PSI - Issue 38

Matthias Hecht et al. / Procedia Structural Integrity 38 (2022) 251–259 Matthias Hecht t al. / Structu al Integrity Procedia 00 (2021) 00–000

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Fig. 2 (a) Normalized stress amplitude, (b) normalized mean stress and (c) R-ratio over the cycles in the load sequence

For reasons of comparability with previous projects, the sequence length used there is used so that it is repeated only approximately three times on the high load horizons. However, no influence is detected, as the Gassner lines does not deflect at high load levels. An � -ratio of 0.01 was chosen with a constant minimum value of stress � min . As all amplitudes come to the same minimum stress � min , the -ratio increases at small amplitudes, so the -ratio varies at each cycle in a range of 0.01 ≤ ≤ 0.4, Fig 2, (c). 3. Results 3.1. Fatigue test results The fatigue test results are presented by Woehler or Gassner lines. All specimens failed in the adhesive bond. Due to the constant stress ratio between normal and shear stress for the multiaxial test series, it is possible to plot these two stresses in one graph with two ordinates, Figure 3. The fatigue tests under constant amplitudes show similar endurable stresses for pure tensile and torsional loading. The endurable stress amplitude with a number of cycles to failure at B = 10 4 is approximately , = 14.7 MPa for pure tension and , = 14.2 MPa for pure torsion, as it is shown via additional lines in Figure 3. The resulting ratio of , / , ≈ 1 indicates that the multiaxial fatigue behavior might be controlled dominantly by normal stresses [15]. Under constant amplitudes combined loading results in lower stress levels compared to pure axial loading or pure torsion. The Woehler lines for multiaxial loading indicate a slight dependence on the phase shift . Non-proportional loading ( = 90°) leads to a slightly higher fatigue life. This behavior is due to the ductility state of the bonding, as observed also at other polymer materials [16]. The slope of the Woehler lines is quite similar in all the load cases investigated and varies in a range 13.5 ≤ ≤ 15.1, Table 1. These comparatively high values resp. shallow slopes are typical for adhesively bonded joints [17]. The Woehler lines do not show a knee point in the range of the load cycles investigated. The determined scatter = 1: ( S =10% ) ( S =90% ) ≥ 1: 1.15 (8) is low. However, a larger scatter can be identified in the case of applied torsional loading.