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Hugh E. Gardenier, IV, Anthony N. Palazotto, Ph.D., P.E., F. ASCE, and Reid A. Larson, Ph.D., P.E.
Journal of Aerospace Engineering, February 2011
Abstract
Quantifying the strain‐rate sensitive mechanical properties of structural materials is an important area of research in the field of solid mechanics. Property evaluation is typically accomplished using dynamic tests which involve rapid loading or impact of specimens. These tests generate inertial forces and wave propagation which make it difficult to accurately record the material response to a loading condition at an equivalent location. Furthermore, dynamic impact tests typically generate high strain rates (in excess of 103 s−1) and an experimental method for generating rates of strain in the intermediate strain rate regime which is simple and reliable is still lacking. This research develops and investigates an experimental technique for generating tensile plastic strain rates up to 102 s−1 in ductile metals. The technique relies on the impact from a vertical drop weight machine capable of delivering suitable impact velocity and energy to globally deform a slotted beam specimen. At impact, a state of predominantly plastic uni‐axial tensile stress is created in the ligament beneath the slot. The ligament is instrumented with an electrical‐resistance strain gauge and the strain history is measured and stored in a digital oscilloscope. Experiments are conducted on Commercially Pure (CP) titanium, 2024‐T3 aluminum and 1018 steel samples at four distinct impact velocities and collected data illustrates the capability to create predominantly uni‐axial tensile strain rates up to 102 s−1. An analysis of the material response identifies where plastic flow initiates. Furthermore, a numerical analysis of the impact event is conducted where the Johnson‐Cook constitutive equation is assumed to reflect the material behavior and published parameters are utilized to illustrate good agreement between experimental strain data and the numerical model.
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