Issue34

Y. Hos et alii, Frattura ed Integrità Strutturale, 34 (2015) 133-141; DOI: 10.3221/IGF-ESIS.34.14 136 Multiaxial Loading Three specimens were tested under proportional loading, 90° out-of-phase loading and 45° out-of-phase loading, namely the specimens R-004, R-005 and R-006 respectively. The amplitudes of the loadings are max 33kN F  , max 382Nm M  and 1 F M R R    . The nominal stresses in the gross section are ,max 101.5MPa n   and ,max 62.6MPa n   . Again, two cracks initiated at the notch. Only for the very early stage of short fatigue crack growth it can be assumed that the crack grows in the uniaxial stress field prevailing at the notch surface. The different crack paths can be seen in Figs. 4 to 6 with a summary in Fig. 7. For proportional loading, Fig. 4, the principal axis calculated from the nominal stresses is 25.5° inclined against the specimen axis. The crack growth direction, however, is only inclined approximately 14° against the cross section plane. Some cyclic mode II loading could have contributed to the fatigue crack growth. The specimen does not show the conventional and expected growth behaviour under pure mode I. Figure 4: Cracks in specimen R-004, proportional loading with max 33kN F  , max 382Nm M  and 1 F M R R    , steel S235. For out-of-phase loading with phase angle 90° the two cracks show unsymmetrical growth behaviour, Fig. 5. While crack 1 grows straight at nearly 45° inclined to the specimen axis, crack 3 starts curving almost immediately after initiation. The crack has at least two options for choice which path to follow and both options are nearly equilibrate. The crack growth life – the comparison is shown in Fig. 8 – under the 90° out-of-phase loading is longest. Figure 5: Cracks in specimen R-005, out-of-phase loading with max 33kN F  , max 382Nm M  and 1 F M R R    , phase angle 90°, steel S235. Under out-of-phase loading with phase angle 45° crack 1 is inclined in average 26° during its first 13 mm of growth and afterwards turns to a direction perpendicular to the specimen axis. Contrary to crack 1 the crack 3 shows an initial steep crack path inclined 58° against the cross section plane. After 12 mm of crack growth a sharp kink to the cross section plane growth occurs. The stepwise crack growth, see Fig. 6, is related to the various kinks in the crack growth direction. After the occurrence of a kink the growth rate is decelerated. As a consequence of the variety of kinks the crack surface becomes very rough. In total, the crack growth is slower than under proportional loading which might be due to increased roughness induced crack closure. The frequency of kinks and the unsymmetrical growth of the two cracks may again indicate that the crack path has at least two options for choice which are nearly equilibrate. In its early stage the mode I growth around the notch surface dominates. With increasing crack length a fracture mode II gains priority and is enforced after approximately 13 mm of crack growth.