Issue34

N.R. Gates et alii, Frattura ed Integrità Strutturale, 34 (2015) 27-41; DOI: 10.3221/IGF-ESIS.34.03 34 Figure 3 : Initial mode II crack length before mode I branching vs. shear stress amplitude for fully-reversed pure torsion tests. Vertical arrows indicate a non-branching condition. The mean stress effect on crack path that was observed in these tests further emphasizes the role of crack face interaction in the crack growth process. For two different values of shear stress amplitude, specimens were tested with a static compressive stress, no normal stress, and a static tensile stress. It should be noted that static normal stresses were used so as to not introduce mixed-mode growth effects through a nonzero mode I SIF range. The fatigue lives for these tests, to various tip-to-tip crack lengths, are shown in Fig. 4. For both shear stress levels, the addition of the tensile normal stress reduced the overall fatigue life by around an order of magnitude. Much of this difference can be attributed to a decrease in crack growth life which would logically stem from a reduction in crack face friction and roughness induced closure effects. As a result, crack paths for these tests were always observed to be in the specimen circumferential direction, perpendicular to the applied tensile stress (Fig. 5(a)). For the tests in which a static compressive stress was applied, the crack growth plane switched from the specimen circumferential direction to the longitudinal direction (Fig. 5(b)). Although both are planes of maximum shear stress, the compressive normal stress acts to increase crack face interaction and inhibit crack growth on the circumferential plane. As a result, longitudinal cracks developed and grew under the same nominal loading and in a similar manner to those for torsion only tests. For specimens tested at the lower shear stress amplitude, crack branching and mode I growth was observed over the same range of crack lengths for both the compressive normal stress and pure torsion cases (although not for the specimen pictured in Fig. 5). The decrease in fatigue life observed in Fig. 4(b) for the static compression tests compared to pure torsion tests may be due to an increase in plastic zone size and crack driving force as a result of the additional compressive tangential stress (T-stress) at the crack tip. Although also present for tests performed at the lower shear stress amplitude, the effect of T-stress is expected to have a larger effect as stress levels increase. Also, the addition of the static stresses at the higher shear stress amplitude results in a general yielding condition throughout the specimen gage section which greatly increases the probability of crack initiation compared to the pure torsion test. For the lower shear stress amplitude, all loading conditions result in nominally elastic stresses throughout the gage section. Figure 4 : Fatigue lives to various crack lengths for smooth specimens tested at shear stress amplitudes of (a) 140 MPa and (b) 188 MPa with and without static axial stresses. Specimens for which no crack growth data were available are shown as solid gray columns. (a) (b)