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The objective of this work is to demonstrate the applicability of quantitative fractography and fractal analysis to fracture processes in materials that exhibit R-curve behavior. In-situ toughened silicon nitride and baria-silica glass ceramics are used as model materials to demonstrate the effect of changes in morphology on toughness. These two tools, i.e., quantitative fracture surface analysis and fractal geometry, provide a new way of quantitatively assessing the mechanisms of toughening due to elongated grains and changes in microstructure.
For example, in silicon nitride the R-curve behavior only has an effect when the crack sizes are larger than about 30 micrometers. In applications where small cracks are expected, i.e., in bearing applications and for other well-polished surface finishes, R-curve materials may not be desirable.
In another example, 3BaO•5SiO2 glass ceramics were fabricated with crystals of aspect ratios (AR) of 1.4, 3.6 and 8.1. Based on fractal analysis, fractography and other accumulated data, it was shown that there was a statistical difference in the toughening behavior between materials with AR = 1.4 & 3.6, as compared to the material with AR = 8.1. The fractal dimensional increment was used to provide a method for determining the amount of toughening increase over that due to crack deflections as 0.4-0.6 MPam1/2. The implication of these results for the fracture process of toughened materials and the role of microstructure are discussed and explained in terms of material design.
Fracture mechanisms, fractography, fractal geometry, R-curve, fracture mechanics, toughness
Professor, University of Florida, Gainesville, FL
Graduate Student, University of Florida, Gainesville, FL
Process Engineer, GE Superabrasives, Worthington, Ohio