Background

Shadow Photography of Specimen after extrusion

Background

Dr Gray’s Test

Dr. Gray’s Dynamic Tensile Test involves firing a spherical shaped specimen at about 400 meters per second into a extrusion block 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. At the moment this mass exits the extrusion block, it is not free of resistance because it pulls trapped material with it. This process of pulling the trapped material from the extrusion block is characteristic of Dynamic Tensile Testing. It looks almost as if the material is squirting out of the extrusion block. 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 photograph of the material’s reaction after it is extruded Failure occurs once the solid mass ejected from the extrusion device goes plastically unstable and necks, and eventually fractures into pieces. With the shadow photography, 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.