Crack path determination for non-proportional mixed-mode fatigue
Highsmith, Shelby, Jr.
MetadataShow full item record
The objective of this work is to study crack path deflection under proportional and non-proportional mixed-mode fatigue and predict crack branching direction based on linear elastic fracture mechanics (LEFM) driving forces. Under proportional in-phase mixed Mode I / Mode II loading conditions, crack growth direction has previously been observed in some materials to shift from tensile-dominated Mode I to shear-dominated Mode II or mixed-mode crack growth at higher proportions of initial Mode II loading, but non-proportional loads are not well-characterized. An LEFM approach is desired in order to implement the model in crack growth software such as the boundary element-based fracture analysis package FRANC3D. A novel specimen configuration has been designed and analyzed for generation of wide ranges of mixed-mode loading conditions in a single test. This specimen and a more conventional thin-walled tubular specimen have been used to test polycrystalline nickel-base superalloy Inconel 718 under proportional in-phase and 3 kinds of non-proportional fatigue loading. Stress intensity factors for the various conﬁgurations have been analyzed with FRANC3D. Modal transition from Mode I (tensile) to Mode II (shear) crack branching has been observed in several load cases. Qualitative microscopy of fracture surfaces was used to characterize the difference between crack branch modes. An LEFM approach based on an effective stress intensity factor range, which incorporates the maximum value and range of each appropriate stress intensity (Mode I or Mode II), has been used to successfully predict the crack deﬂection angles, and in some cases to quantify modal transition, within each load case considered. Variability between load cases and specimen configurations points to the limitations of LEFM in providing a general predictor of crack path behavior across all types of non-proportional mixed mode loading.