Kolsky bar impedance mismatch effect on stress equilibrium and strain rate constancy

Hangjie Liao, Purdue University

Abstract

The split Hopkinson pressure bar or Kolsky bar is widely used to determine dynamic material behavior. Stress equilibrium and strain rate constancy condition must be satisfied for a valid test result. These two critical conditions have different requirements on the impedance mismatch between bars and sample. The first part of this thesis is focusing on how the impedance mismatch affects the time to achieve constant strain rate and to achieve stress equilibrium by both analytical and experimental methods. The second part is testing the compressive stress-strain response of a shock-mitigation soft rubber. Analytical investigation is based on the method introduced by Frew (2001) to simulate the wave propagation inside the sample. A computer program is developed to expand the application to non-linear loading case such as rectangular step loading and step loading with an initial rise ramp. Aluminum, titanium and steel bar testing are performed in experimental investigation using ramp loading and elastic sample. The experimental results generally match model predictions. An optimum impedance mismatch range is suggested for linear loading. The compressive stress-strain response of the soft rubber compound is obtained experimentally at quasi-static, intermediate and high strain rates under uniaxial stress and uniaxial strain stress states. Kolsky bars with modifications for characterizing soft material under the two specific stress states and a long Kolsky bar are used to conduct the dynamic experiments; and an MTS load frame is used for conducting tests at quasi-static rates. Experiments are conducted at every decade in the strain-rate scale without any gap typically seen in the intermediate range. The experimental results show a significant strain-rate effect on the mechanical responses for this soft material, which are summarized by a rate-dependent constitutive model.

Degree

M.S.A.A.

Advisors

Chen, Purdue University.

Subject Area

Mechanics|Aerospace engineering

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