OPTIMIZATION OF TEST PARAMETERS FOR QUANTITATIVE STRESS MEASUREMENTS USING THE MINIATURIZED DISK-BEND TEST

A recently-developed miniaturized disk-bend test (MDBT) has been succe ssfully used to evaluate the mechanical properties of a variety of mat erials, using specimens 3 mm in diameter. The load is applied either b y a solid ball (the ball-on-ring (BOR) mode), or by a hollow cylinder (the ring-on-ring (ROR) mode). We have reproduced the yield stresses o f ordered intermetallic compounds and the fracture toughnesses of seve ral ceramics using analytical solutions to the equations of elasticity theory. Despite this success there are several curious features invol ved in the analysis of data; for example, in previous tests conducted in the BOR mode correct values of the yield stress were obtained using the equations appropriate to clamped specimens, whether or not they w ere actually clamped in the test fixture. We show that this is ubiquit ous to tests in the BOR mode, and does not arise because of frictional constraints at the supporting ring. We have also completed a thorough evaluation of testing in the ROR mode, in which the yield stresses of cold-rolled or annealed AISI type 302 stainless steel were measured u sing various combinations of specimen thickness and radii of the loadi ng and supporting rings, and compared to those of tensile specimens ma chined from the same material. The most accurate and reproducible meas urements of the yield strength were obtained for specific combinations of specimen thickness and geometry of the apparatus. We describe thes e conditions and demonstrate that they provide values that are always within 10% of the tensile results. The errors induced by potential mis alignments in the MDBT are also discussed, and are shown to cause no m ore than a 5% deviation in the measured yield stress.