High Rate Tensile Strength Measurements of Frangible Bullets Using a Kolsky Bar | NIST
The tensile strength of frangible bullets is measured by a high rate diametral compression test (DCT) performed with a Kolsky Bar. Frangible bullets, meant to disintegrate on impact by brittle failure, also exhibit significant plasticity in compression. As a result, the elastic stress analysis usually used for the DCT cannot accurately deduce a tensile strength from the test results. To account for plasticity effects, finite element analysis is used to calculate the stress throughout the disk as it deforms under diametral loading prior to failure. Two cases are examined. The first involves a standard disk-shaped sample, and the second involves a disk with its contact points pre-flattened to reduce the amount of plastic strain prior to failure. With the standard sample, plastic flow produces tensile stresses at failure that are 80 % larger than those predicted by the usual elastic analysis of the DCT. Stresses at failure with the pre-flattened sample are 33 % lower than the elastic prediction, owing to load distribution and three dimensional effects. Results with the pre-flattened sample are considered more reliable since the finite element calculation does not account for crushing damage that is observed at the contacts points with the standard sample but is avoided with the pre-flattened sample.
Stresses and failure in the diametral compression test
The validity of the diametral disc test with a small flat ground has been established. The stress distribution was determined by a finite element method for a range of loading conditions. The results show that for the case of a point load, the failure is due to shear and compressive stresses at the loading point. Application of the diametral disc test for a ground flat was proposed and tested. The width of the flattened area must be less than 0.2 times the diameter of the disc to obtain accurate tensile strength.
Evaluation of Lapping Grains based on Shape Characteristics.
The shape and size of abrasive grains during lapping varies complicately and irregularly. In this study, the aspect ratio of grain size and the fractal dimension of the grain external form were used as shape characteristics of grain in order to evaluate grains with lapping time. The results obtained are summarized as follows : (1)The fractal dimension increases due to microscopic fracture and decreases due to macroscopic fracture or attritious wear. (2)The aspect ratio and the fractal dimension, which are grain shape characteristicsis of the macroscopic shape and the microscopic surface complication of grain respectively, can be used for evaluating grain shape variation during lapping.