Biaxial tensile strength test for brittle rectangular plates

A new biaxial tensile strength test (ball on three balls test, B3B-test) for brittle materials has been proposed by some of the authors several years ago. A disc-shaped plate is symmetrically supported by three balls at one plane and loaded by a fourth ball in the centre of the opposite plane. This new test has several advantages compared to conventional bending tests or other biaxial strength tests, especially the well defined load transfer geometry and a high tolerance against measurement uncertainties. This makes the testing of as-sintered (slightly buckled) specimens possible. In the past years the test has proved to be very practicable to test as-sintered disc specimens as well as as-sintered specimens which are shaped like a disc. Although discs are-in principle-easy to produce, in some cases it is more convenient to produce specimens having the shape of rectangular plates. Also many components have the shape of rectangular plates. Therefore, in this paper, the B3B-test is extended to test specimens which are that shape. An FE analysis of the stress fields in the specimens is performed and the maximum tensile stress amplitude and the effective volume in the specimens are determined. Specimens of different size and of several advanced ceramic materials were tested. It is shown that there exists a size effect on strength which can be explained in the framework of the Weibull theory. The B3B strength data determined on rectangular plate specimens fit to tests results determined on disc specimens and to conventional bending test results. The smallest tested specimen had a volume of some tenth of a cubic millimetre. To demonstrate the practicability of the method a strength distribution of an electroceramic component is determined by testing 494 individual rectangular B3B test specimens.