Issue 37

N.R. Gates et alii, Frattura ed Integrità Strutturale, 37 (2016) 166-172; DOI: 10.3221/IGF-ESIS.37.23 168 tests can be found in [5]. In addition to the variable amplitude fatigue tests, a variety of constant amplitude fatigue tests were also performed on notched specimens under fully-reversed axial, torsion, and combined axial-torsion loading. Some results from these tests are presented herein as a basis for evaluating the variable amplitude crack growth predictions. Crack initiation and growth were monitored by using a digital microscope camera, capable of 10–230x optical zoom levels. Crack lengths were then measured using digital image analysis software. Figure 1 : Representative loading segment from the variable amplitude service loading history in terms of (a) applied stresses vs. time and (b) axial-shear stress path. M ODELING PROCEDURES ecause constant amplitude crack growth rate data for the specimens tested in this study were only generated under fully-reversed loading conditions, they are not ideal for computing crack growth properties for either the UniGrow or FASTRAN software. As a result, the UniGrow analyses performed in this study (version 2014-02-09) utilized material properties for a 2024-T351 aluminum alloy, which were already included in the UniGrow material library. Similarly, FASTRAN (version 5.42) analyses were based on crack growth properties reported in literature for 2024-T3 [6]. The mechanical and crack growth properties for the materials used in both programs were found to be very similar to those measured experimentally for the 2024-T3 alloy used in this study. Consequently, using the material data from literature is not expected to have a significant effect on the accuracy of crack growth predictions presented in the following section for either program. Crack growth was only measured on the outer surface of the specimens tested in this study. Therefore, it is reasonable to assume that while cracks were short, their geometry was either that of a through thickness crack or corner crack growing from the edge of the hole. Additionally, since cracks were observed to grow from both sides of the hole, the assumption of diametrically opposite symmetric cracks was considered reasonable. As a result, two different specimen geometries were used in the following analyses: a circumferential through crack in a tube (TT), and two symmetric semi-elliptic corner cracks at a hole in a plate subjected to remote tensile stress (CCH). The CCH geometry was assumed to transition to the TT geometry after cracks became through thickness. The same SIF solutions were defined as custom inputs in each crack growth program. Because stress intensity factors for short cracks are lower for the CCH geometry, the predicted crack growth lives are longer than those based on the TT crack geometry assumption. Since it is possible for actual cracks to assume either of these geometries, or a combination of both, crack growth predictions based on both TT and CCH SIF solutions should provide an approximate upper and lower bounds for experimentally observed crack growth curves, so long as the crack growth models are accurate. For each nominal loading history, stresses were projected onto the maximum principal stress plane (consistent with observed crack growth planes) and input into the crack growth programs for analysis. This corresponds to the 0° plane (perpendicular to the specimen axis) for axial and combined variable amplitude loading histories, and the 45° plane for pure torsion loading. Crack growth lives were compared from an initial half crack length of 1.8 mm (0.2 mm excluding the hole radius) to a length of 7.5 mm. B (a) (b)