Influence of finite notch root radius and optically determined crack length on the measured fracture toughness of brittle materials

Abstract

When measuring the fracture toughness of ceramics, for instance with the Single Edge VNotched Beam (SEVNB) method, a known problem is given by the finite notch root radius. If this radius is larger than a certain small value of a few micrometers, the measured stress intensity factor is higher than the true intrinsic value. Another problem is that due to technical and physical reasons the optically measured crack length at stable crack growth is smaller than the true crack length, which might lead to an underestimation of the fracture toughness.In the given paper an experimental approach is presented to register these effects quantitatively with stable crack advance and to finally obtain the correct fracture toughness. This is achieved by applying a new computer aided automatic control system for stable crack advance and by analyzing the results in force -displacement and crack length -force diagrams. The used material is zirconia-toughened alumina.

Sub-10-micrometer toughening and crack tip toughness of dental enamel

“…It is well-known from ceramic materials that toughness measurements with SingleEdge-Notch-Beam (SENB) or CompactTension (CT) specimens are usually not able to measure the crack tip toughness, K I0 , in particular when steep crack resistance curves prevail (Fett et al, 2008;Fünfschilling et al, 2009;Özcoban et al, 2010).…”

Computer-Controlled Stable Crack Growth as a Reliable and Fast Method to Determine Subcritical Crack Growth

“…A key feature of this device is a computer-aided control system that evaluates the slope of the force-displacement curve during the measurement online. Because the compliance of the sample increases as the crack propagates, the information of the actual slope of the force-displacement curve can be used to detect crack propagation even before it can be observed optically on the sample surface [8]. The user sets a critical slope value (representing a well-defined compliance), which is above the initial compliance.…”

“…Herewith, this technique provides the opportunity to perform several loading cycles on one specimen. It should also be mentioned that performing controlled fracture tests on particular brittle materials (like Si 3 N 4 [6,7] and zirconia-toughened alumina [8]) were only possible by means of this computer-aided technique and the high stiffness of the device. Especially, materials with constant fracture toughness like glass require an even stiffer device and a faster response time to unload to perform stable crack growth.…”

“…This enables automatic stable crack growth measurements without much operation of the user. It was already successfully applied to silicon nitride [6,7] to measure extremely steep R-curves and to zirconia-toughened alumina [8] to determine the fracture toughness with automated control of stable crack growth. The advantage of this computer-controlled system is that manifold routine measurements without the necessity of highly qualified research assistants are possible.…”

Sinterização e caracterização mecânica e microestrutural de cerâmicas de SiC aditivadas com misturas de AIN/Dy2O3 e AIN/Yb2O3

“…Da mesma forma, Fischer também observou que é muito importante diminuir o raio do entalhe da amostra para se determinar valores "verdadeiros" de tenacidade à fratura, pois se os valores do raio estão acima de um valor crítico, os valores de tenacidade à fratura serão superestimados (104) . Entretanto, também podem ocorrer valores subestimados se por razões técnicas e físicas da medida em microscópio óptico, o valor do comprimento da trinca for menor que o comprimento verdadeiro da trinca (109) .…”