Automated Control of Stable Crack Growth for R-Curve Measurements in Brittle Materials

Stable crack advance is required for a reliable crack growth resistance (R-Curve) measurement. In bending experiments, manual control of the mechanical load and observation of the growing crack is still being done by the operator. This work presents an approach to partly and fully automated R-curve measurements, where stable crack growth is achieved solely via computer control. The experimental setup in conjunction with an intelligent control algorithm leads to reliable results, even for brittle materials like alumina ceramics, silicon nitride, and glass. Furthermore, it allows for a novel type of measurement, because the device detects any kind of energy release in the specimen, actually also without visible crack extension. The setup has been used successfully for about 3 years. The operating principle is explained and some of the results are presented exemplarily. The method is realized in 4-point-bending, but can be implemented also for other types of specimen and loading to automatically achieve stable crack growth.