Extrusion Block- Background

Extrusion Block-Background

Dr. Gray’s Dynamic Tensile Test involves firing a spherical shaped specimen at about 400 meters per second into a die with a reduced conical extrusion cut out of it. The specimen enters the conical section and as a result experiences a very high strain rate over an extremely short period of time. The outer edge of the sphere makes a line contact with the inner wall of the conical section. This line contact slows the outermost mass of the sphere, trapping it in the die, without resisting the innermost mass of the sphere. As the innermost mass continues traveling through the extrusion, it makes new lines of contact with the inner wall of the conical section until that innermost mass is ejected out of the reduced end of the die. At the moment this mass exits the die, it is not free of resistance because it pulls trapped material with it. This process of pulling the trapped material from the die is characteristic of Dynamic Tensile Testing.

Shadow Graph Photography

The extrusion process looks almost as if the material is squirting out of the extrusion die. This process is recorded using high speed photography equipment that captures images in microseconds. A high intensity light source is aimed at the exit of the extrusion device and into the lens of the camera, resulting in a shadow video of the material’s reaction after it is extruded (See above figure). Failure occurs once the solid mass ejected from the extrusion device goes plastically unstable and necks, and eventually fractures into pieces. With the shadow video, analysis prior to and after failure was used to validate a finite element program that is capable of predicting the dynamic mechanical properties (plasticity and failure) of the material.