A SIMPLE, VERSATILE MINIATURIZED DISK-BEND TEST APPARATUS FOR QUANTITATIVE YIELD-STRESS MEASUREMENTS

A miniaturized disk-bend test (MDBT) apparatus for testing specimens 3 mm in diameter and a range of thicknesses is described. It incorporates several features not generally found in previous designs, including direct measurement of specimen displacement, noncritical alignment, and the flexibility to conduct tests on clamped or unclamped samples. The test is similar to a microbulge test in that load is applied through a ball bearing 1 mm in diameter. Displacement of the center of the specimen is measured using a linear variable differential transformer (LVDT), the core of which is attached to a rod maintained in contact with the specimen through the use of a weak spring. Results on four materials are presented, with sample thicknesses ranging from 25 to 250-mu-m: annealed OFHC copper, fine-grained Ni3Al, irradiated and unirradiated Zr3Al, and an amorphous Fe78Si9B13 alloy ribbon. A simple procedure for measuring the contact area at yielding between the ball bearing and the specimen is described. It enables accurate values of the yield stress to be calculated from equations for the elastic deformation of clamped specimens; previously, measurements of the yield stress from the MDBT relied on a finite element model. The accuracy was confirmed by tests on fine-grained specimens of Ni3Al which were cut from the grip sections of previously tested tensile samples. The effects of varying the clamping force on the load-displacement curves are described, and the sensitivity of the apparatus to near-surface microstructural effects is demonstrated by comparison of the results on two of the materials tested: Zr3Al, the irradiated samples of which are affected microstructurally only within 2-mu-m from the surface, and Fe78Si9B13, in which the loads at fracture depend on which surface of the ribbon experiences tensile loading during the test.