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.
Experimental investigation of the geotechnical properties and microstructure of lime-stabilized saline soils under freeze-thaw cycling
Abstract The Song-Nen Plain in northeastern China holds one of the three largest expanses of soda saline-alkali soil in the world. Due to its disadvantageous geotechnical properties, the saline soil in this area is unable to meet the needs of construction projects. This study evaluates the stabilizing effect of adding lime to such soils. Specifically, it investigates how the physicochemical and mechanical properties and microstructure of soda saline-alkali soil are altered by lime and how these changes are affected by both freeze-thaw cycling and the duration of curing. The results reveal that adding lime to saline soil transforms clay particles into sand and silt particles and causes the pore size distribution (PSD) to become bimodal, with small-pore and macropore populations. An increase in lime content leads to an increase in the unconfined compressive strength (UCS) but a gradual decrease in both the cation exchange capacity (CEC) and plastic index. Freeze-thaw cycles break up coarse particles and aggregate finer particles, increasing the diameter of macropores and decreasing the USC; however, the effect on specimens with >9% lime is negligible. It is further shown that the geotechnical properties of the soil change immediately after the addition of lime, indicating that the cation exchange reaction takes place very quickly. Furthermore, the pozzolanic reaction leads to an increase in the percentage of sand-sized particles and a decrease in porosity after 28 days of curing. Overall, these results indicate that it is feasible to use lime to improve the engineering properties of soda saline-alkali soil in this area.