M.Sc Student | Avriel Eyal |
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Subject | Investigating Strength Characteristics of Materials at Very high strain rates using magnetically driven Expanding Cylinders |
Department | Department of Design and Manufacturing Management | Supervisors | Professor Daniel Rittel |
Dr. Zev Lovinger | |
Full Thesis text | ![]() |
Dynamic
characterization of strength properties is done, in common practice by the
means of a Split-Hopkinson Pressure Bar (also named Kolsky-Bar) apparatus. In
such systems, strain rates are limited up to ~5? 103
sec-1. For higher strain rates, the strain rate hardening is assumed
to be the same as that measured at lower rates, with no direct measurement to
validate the assumptions used for this extrapolation. In fact, for most
metallic materials a sharp change in strain rate sensitivity is reported above
strain rates of 5? 103
- 104 sec-1, thus the importance to measure strength at
strain rates above this point.
Different
experimental methods are reported in the literature for measuring strength at
very high strain rates. Expanding ring tests (ERT) using explosives or
Electro-Magnetic driving techniques reach strain rates of 103-104
sec-1, Plate Impact tests, using different methodologies and
interpretations reach 106 sec-1 and RayleighRaleigh-Taylor/Richtmayer-Meshkov
instability growth tests can reach strain rates of 106 sec-1
using explosives and up to 109 sec-1 with laser ablation
techniques. However, the interpretation of these tests is controversial as
coupled effects such as pressure hardening and thermal softening together with
the effects of high strain rates, do not enable a "stand alone"
determination of the part of high strain rate in the actual measured strength.
This research
deals with a new methodology to measure strength of materials at very high
strain rates, above 5?*104
sec-1 using magnetically driven expanding cylinder experiments. We
used a Pulse current Current gGenerator
(PCG) to create the magnetic forces on the cylindrical specimens and measured
the expanding motion using velocity interferometery.
To investigate
the dynamic behavior of the specimens and for the determination of their
strength, we used numerical simulations. We conducted 2D hydrodynamic
simulations for the design of the specimens and 1D MHD simulations for
simulating the actual tests as we found the coupled physics to be essential for
the accuracy we needed in the process of calibration. In this work we analyze
nine experiments of OFHCOFHC copper in three
different mechanical configurations, reaching strain rates up to 7.5? 104 sec-1.
We found a significant strain rate hardening for the OFHCOFHC
copper and present a calibration of the Modified Johnson Cook (MJC)
constitutive model for our data. Preliminary fragmentation analysis was conducted
showing combinations of shear and tension failure mechanisms. Results
of this work are discussed and compared with previous studies reported in
literature.