Microscale Model for Intergranular and Transgranular Damage and Fracture in Polycrystalline Ceramics
Microscale Model for Intergranular and Transgranular Damage and Fracture in Polycrystalline Ceramics
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Intergranular and transgranular fracture play a critical role in determining the fracture behavior and toughness of polycrystalline materials. These mechanisms are governed by microstructural features, including grain size, grain shape, grain crystallographic orientations, and grain boundary properties. We present a microstructure-explicit and fracture process-explicit model for elucidating the relationships between the fracture mechanisms, microstructural features, and macroscopic fracture behavior. This cohesive finite element method (CFEM) based model accounts for anisotropic grain constitutive and fracture behaviors and misorientation angle-dependent grain boundary fracture behavior, enabling the explicit resolution of complex crack paths and patterns. The material considered is Silicon Carbide (SiC), for which the model is calibrated using experimental and molecular dynamics data. Simulations under impact loading reveal dependencies of spall strength on grain size and grain shape. Specifically, the spall strength increases with grain size. The grain shape, characterized by the aspect ratio, also exhibits a strong influence on the spall strength, with grains elongated in the direction of impact loading providing up to two-fold increases in the spall strength over aspect ratios in the range of 0.2–10. Analyses reveal that the interplay between intergranular fracture and transgranular fracture is responsible for the observed trends. The promotion of transgranular fracture, particularly in grain fracture sites with high orientation-dependent fracture energies, is essential for the strength enhancement. The findings can be used to identify microstructural configurations that maximize the spall strength under specific conditions. The model presented can also be used to explore microstructure design of other ceramics and ceramic composites.
Intergranular and transgranular fracture play a critical role in determining the fracture behavior and toughness of polycrystalline materials. These mechanisms are governed by microstructural features, including grain size, grain shape, grain crystallographic orientations, and grain boundary properties. We present a microstructure-explicit and fracture process-explicit model for elucidating the relationships between the fracture mechanisms, microstructural features, and macroscopic fracture behavior. This cohesive finite element method (CFEM) based model accounts for anisotropic grain constitutive and fracture behaviors and misorientation angle-dependent grain boundary fracture behavior, enabling the explicit resolution of complex crack paths and patterns. The material considered is Silicon Carbide (SiC), for which the model is calibrated using experimental and molecular dynamics data. Simulations under impact loading reveal dependencies of spall strength on grain size and grain shape. Specifically, the spall strength increases with grain size. The grain shape, characterized by the aspect ratio, also exhibits a strong influence on the spall strength, with grains elongated in the direction of impact loading providing up to two-fold increases in the spall strength over aspect ratios in the range of 0.2–10. Analyses reveal that the interplay between intergranular fracture and transgranular fracture is responsible for the observed trends. The promotion of transgranular fracture, particularly in grain fracture sites with high orientation-dependent fracture energies, is essential for the strength enhancement. The findings can be used to identify microstructural configurations that maximize the spall strength under specific conditions. The model presented can also be used to explore microstructure design of other ceramics and ceramic composites.
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(a) Anisotropic grain fracture envelop in the form of an ellipsoidal fracture energy surface. The radial vector Γ indicates the fracture energy in the n crystallographic direction. The components Γ₁ = Φ₁n₁g₁, Γ₂ = Φ₂n₂g₂, and Γ₃ = Φ₃n₃g₃ represent the projections of Γ along the local grain basis vectors g₁, g₂, and g₃, respectively. Here, Φ₁, Φ₂, and Φ₃ are the principal axes of the ellipsoid, representing the fracture energy of the crystal in its major crystallographic directions. (b) The fracture energy vector Γ and normal vector n in the global basis eᵢ.
The study captures crack evolution in SiC microstructures at t=0.022 μs under impact, contrasting grain sizes of (a) 4 μm and (b) 10 μm. Intergranular cracks (blue) snake along boundaries, while transgranular ones (red) slice through grains, with grayscale shading revealing orientation-dependent fracture energies—darker for weaker resistance. Physically, it unveils how larger grains curb boundary density, shifting failure from easy intergranular paths to energy-hungry transgranular fractures, fostering tortuous cracks and bridging. This boosts spall strength by up to 50% as size grows, guiding tougher ceramic designs for armor and extremes.
Spallation behavior of polycrystalline 6H-SiC was investigated through finite-element simulations performed in Abaqus.
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