Issue34

N.R. Gates et alii, Frattura ed Integrità Strutturale, 34 (2015) 27-41; DOI: 10.3221/IGF-ESIS.34.03 30 factory roof fracture surfaces were observed, it was speculated that the mode I loading of favorably oriented microcracks in the crack tip plastic zone would produce higher crack growth rates than the effective mode III loading of the main crack. Therefore, whichever loading mode produced the highest crack growth rate values for a given combination of loading conditions and crack length was considered to control the crack growth process. Using a similar “maximum growth rate” approach, Doquet and Bertolino [8] addressed the fracture mode issue by considering a local approach based on elastic-plastic crack tip stresses and strains derived via non-linear incremental FEA. Two multiaxial fatigue damage parameters, one predicting tensile dominated failure (Smith-Watson-Topper) and one predicting shear dominated failure (Fatemi-Socie) were then applied to predict crack path based on which mode would produce the most damage (based on the shortest predicted life and highest growth rate). The damage parameter values were averaged over an arbitrary length (between 0.020 – 0.100 mm) along a radial line emanating from the crack tip along either plane experiencing the maximum normal strain or maximum shear strain range. A transition in growth mode from tensile dominated to shear dominated was successfully predicted at higher applied loading, but no quantitate comparison was made to experimental results. A major drawback of this approach, however, is the need to perform non-linear FEA over an entire cycle for each crack length and loading level being considered. Tanaka [9], in an investigation on shear crack growth in circumferentially grooved specimens of a stainless and carbon steel, speculated that a criterion for coplanar versus branched crack growth could be related to the shear strain range ahead of the crack tip. For higher loading levels, it was suggested that increased plasticity at the crack tip could lead to the initiation of local shear oriented microcracks ahead of the crack tip which would encourage coplanar crack growth through local coalescence. However, no quantitative analysis was provided. This study is similar in some regards to those mentioned previously, but presents some aspects typically not considered in shear-mode crack growth studies. For example, nearly all of the aforementioned studies deal exclusively with mode II cracks growing from precracks or circumferential notches. This study, on the other hand, places a strong emphasis on the behavior of naturally initiated fatigue cracks in smooth specimens. One problem with studying growth from precracks is that residual stresses in the crack tip region, resulting from the precracking procedure, have the potential to influence the subsequent mode II crack growth. Even if specimens are annealed following precracking to eliminate these effects, the crack face topography still varies from that of a naturally occurring crack. In addition, growing a mode II crack from a precrack can significantly alter the length scale required for branching mechanisms to occur and eliminates any influence on crack path from microcrack coalescence. The present study compares crack path evolution between both natural cracks and cracks growing from artificial precracks in order to evaluate the effect of microcrack coalescence on overall crack path. The role of crack face friction and roughness induced crack closure on shear-mode crack growth is evaluated as well. An emphasis is placed on crack paths in the low to intermediate fatigue life regime where mode II cracks may or may not branch to grow in mode I. In an attempt to quantify the experimental observations, a model is proposed to account for reductions in effective mode II SIF due to crack face interaction effects. M ATERIAL AND TESTING PROCEDURES ll tests performed for this study utilized thin-walled tubular specimens of 2024-T3 aluminum alloy, a common aerospace alloy since the 1930s. Mechanical properties for the material were generated experimentally and include a yield strength (0.2% offset) of 330 MPa, ultimate tensile strength of 495 MPa, and modulus of elasticity of 73.7 GPa. The specimens, machined from drawn tubing with nominal dimensions of 34.9 mm outside diameter and 4.75 mm wall thickness, were designed in accordance with ASTM Standard E2207 [22] and feature a 30 mm long gage section with an outside diameter of 29 mm and an inside diameter of 26 mm, resulting in a wall thickness of 1.5 mm. The specimen geometry and dimensions are shown in Fig. 1(a). For precracked specimens, the term precrack is used to refer to a small notch machined in the specimen meant to resemble, as closely as possible, the profile of a naturally occurring fatigue crack. No actual precracking procedure was performed on this notch prior to applying the testing loads. It is referred to as a precrack simply as a means of differentiating it from other notched specimens, containing non crack-like notches, which are referred to in the introduction. All precrack notches were machined by means of a 0.127 mm diameter ball mill and were semi-elliptic in shape. The precracks were of a length and depth equal to approximately 1 mm and 0.2 mm, respectively, and were aligned with a plane of maximum shear stress for each applied loading condition. A sectioned view of a precrack notch is shown in Fig. 1(b). All smooth specimens were fully polished, inside and out, to eliminate any adverse effects from machining marks. Final polishing was performed with a 3 micron lapping film. Precracked specimens were polished similarly, but external polishing was only performed in the crack growth region surrounding the precrack. A