Carbon Fiber Rod-Inserted Ballistic Sandwich Panel
United Technologies Research Center and Pepin Associates, Inc. have developed a hybrid sandwich panel using IM7 / 8552 DiscoTex® for face and back plates, IM7 / 8552 slit tape for penetrating rods, and Dyneema® 50 for the core . The resulting panel is both a structural material and a ballistic panel.
PANEL CONFIGURATION
Pepin Associates built a number of test panels for UTRC which consisted of IM7 / 8552 DiscoTex formable carbon fiber fabric for strike and back plates, Dyneema 50 for the core, and IM7 / 8552 slit tape from which the through thickness penetrating rods were formed.
The carbon fiber-reinforced “skins” of the 6” x 6” panels used 7 plies on the strike plate and 3 plies on the back plate, each applied in a 0° layup: the 8” x 2” panels used 3 plies on each plate also using just 0° layup.
The Dyneema 50 prepreg was applied in a 0°/90° degree pattern, each of the 6” x 6” panels having 48 plies and each of the 8” x 2” panels having 35 plies.
The rods were made from slit tape cut from HEXCEL’s IM7 / 8552 unidirectional prepreg reinforcement tape to a common width of 0.120”. The slit tape was inserted into the eye of a steel needle, doubled over, pulled through the panel, and then cut off to leave 0.25” of the rod extending beyond the face of the plates on either side (1-2 plies of DiscoTex were penetrated by the rods on each panel – the remaining plies were layed up over the rod ends as a cap).
PENETRATING RODS GEOMETRY
Using a metal frame to hold the panel layup in a Bridgeport milling machine, our technician would angle the lay-up to 45-degrees and use the table power feed to push the threaded steel needle through the lay-up. Grid paper was applied to the working face of the panel to guide placement of the rods. Once pushed through the panel, the needle would be pulled through the rest of the way with pliers and the slit tape “rod” cut off to the appropriate length. The steel needle diameter of 0.056” would define the hole in the Dyneema core and also the diameter of the rod itself: different diameters of needles and widths of slit tape may be substituted as desired. The rods are inserted every 0.25” with opposing rows of 45°+/- angled insertions. Each row is off-set by 0.0125” to place the rods equi-distant from each other throughout the panel.
PANEL CURING
Hexcel’s IM7 / 8552 is typically cured in an autoclave at 350°F for 3 hours, but in this application, the entire panel was cured in a heated platen press at 250°F for 6 hours under the appropriate amount of pressure per square inch to cure the Dyneema. The panel was held in a vacuum using release film and minimal amount of breather material. A HEXCEL study shows that IM7 / 8552 cured outside the autoclave in this manner retains most if not all of its mechanical properties. Curing the panels in this manner allows the various elements (rods, substrate, and skins) to bond together.