Fused quartz shows three distinct regimes during nanoindentation, i.e. Plastic deformation, inelastic densification and cracking. Cohesive zone finite element modelling (FEM) is used to study these regimes for different indenter geometries. In a three dimensional model, the median/radial cracking is considered by introducing cohesive element planes that are aligned along the indenter edges perpendicular to the indented surface. A pressure independent Von Mises and a pressure dependent Drucker-Prager-Cap constitutive model are used to describe deformation behavior of silicates with and without densification. Simulations were performed with rigid, three-sided pyramidal indenters having centerline-to-face angles of 35.3° (Cube Corner), 55° and 65.3° (Berkovich) as well as one four-sided indenter with a centerline-to-face angle of 68° (Vickers).
The results show that the Drucker-Prager-Cap constitutive model delivers a remarkable accurate description of the elastic-/plastic deformation conditions for Vickers and Berkovich indenter, while shear deformation becomes more dominant with decreasing centerline-to-face angle and increasing indenter sharpness. In addition to comparing indentation cracking data with experimental data, the role of densification on indentation crack growth is critically examined. Material densification leads to smaller crack lengths and thus larger fracture toughness values. However, once the crack was initiated crack propagation was found to be comparable with both constitutive models, which lead to the conclusion that densification primary influences crack initiation rather than the propagation.
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