J. Duffy

Bio: J. Duffy is an academic researcher from Brown University. The author has contributed to research in topics: Strain rate & Stress–strain curve. The author has an hindex of 8, co-authored 10 publications receiving 704 citations.

The Dynamic Stress-Strain Behavior in Torsion of 1100-0 Aluminum Subjected to a Sharp Increase in Strain Rate

Abstract: : A modification of the torsional split Hopkinson bar is described which superimposes a high rate of shear strain on a slower 'static' rate. The 'static' rate of 0.00005/sec is increased to 850/sec at a predetermined value of plastic strain by the detonation of small explosive charges; the rise time of the strain rate increment is about 10 microseconds. During deformation at the dynamic rate, direct measurement is made of the excess stress above the maximum static stress attained. Results for 1100-0 aluminum show that the initial response to the strain rate increment is elastic, followed by yielding behavior reminiscent in appearance to an upper yield point. The magnitude of the stress measured at this yield point is always less than the stress obtained at the same strain in a wholly dynamic test; as the stress-strain curve asymptotically. It is concluded that the material behavior is a function of strain, strain rate and strain rate history. (Author)

Physics of the Granular State

Abstract: Granular materials display a variety of behaviors that are in many ways different from those of other substances. They cannot be easily classified as either solids or liquids. This has prompted the generation of analogies between the physics found in a simple sandpile and that found in complicated microscopic systems, such as flux motion in superconductors or spin glasses. Recently, the unusual behavior of granular systems has led to a number of new theories and to a new era of experimentation on granular systems.

Pulse shaping techniques for testing brittle materials with a split hopkinson pressure bar

Abstract: We present pulse shaping techniques to obtain compressive stress-strain data for brittle materials with the split Hopkinson pressure bar apparatus. The conventional split Hopkinson pressure bar apparatus is modified by shaping the incident pulse such that the samples are in dynamic stress equilibrium and have nearly constant strain rate over most of the test duration. A thin disk of annealed or hard C11000 copper is placed on the impact surface of the incident bar in order to shape the incident pulse. After impact by the striker bar, the copper disk deforms plastically and spreads the pulse in the incident bar. We present an analytical model and data that show a wide variety of incident strain pulses can be produced by varying the geometry of the copper disks and the length and striking velocity of the striker bar. Model predictions are in good agreement with measurements. In addition, we present data for a machineable glass ceramic material, Macor, that shows pulse shaping is required to obtain dynamic stress equilibrium and a nearly constant strain rate over most of the test duration.