An artificial neural network is used to evaluate the effectiveness of six metrics and their combi... more An artificial neural network is used to evaluate the effectiveness of six metrics and their combinations to assess whether slip transfers across grain boundaries in coarse-grained oligocrystalline Al foils <cit.>. This approach extends the one- or two-dimensional projections formerly applied to analyze slip transfer. The accuracy of this binary classification reaches around 87 for the best single metric and around 90 when considering two or more metrics simultaneously. The results suggest slip transfer mostly depends on the geometric relationship between grains. Training a double-layer network having 10 nodes per hidden layer with 40 measurements is sufficient to render the maximum accuracy.
Abstract A new electron channeling contrast imaging (ECCI) based approach for the characterizatio... more Abstract A new electron channeling contrast imaging (ECCI) based approach for the characterization of heterogeneous plastic deformation across multiple grains in a polycrystal is presented. The nature of the shear accommodation at grain boundaries has been assessed by evaluating the spatial distribution of dislocations and slip system activities near the boundaries and relating these observations to standard slip transfer criteria and the global state of stress. The direction of slip band propagation, and consequently the location of dislocation slip band nucleation, was determined by characterizing the slip band widths across individual grains. This assessment leads to a better understanding of the factors involved in polycrystalline deformation, including the grain boundary shear accommodation associated with both dislocation pile-ups and grain boundary dislocation nucleation. Because ECCI allows study of dislocation structures over large surface areas, and thus larger overall volumes than typically are accessible by other techniques, the overall approach allows the dislocation distribution across individual grains to be understood in terms of the broader polycrystalline constraints.
Abstract The feasibility to determine the adjustable parameters of single crystal plasticity cons... more Abstract The feasibility to determine the adjustable parameters of single crystal plasticity constitutive laws by an inverse approach that minimizes the deviation between the measured and simulated indentation response of individual grains (of a polycrystalline sample) is investigated for the case of face-centered cubic (fcc) lattice structure. Optimization uses the Nelder–Mead (NM) simplex algorithm, which was modified to navigate parameter space regions where objective function evaluations fail. A phenomenological power-law is assumed as the constitutive description for crystal plasticity. Simulated cases of indentation with prescribed constitutive parameter values serve as the virtual reference. A sensitivity analysis revealed that the initial and saturation slip resistance τ 0 and τ sat are the most dominant parameters while the hardening slope h 0 has less influence. Reproducibility and robustness are analyzed for different objective functions involving the load–displacement response and residual surface topography for several indentation crystal orientations . Concurrent optimization of load–displacement and topography consistently provided the least scatter in the optimized parameter values from the target solution compared to either one individually and essentially independent of indentation crystal orientation. Deviations in slip activity were typically of the same order of magnitude as the combined deviations of load–displacement and surface topography response. Optimization of more than one crystallographic indentation response at once did not improve the parameter estimation quality but proportionally increases the evaluation effort. It is concluded that for fcc materials one single crystal indentation experiment suffices to closely quantify the two most influential parameters of a phenomenological constitutive plasticity law when the objective function of the modified NM simplex algorithm proposed herein combines the load–displacement response and residual surface topography.
Many of the three-dimensional photonic crystals occurring in the scales of insects have bicontinu... more Many of the three-dimensional photonic crystals occurring in the scales of insects have bicontinuous cubic structures. Their optical properties have been studied extensively, however little is known about their mechanical properties and their optical response under deformation. We demonstrated a mechanochromic effect by deforming the scales of a weevil and calculated the elastic, optical and mechanochromic (assuming homogeneous deformation) properties of the three types of bicontinuous cubic structures occurring in nature: P-structure (primitive), G-structure (gyroid) and D-structure (diamond). The results show that all investigated properties of these three structure types strongly depend on their geometry, structural parameters such as volume fractions of the two constituting phases and the directions of the incident light or applied stress, respectively. Interestingly, the mechanochromic simulation results predict that these structures may show blue-shift or even red-shift under ...
