The experiments have been conducted using a servo-hydraulic multi-axial test rig with testing frequencies of 8-10 Hz for uni-axial and 1-2 Hz for multi-axial loading conditions. The experimental set-up is depicted in Fig. 2. Prior to testing the specimens were sprayed using a scan spray in order to ease the optical detectability of both formation and growth of fatigue cracks after the test. In Fig. 2a scan of the specimen in the sprayed welded area comprising start (left) and end points (right) is shown. During testing, the fatigue cracks were monitored by taking photographs of the four existing weld start and end points at predefined numbers of cycles. If predefined upper and lower limits for deformation values were exceeded, the experiment was terminated. The limits for deformation values were set to ±8 mm and ±5 degrees. The corresponding number of cycles is defined as the failure criterion. series condition R F ٣ M T M T,a / F ٣ ,a in Nm/kN  a /  a   number of specimens 01 stress-relieved -1 + - - - - 7 02 stress-relieved -1 - + - - - 7 03 stress-relieved -1 + + 28.0 1.40 0° 7 04 stress-relieved -1 + + 17.9 0.895 0° 7 05 stress-relieved -1 + + 28.0 1.40 90° 7 06 stress-relieved -1 + + 17.9 0.895 90° 7 07 as-welded -1 + - - - - 7 08 as-welded -1 - + - - - 7 09 as-welded -1 + + 28.0 1.40 0° 7 10 as-welded -1 + + 28.0 1.40 90° 7 Table 1 : Test program. Figure 2 : Experimental set-up a) , scan of the weld b) . Results Typical failures of specimens under pulsating uni-axial and multi-axial loading are shown in Fig. 3. Fatigue cracks initiated at the transition area between weld toe and root in all the tested specimens, either at the weld start or at the weld end