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 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.  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.  It 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   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

 

 

 

 

 

 

 

 

 

 

   Sabot

           A Sabot is a device used in ballistics to carry the sub-caliber projectile and to form a gas seal between the specimen or payload being fired and the barrel of the firing device (Figure 5: Types of Sabots from Appendix E).  This seal directs all of the energy from the ignition of the propellant into the specimen being launched.  For instance, consider having a muzzle that has an inner diameter of a half an inch and the projectile that will be fired is a quarter of an inch.  By simply dropping the projectile into the muzzle and igniting the charge, you will lose significant amounts of energy from that charge since most of that energy is allowed to travel around the projectile in the gap between it and the bore of the gun tube.  Using a sabot gives assurance that most of the propellant energy is used to accelerate the sabot/projectile package and improves the repeatability of the velocity from experiment-to-experiment. 

 

 

Various Sabots