An artificial neural network is used to evaluate the effectiveness of six metrics and their combi... more An artificial neural network is used to evaluate the effectiveness of six metrics and their combinations to assess whether slip transfers across grain boundaries in coarse-grained oligocrystalline Al foils <cit.>. This approach extends the one- or two-dimensional projections formerly applied to analyze slip transfer. The accuracy of this binary classification reaches around 87 for the best single metric and around 90 when considering two or more metrics simultaneously. The results suggest slip transfer mostly depends on the geometric relationship between grains. Training a double-layer network having 10 nodes per hidden layer with 40 measurements is sufficient to render the maximum accuracy.
Abstract A new electron channeling contrast imaging (ECCI) based approach for the characterizatio... more Abstract A new electron channeling contrast imaging (ECCI) based approach for the characterization of heterogeneous plastic deformation across multiple grains in a polycrystal is presented. The nature of the shear accommodation at grain boundaries has been assessed by evaluating the spatial distribution of dislocations and slip system activities near the boundaries and relating these observations to standard slip transfer criteria and the global state of stress. The direction of slip band propagation, and consequently the location of dislocation slip band nucleation, was determined by characterizing the slip band widths across individual grains. This assessment leads to a better understanding of the factors involved in polycrystalline deformation, including the grain boundary shear accommodation associated with both dislocation pile-ups and grain boundary dislocation nucleation. Because ECCI allows study of dislocation structures over large surface areas, and thus larger overall volumes than typically are accessible by other techniques, the overall approach allows the dislocation distribution across individual grains to be understood in terms of the broader polycrystalline constraints.
Abstract The feasibility to determine the adjustable parameters of single crystal plasticity cons... more Abstract The feasibility to determine the adjustable parameters of single crystal plasticity constitutive laws by an inverse approach that minimizes the deviation between the measured and simulated indentation response of individual grains (of a polycrystalline sample) is investigated for the case of face-centered cubic (fcc) lattice structure. Optimization uses the Nelder–Mead (NM) simplex algorithm, which was modified to navigate parameter space regions where objective function evaluations fail. A phenomenological power-law is assumed as the constitutive description for crystal plasticity. Simulated cases of indentation with prescribed constitutive parameter values serve as the virtual reference. A sensitivity analysis revealed that the initial and saturation slip resistance τ 0 and τ sat are the most dominant parameters while the hardening slope h 0 has less influence. Reproducibility and robustness are analyzed for different objective functions involving the load–displacement response and residual surface topography for several indentation crystal orientations . Concurrent optimization of load–displacement and topography consistently provided the least scatter in the optimized parameter values from the target solution compared to either one individually and essentially independent of indentation crystal orientation. Deviations in slip activity were typically of the same order of magnitude as the combined deviations of load–displacement and surface topography response. Optimization of more than one crystallographic indentation response at once did not improve the parameter estimation quality but proportionally increases the evaluation effort. It is concluded that for fcc materials one single crystal indentation experiment suffices to closely quantify the two most influential parameters of a phenomenological constitutive plasticity law when the objective function of the modified NM simplex algorithm proposed herein combines the load–displacement response and residual surface topography.
Many of the three-dimensional photonic crystals occurring in the scales of insects have bicontinu... more Many of the three-dimensional photonic crystals occurring in the scales of insects have bicontinuous cubic structures. Their optical properties have been studied extensively, however little is known about their mechanical properties and their optical response under deformation. We demonstrated a mechanochromic effect by deforming the scales of a weevil and calculated the elastic, optical and mechanochromic (assuming homogeneous deformation) properties of the three types of bicontinuous cubic structures occurring in nature: P-structure (primitive), G-structure (gyroid) and D-structure (diamond). The results show that all investigated properties of these three structure types strongly depend on their geometry, structural parameters such as volume fractions of the two constituting phases and the directions of the incident light or applied stress, respectively. Interestingly, the mechanochromic simulation results predict that these structures may show blue-shift or even red-shift under ...
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