Issue 37

N.R. Gates et alii, Frattura ed Integrità Strutturale, 37 (2016) 166-172; DOI: 10.3221/IGF-ESIS.37.23 167 Integral, stress intensity factor (SIF), crack tip opening displacement (CTOD), etc.), the state of stress surrounding the crack tip is uniquely described by the value of that parameter. However, for variable amplitude loading, load history dependence may alter the local stress state at a crack tip allowing the possibility for multiple crack growth rates to occur at the same nominal driving force. Because of this, the amount of crack growth experienced in a variable amplitude loading history cannot be accurately assessed by simply summing the nominal crack growth increment for each applied cycle. Load sequence effects have been attributed to a number of mechanisms including: crack blunting, compressive residual stresses in front of the crack tip, changes in plasticity induced crack closure due to varying amounts of residual deformation in the crack wake, crack deflection (i.e. increased roughness induced closure), and strain hardening effects. Although all of these mechanisms may contribute in some degree, residual stresses and changes in closure levels tend to be the favored explanations [1]. In addition to load sequence effects on mode I crack growth under uniaxial nominal loading, multiaxial nominal stress states can also affect growth rates. For example, Gladskyi and Fatemi [2] studied axial and torsion load sequence effects on mode I crack growth and found that for cracks growing in mode I under pure torsion and pure axial loadings, cracks in pure torsion tests grew faster despite the same maximum principal stress range. Additionally, the insertion of blocks of pure torsion cycles into an otherwise uniaxial loading history was shown to increase crack growth rate, while the insertion of uniaxial cycles into a pure torsion loading history was shown to decrease crack growth rate. Most of these effects were explained in terms of the stress state at the crack tip. For mode I cracks growing under nominal torsion loading, it was speculated that the presence of a tangential stress (T-stress) component, acting parallel to the direction of crack growth, increased crack tip driving force for a given value of nominal SIF. Crack growth rate correlations for the various loading histories were found to improve when considering the effects of T-stress on plastic zone size. In this study, crack growth data were generated under a variety of variable amplitude loading conditions using notched tubular specimens of 2024-T3 aluminum alloy. Given the complexity of such an analysis, two state-of-the-art analysis codes were used in order to compare the experimentally measured crack growth lives to predictions based on two fundamentally different crack growth models. UniGrow [3] is based on the idea that residual stress distributions surrounding the crack tip are responsible for causing load sequence effects in variable amplitude crack growth, while FASTRAN [4] attributes these effects to varying degrees of plasticity induced closure in the crack wake. Since both analysis programs are meant for application to crack growth under uniaxial loading conditions, variable amplitude crack growth trends for multiaxial nominal loadings are compared with those for constant amplitude loading conditions in order to help interpret the analysis results. M ATERIAL AND TESTING PROCEDURES he material chosen for all fatigue tests performed in this study was aluminum alloy 2024-T3. Tests were performed using notched specimens of a thin-walled tubular geometry. The specimens feature a 30 mm long gage section with an outside diameter of 29 mm and an inside diameter of 25.4 mm, resulting in a wall thickness of 1.8 mm. To serve as a stress concentrator, a 3.2 mm diameter circular transverse hole was produced using a drilling and reaming operation through one side of the specimen gage section. Material properties and complete specimen geometry can be found in [5]. All variable amplitude fatigue tests were based on a single stress-based simulated service loading history representing the nominal axial and shear loading conditions on the lower wing skin of a long-range military patrol aircraft. A variety of take-off, landing, and in-flight maneuvers are represented in the history. In its entirety, the loading history contains around 915000 data points, with each point approximately corresponding to one loading reversal on the axial stress channel. The maximum and minimum axial stresses in the unscaled history are 144.8 MPa and -51.3 MPa, respectively, while the maximum and minimum shear stresses are 67.0 MPa and -15.9 MPa, respectively. Plots showing the time history of a 1000 reversal segment taken from the loading history, along with the axial-shear stress path for this same segment, are shown in Fig. 1. The loading segment shown in this figure is representative of the loading patterns repeated throughout the remainder of the full history. From the stress path, it can be seen that the loading history contains significant non- proportional loading events. Different loading conditions were obtained in testing by using the axial loading channel only, the torsion channel only, or the combined axial-torsion loading. Variable amplitude fatigue tests were performed using a closed loop servo-hydraulic axial–torsion load frame by repeatedly applying the entire load history in nominal load control until a tip-to-tip crack length of 15 mm was reached. For each test, the entire loading history was scaled by an appropriate factor to obtain stress levels that would produce fatigue lives ranging from less than one block to around 10 blocks. A summary of the applied loading conditions for all T