During cyclic loading, localization of intragranular deformation due to crystallographic slip acts as a precursor for crack initiation, often at coherent twin boundaries. A suite of high-resolution synchrotron X-ray characterizations, coupled with a crystal plasticity simulation, was conducted on a polycrystalline nickel-based superalloy microstructure near a parent-twin boundary in order to understand the deformation localization behavior of this critical, 3D microstructural configuration. Dark-field X-ray microscopy was spatially linked to high energy X-ray diffraction microscopy and X-ray diffraction contrast tomography in order to quantify, with cutting-edge resolution, an intragranular misorientation and high elastic strain gradients near a twin boundary. These observations quantify the extreme sub-grain scale stress gradients present in polycrystalline microstructures, which often lead to fatigue failure.
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Abstract -
Free, publicly-accessible full text available November 1, 2024
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Free, publicly-accessible full text available October 1, 2024
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Mechanophores (MPs) undergo chemical reactions to become fluorescent in response to a mechanical stimulus that reflects the magnitude and distribution of applied stress. MPs are an emerging technology for self‐reporting damage sensing applications in polymeric materials in the aeronautical, energy generation, and automotive industries. However, quantitative calibration of the MP response to local stresses remains an outstanding challenge. Herein, a method to calibrate the intensity of the MP fluorescent activation (
I ) with local hydrostatic stresses (σ h ) is presented. Uniaxial tension is applied to a simple composite comprised of a rigid sphere (silica) embedded in a MP‐functionalized elastomeric matrix (spiropyran (SPN) functionalized polydimethylsiloxane (PDMS)). By monitoring the fluorescence intensity with a confocal microscope while a quasi‐static deformation is applied, in situ observations of MP activation as a function of applied uniaxial strain are obtained. To calculate the associated stress fields, a finite element analysis (FEA) with cohesive zone elements is employed. By comparingσ h , calculated through FEA with theI of the PDMS/SPN system, a linear relationship betweenI andσ h is directly determined. The technique presented can be employed for many MP‐containing materials systems to calibrateI toσ h .
