Masters Theses

Date of Award

12-1992

Degree Type

Thesis

Degree Name

Master of Science

Major

Engineering Science

Major Professor

Raymond D. Krieg

Committee Members

Jerry E. Stoneking, John D. Landes

Abstract

In medical as well as military applications it is necessary to transport modest amounts of nuclear material. The statistically safest way to transport these materials is by airplane. The nuclear material must be transported in a container that will not expose the public to any radiation even if the airplane crashes. The tests to verify the survivability of this type of package are very difficult and expensive. Because they are so difficult and expensive, it is useful, perhaps even necessary, to supplement the tests with numerical simulations.

The finite element method is well suited to perform this type of analysis if the proper material description or constitutive model is available. This thesis develops a material model that yields both volumetrically and deviatorically like the packaging materials used in air transportable containers for nuclear materials. The model is tailored to a new design under consideration that uses wrapped screen wire and Kevlar cloth. As an alternative material to the wrapped screen wire and Kevlar, perforated plate is also considered.

Material tests are performed on screen wire and Kevlar cloth to determine the necessary material properties. The material properties of a perforated plate are determined computationally using the finite element method. These properties are then fit to the constitutive model developed in this work.

The constitutive model consists of two parts. The first is an isotropic crush model which consists of a yield cone capped by an ellipsoidal end described in the pressure-√J2 stress space. The elastic material constants are variable and depend on how much plastic volume crushing has taken place locally in the material. The isotropic crush model yields both volumetrically and deviatorically. Non-associative flow rule are used on both surfaces, although both flow rules are associative in the limited deviatoric stress space. The second part of the constitutive model simulates the in-plane tensile behavior of the screen wire and Kevlar fibers. The in-plane tensile model is treated independently of the isotropic crush model. The in-plane tension model is subjected to the same deformations as the isotropic part of the model and the resulting stress fields from the two parts are superimposed giving an anisotropic material description. The constitutive model is implemented and verified as a stand alone model first and then introduced into a non linear large strain finite element code.

It was concluded that the constitutive model developed in this work could be applied to a wide class of volumetrically yielding materials.

It was also concluded that the test procedures developed for testing the material properties of screen wire and Kevlar could also be applied to other similar materials. Finally, future areas of study were recommended.

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