| The objective of this research is to analyze and evaluate the dynamic flow function of metals under impact loading at both high strain rate (over 1000/s) and large strain (over 100%). A test method for dynamic compression of metal disc is described. A long steel striker (20 inches long, 1 inch diameter) impacts a small copper specimen placed on a massive anvil. The velocity of the striker face and the force on the anvil are measured during the impact period. From these primitive data the axial stress, strain, and strain rate of the disc are obtained. The strain rate is determined by the striker velocity divided by the specimen height. This gives a slightly increasing strain rate over most of the deformation period. Strain rates of 100 to 10,000 per second are achieved. Attainable final strains are 150%.; The advantages of this test method over the split Hopkinson pressure bar (SHPB) are: (1) the velocity of the striker and the attendant strain rate in the specimen can be considerably greater than the particle velocity and attendant strain rate in the SHPB; (2) the energy of the striker can be larger than the kinetic energy of the SHPB such that strains of several hundred percent can be achieved.; A discussion of several problem areas is presented. The friction on the specimen surfaces, the determination of the friction coefficient, the influence of the specimen geometry (h/d ratio) on the friction effect, the lock-up condition for a given configuration caused by friction, temperature rise during deformation, the friction correction, and the evaluation of several lubricants are given. The flow function (stress versus strain) is dependent on the material condition (e.g., prior cold work), specimen geometry, strain rate, and temperature. |
