Development of an Equibiaxial Test Fixture to Determine the Strength of Ceramics and Composites
Authors:Shane Cioni, Nathan Schultz
Mentor:Vilupanur Ravi, Professor and Chair of Chemical and Material Engineering, California State Polytechnic University Pomona
The most commonly used procedures for determining the strengths of ceramics are three- and four-point bending tests. Problems encountered with these types of tests include edge effects that can lead to noisy test data. A relatively new type of test, the equibiaxial test, avoids edge effect issues and leads to more reliable data. This test procedure involves stress development in two different axes which gives a more accurate representation of stress states that the material will likely face in service. In the test method, a thin circular or square plate is supported on a ring and is loaded by a smaller ring, giving rise to the name “ring-on-ring” tests. The thickness and the diameter of the specimen are dictated by the ring diameter of the fixture as well as the estimated strength and stiffness of the test material. All of these relationships are given in detail in the ASTM C1499-09 standard with ranges of dimensions and equations. The goal of this geometry is to induce the loading in the center of the thin plate, away from the edges. Small deflection plate theory is used to find the biaxial stresses. A working test fixture has been successfully fabricated and was used to obtain the flexural strength of borosilicate glass. A full Weibull analysis revealed that the characteristic strength of borosilicate glass was 190 MPa and a Weibull modulus for this material of 4.5 (N=38). A separate study was done in “Dynamic Ring-on-Ring Equibiaxial Flexural Strength of Borosilicate Glass” found the flexural strength to be 146 MPa. We are currently refining our test fixture with the objective of operating and maintaining a working equibiaxial testing capability bench-marked using standard materials. This will allow us to gather reliable data on advanced ceramics and composite materials that are being developed at Cal Poly Pomona.