Search Results (3)

Subsurface damage of fused silica optics is one of the major factors restricting the performance of optical systems. The densification-affected deformation and fracture in fused silica under a sliding contact are investigated in this study, via three-dimensional finite element analysis (FEA). The finite element models of scratching with 70.3° conical and Berkovich indenters are established. A refined elliptical constitutive model is used to consider the influence of densification. The finite element models are experimentally verified by elastic recovery, and theoretically verified by hardness ratio. Results of densification and plastic deformation distributions indicate that the accuracy of existent sliding stress field models may be improved if the spherical/cylindrical yield region is replaced by an ellipsoid/cylindroid, and the embedding of the yield region is considered. The initiation sequence, and the locations and stages of radial, median, and lateral cracks are discussed by analyzing the predicted sliding stress fields. Median and radial cracks along the sliding direction tend to be the first cracks that emerge in the sliding and unloading stages, respectively. They coalesce to form a big median–radial crack that penetrates through the entire yield region. The fracture behavior of fused silica revealed in this study is essential in the low-damage machining of fused silica optics. Full article

In this paper, a series of indentation tests in which the maximum normal force ranged from 0.4 to 3.3 N were carried out to determine the fracture toughness of 4H-SiC single crystals. The results indicated that an appropriate ratio of the distance from the indentation center to the radial crack tip to the distance from the indentation center to the indentation corner is significant to calculate fracture toughness of 4H-SiC single crystals. The critical condition with no cracks on the edge of the indentation was obtained through a fitting method. The surface morphologies of the groove were analyzed by scanning electron microscopy (SEM). Plastic deformation was observed and characterized by the smooth groove without cracks and ductile chips on the edge of the groove in the initial stages of scratch. With increased normal force, median cracks, radial cracks, and microcracks appeared in turn, followed by the crack system no longer being able to stably extend, causing the brittle fracture to dominate the material removal. The size of the edge damages were measured through SEM and the experimental data highly agreed with the predicted curve. A modified calculation model considering elastic recovery of the sample by the indenter during the scratching process was suggested. These results prove that elastic recovery of 4H-SiC single crystals cannot be ignored during ultra-precision machining. Full article

In this paper, the fracture toughness K_Ic of near-nano and nanostructured WC-Co cemented carbides by Vickers indentation fracture toughness (VIF) was investigated. The aim was to research the type of cracking occurring in near-nano and nano-grained WC-Co cemented carbides with respect to the Co content and, consequently, to evaluate the appropriateness of different models for the fracture toughness calculation. The mixtures with different binder content—4, 6, and 9 wt. % Co—were consolidated by sintering in a hydrogen atmosphere. Vickers indentation using a test force of 294 N was used for the determination of fracture toughness. The type of crack that occurred as a consequence of the applied load on the corners of the Vickers indentations was analysed with optical microscopy before and after repolishing the samples. Different crack models, Palmqvist and radial-median, were applied for the calculation of K_Ic. Instrumented indentation testing was used to determine the modulus of elasticity of the consolidated samples. From the research it was found that near-nano and nanostructured cemented carbides with 9 and 6 wt. % Co do not exhibit median cracking and the indenter cracks remain radial in nature, while near-nano and nanostructured cemented carbides with 4 wt. % Co exhibit both radial and median cracking. Accordingly, it was concluded that the critical amount of the binder phase in near-nano and nanostructured WC-Co at which the crack changes its geometry from Palmqvist to radial-median is around 4 wt. % Co. Comparing different models it was found that K_Ic values are not consistent and differ for each method used. Models from Exner crack resistance for the Palmqvist crack showed good agreement. Radial-median crack models showed significant K_Ic deviations for the same testing conditions for all samples. Full article