3.2.1.7 Ring-loaded Disc Test
The ring-loaded disc strength test method was developed for the ceramics industry as a means of testing the strength of a range of brittle ceramic materials such as alumina and zirconia. Godfrey [1987] used the test as an alternative to the traditional flexural three point bend test as it required less time and skill to produce test specimens and the need for a good edge surface finish was eliminated. In addition to these sample preparation advantages is the convenience of the test procedure itself, which consists of placing a disc (19mm in diameter and 3mm thick) in a brittle flexure test rig on a load frame and applying a steady load until the sample fractures. In the test the disc sample is supported on a ring of ball bearings accurately seated in a platten. A smaller diameter ring of ball bearings is then used to apply a load to the sample from above at a fixed rate. A theoretical relationship was derived between the stress and the geometric properties of the specimen based on the use of toroidal rings to support the test piece and apply the load. The simpler ring support system was shown to give the same results as a toroidal design and has been adapted in the current work. Using this arrangement, the load at fracture is recorded and the values are substituted into equation 1 below in order to calculate the stress:-
Where :
r = Stress in Radial Direction Mpa
W = Load Newtons
T = Thickness Metres
= Poisson Ratio
Rl = Radius of load circle Metres
Rs = Radius of circle support Metres
Rd = Radius of disc Meters
The method was first applied to mineral systems by Kingman et al [1996] as a means of determining the effect of dielectric heat treatments on the strength of Norwegian ilmenite and showed that the strength of the mineral decreased with increasing exposure time to microwave radiation. Work by Young [1998] showed a positive correlation between data from the ring-loaded disc test and the Bond Work Index for different materials that were tested.