Recently, the Notched Ball Test (NBT) was established at the Institut für Struktur- und Funktionskeramik to measure the strength of ceramic balls for structural applications. In this test, a notch is cut along the equatorial plane of the sphere with a length of about 80 % of the ball diameter. The ball is then squeezed together perpendicular to this plane in order to generate tensile stresses at the ball surface opposite to the notch. The fracture load and the geometry of the notch are used to calculate these stresses which are in accordance to the strength of the ball. In this work a test similar to the Surface Crack in Flexure-method is developed to determine the fracture toughness KIc of the notched ball. With a Knoop indenter a semi-elliptical surface crack is produced at the location where the maximum stress occurs. The NBT is used to generate a well dened stress eld around the crack. This crack is inltrated with uorescent dye penetrant to determine exactly the length of the crack by means of ultraviolet light. With the information of the crack geometry and the approximation of the ligament as a bending beam the geometric factor can be calculated with the Newman-Raju model. Measurement uncertainties and the feasibility of this new method are studied for dierent crack and sample dimensions. Using ball and beam samples of a reference material it is possible to compare the results obtained with NBT and with accredited methods for toughness measurement. It is shown here that the fracture toughness can be correctly determined with this new method. The new test is also applied to other common structural ceramics to investigate its possibilities and limits for a wide range of materials. This method can be used to test ball-shaped components without the need to produce any special samples. Thus it can be regarded as a component test and offers new possibilities for material selection and characterisation.
|Translated title of the contribution
|Determination of fracture toughness of ceramic balls
|25 Jun 2010
|Published - 2010
Bibliographical noteembargoed until 16-02-2011
- fracture toughness measurement ceramic balls ceramic bearing balls strength fractography