This investigation focuses on mode I delamination propagation in a unidirectional (UD) carbon fiber reinforced polymer (CFRP) composite laminate. Delamination propagation in this type of material may be accompanied by fiber bridging, a phenomenon where fibers from one face of the delamination cross over to the other face, such that the fibers are simultaneously pulled from both faces, thus, bridging the delamination. This increases the material’s apparent resistance to further propagation of the delamination. This phenomenon occurs mostly in beam-type test specimens commonly used to characterize failure of composite materials, but does not generally occur in structures with the exception a few structures such as rotor blades. The aim of this investigation is to quantify the effect of fiber bridging for quasi-static and fatigue testing of DCB specimens so as to eliminate it from the fracture and fatigue delamination propagation properties.
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Themes: Beyond Similitude: Role of Multiscale Heterogeneity in Fracture Prognosis
ANALYSIS OF RIGID CURVED INCLUSION EMBEDDED IN A SOFT MATRIX: EXPERIMENTAL INSIGHTS
The role of fiber curvature in short-fiber thermoplastics can be explored by studying a rigid curved inclusion embedded in an epoxy matrix. Although inclusion enhances global stiffness, it also acts as a source of stress singularity, which leads to failure. The current study employs the 2D digital image correlation (DIC) technique to obtain full-field strain fields over a rigid curved inclusion embedded in a soft matrix. The experiment is performed on a rigid curved inclusion specimen subjected to remote tensile loading of 350N. The experimentally obtained strain field is verified using the finite element technique, and a good match is observed. Finally, the stress intensity factor is defined for the rigid curved inclusion, and it is estimated along with the geometric correction factor.
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EXAMINING SUB-GRAIN DRIVING FORCES FOR SMALL CRACK GROWTH [Keynote]
High energy X-ray diffraction microscopy (HEDM) techniques and micro-computed tomography were combined with in-situ cyclic loading to examine the evolution of sub-grain-level fatigue crack growth within a Ni-base superalloy at room temperature. A focused-ion beam notch was introduced within the specimen to concentrate damage within the characterized microstructure region of interest. The test specimen was subjected to fatigue cycling with pauses for periodic micro-computed tomography and HEDM measurements to characterize the sporadic growth of the crack front and grain-level strains ahead of the crack front. The HEDM data was used to instantiate a crystal plasticity finite element model and compared to experimentally determined grain-level strains, sub-grain reorientation, and crack path.
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PREDICTING MICROSTRUCTURALLY SENSITIVE FATIGUE-CRACK PATH IN WE43 MAGNESIUM USING HIGH-FIDELITY NUMERICAL MODELING AND THREE-DIMENSIONAL EXPERIMENTAL CHARACTERIZATION
Microstructurally small fatigue-crack growth in polycrystalline materials is highly three-dimensional due to sensitivity to local microstructural features (e.g., grains). One requirement for modeling microstructurally sensitive crack propagation is establishing the criteria that govern crack evolution, including crack deflection. Here, a high-fidelity finite-element modeling framework is used to assess the performance and validity of various crack-growth criteria, including slip-based metrics (e.g., fatigue-indicator parameters), as potential criteria for predicting three-dimensional crack paths in polycrystalline materials. The modeling framework represents cracks as geometrically explicit discontinuities and involves voxel-based remeshing, mesh-gradation control, and a crystal-plasticity constitutive model. The predictions are compared to experimental measurements of WE43 magnesium samples subject to fatigue loading, for which three-dimensional grain structures and fatigue-crack surfaces were measured post-mortem using near-field high-energy X-ray diffraction microscopy and X-ray computed tomography. Findings from this work are expected to improve the predictive capabilities of numerical simulations involving microstructurally small fatigue-crack growth in polycrystalline materials.
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EFFECT OF LOCAL HETEROGENEITY ON FRACTURE DRIVING FORCES
Traditional fracture theories infer the local crack growth driving forces by surveying the mechanical response far from the crack. Although this approach has successfully predicted fracture by assuming isotropic and homogeneous materials, local heterogeneity such microstructural heterogeneity can affect fracture response. This presentation will evaluate the differences between the local and far field driving forces using different microstructure-sensitive modelling approaches. We will demonstrate the effects of grain size and crystallographic orientation gradients on crack tip blunting and microplasticity variability. We will also explore the role of microstructures as a buffer between the local and far fields considering the propagation of uncertainty from constitutive models into fracture prognosis. To conclude, we will discuss the implications for traditional experimental methods based on far field measurements smearing out important crack tip variability.
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