Issue34

N.R. Gates et alii, Frattura ed Integrità Strutturale, 34 (2015) 27-41; DOI: 10.3221/IGF-ESIS.34.03 31 All fatigue tests were carried out in a closed loop servo-hydraulic axial–torsion load frame with a dynamic rating of 100 kN axial load and 1 kN·m torsional load. Load train alignment was verified in accordance with ASTM Standard E1012 [23] prior to the beginning of, and periodically throughout, testing. Crack initiation and growth were monitored via cellulose acetate replication for smooth specimen tests, and by using a 2.0 megapixel digital microscope camera, capable of 10–230x optical zoom levels, for precracked specimens. Crack lengths were measured using an eyepiece scale affixed to an optical microscope for the surface replicas, and by means of image analysis software for the digital microscope images. (a) (b) Figure 1 : Test specimen details for (a) thin-walled tubular specimen geometry and (b) sectioned view of machined precrack notch. All dimensions are in mm. All tests were performed in load control and include fully-reversed ( R = -1) pure torsion and in-phase axial-torsion tests for both smooth and precracked specimens. Additional smooth specimen tests were also performed for pure torsion loading, with and without the addition of a static tensile or compressive stress, to evaluate the effect of mean stresses on mode II crack growth. E XPERIMENTAL RESULTS AND DISCUSSION lthough surface replicas have shown the development of small microcrack networks in a number of the smooth specimen fatigue tests performed in this study, crack coalescence was only observed in a limited number of these tests and usually occurred relatively early in the crack growth life. Therefore, questions were raised on whether or not crack coalescence played a large role in determining overall crack path for these tests. Thus the effect of microcrack networks and coalescence was investigated through the testing of smooth and precracked specimens under identical loading conditions. Two load levels each were used for tests under fully-reversed pure torsion loading, in-phase axial- torsion loading, pure axial loading, and 90° out-of-phase axial-torsion loading. However, results from the latter two loading conditions are not included in this study because the difference between mode I and mode II growth cannot be observed from the outer surface of the specimen. A future three-dimensional analysis would be required to interpret those results. Therefore, only the experimentally observed crack paths for the two torsion loading levels and two in-phase axial- torsion load levels are shown in Fig. 2. By comparing crack paths between the smooth and precracked specimens in Fig. 2, it is easy to see that they are very similar. This is true even for complex crack paths where cracks initiate in mode II, branch into mode I cracks, and eventually transition back to mode II after growing for some distance (Fig. 2(b-d)). This suggests that microcrack networks and their coalescence did not play a significant role in determining the crack paths for these tests. If dominant cracks, growing without the influence of crack coalescence, were expected to always branch into mode I growth regardless of the applied loading (as predicted by traditional crack growth direction criteria), then the crack path for the precracked specimen subjected to the higher level pure torsion load (Fig. 2(a)) would certainly not have remained vertical for its entire growth life. This, combined with the transition back to shear-mode growth after a period of mode I growth observed for the other precracked specimen tests, supports findings from many smooth specimen surface replicas where shear dominated crack growth occurred even in the absence of any observable crack coalescence. Although the existence of the type R crack growth mechanism is certainly not being rejected, it is clear that in the case of this study, coalescence was most likely not responsible for the shear dominated crack paths observed in the smooth specimen tests. Therefore, there must be some other mechanism by which crack growth behavior transitions from being mode I to mode II dominated. By studying the crack paths in Fig. 2, a correlation between the loading level and/or SIF and the crack growth direction is observed. To help illustrate this point, Tab. 1 was constructed and contains all available A