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Critical comparison of moving bar impact with gas expansion machines for dynamic loading of metals
Author(s) -
Krafft J. M.
Publication year - 1960
Publication title -
journal of applied polymer science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.575
H-Index - 166
eISSN - 1097-4628
pISSN - 0021-8995
DOI - 10.1002/app.1960.070041209
Subject(s) - bar (unit) , piston (optics) , stiffness , materials science , strain rate , mechanics , strain (injury) , universal testing machine , stress (linguistics) , spark plug , amplitude , head (geology) , structural engineering , mechanical engineering , composite material , engineering , physics , optics , medicine , linguistics , philosophy , wavefront , ultimate tensile strength , meteorology , geomorphology , geology
Achievement of high head speed in a testing machine requires that two conditions be met: ( 1 ) a high velocity input to the machine, and ( 2 ) a reasonably high stiffness. The machines discussed here, while excelling in requirement ( 1 ) are characterized by a rather deficient stiffness. Because of this softness, the strain rate during a test will vary markedly with the amplitude and shape of the stress strain curve; indeed, so much so that it must be considered as a variable in the data. This paper gives an analysis of the motional characteristics of a typical gas‐pressurized testing machine. This machine is driven by a pressure source obtained from a chambered burning of gunpowder; head speeds up to 100 in./sec. are reached by a restricted expansion against a piston. It is found that observed strain rate vs. strain behavior can be predicted from the analysis by taking into account the decrease in driving pressure due to gas cooling and to its expansion, and the decrease in stiffness with strain in the specimen or stroke of the piston. Comparison of this type of machine with impact bar loaders previously developed at NRL shows a comparable degree of strain‐rate variation for both machines. However, with mild steel, the stress–strain data from the two machines can be correlated by considering instant strain rate as a variable.