Materials for dental applications do not only have to satisfy optical demands, they also have to provide sufficient structural integrity. Parts must be hard, strong and tough. Ceramic materials offer an interresting combination of these properties and are thus of specical interrest, also due to their excellent biocompatibility.
Since the advent of genereric manufacturing methods as feasable means for ceramic shaping, ceramics manufactured by conventional routes have to compete with parts manufactured by additive manufacturing (AM) technologies. While restorations like crowns and bridges can also be reliably produced using conventional subtractive techniques, ultra-thin walled veneers for minimal invasive restorations can be manufactured beneficially using AM, especially by the lithograpgy-based ceramic manufacturing (LCM) technology. Hand in hand with emerging AM-technologies comes an increased demand for characterization, specifically for ceramics, where details of the manufacturing route and properties are strongly linked.
Mechanical testing of materials and parts for relevant properties is thus an indispensable need in material development, process control and quality assurance. For the measurement of strength of ceramic materials various standardized tests are available: flexural tests on bars and a variety of bi-axial strength tests on discs. Some of them require test piece geometries that are unfavourable when it comes to typical amounts of material that are available for dental parts. The additive manufacturing technologies also introduce new aspects to the topic that have to be addressed: the layerwise build-up process may create material states and/or surface structures that influence strength and may lead to orientation dependent properties. Such phenomena have to be considered in the choice of the test method and when specimens are fabricated.
Practical and theoretical aspects of biaxial strength tests like the Ring-on-Ring test, the Piston-on-Three-Balls test or the Ball-on-three-Balls test will be discussed with respect to specimen size, possible sources of errors, achievable accuracy, comparability and user friendliness. Special attention will be paid to the influences of the typically low Young’s modulus of dental materials and multi-axial loading scenarios. A convenient test body for AM-manufactured materials which is designed to capture the orientation dependent strength of AM-parts will be introduced.