טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentEyal Avriel
SubjectInvestigating Strength Characteristics of Materials at Very
high strain rates using magnetically driven
Expanding Cylinders
DepartmentDepartment of Design and Manufacturing Management
Supervisors Full Professor Rittel Daniel
Dr. Lovinger Zev
Full Thesis textFull thesis text - English Version


Abstract

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.