(14) 3D Characterization and Modeling of Fatigue Cracks (slides)
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Rollett, Anthony D.
Enormous strides have been made in quantifying the growth of fatigue cracks over the years and incorporating that understanding in predictions of component lifetime. Nevertheless, it is clear that the behavior of short cracks is less well quantified, where short is relative to the length scale(s) found in materials microstructure, e.g. grain size. Ultimately, materials science seeks to predict the location and growth of fatigue cracks in order to design materials microstructure to maximize fatigue lifetime. Towards that end, it is interesting to study the relationship between cracks and microstructure near the initiation point. Short fatigue cracks in nickel-based superalloys have been characterized using conventional SEM and orientation mapping. 3D characterization used High Energy Diffraction Microscopy (HEDM), and computed tomography (CT) to map out the crack positions within their embedding grain structure. The main finding is that cracks develop most readily along long twin boundaries with high resolved shear stress on the slip systems parallel to the twin plane. Also, both halves of a different superalloy, fully fractured sample have been fully characterized in 3D using the same tools. The HEDM and CT were performed with high energy x-rays on beamline 1ID at the Advanced Photon Source (APS). The 3D orientation maps are used as input to computations of the full field stress-strain response. The fracture surface is analyzed with respect to local orientation and inter- versus trans-granular character. The likely origins of fatigue crack initiation in these cases are discussed.
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