Creation of a structural joint for a heat shield for extreme entry environments requires structural fibers penetrating through the thickness of the shield at joint locations. The structural fibers must be made of carbon to withstand extremely high temperatures, i.e. 2000 ºC. Carbon fibers, due to their relatively high modulus (stiffness), are easily damaged and broken when handled by a conventional sewing machine. Special coatings such as nylon are required to increase the durability of the fiber to enable its use in a sewing or tufting process.
The extreme material thickness, i.e. 50-l00 mm, of the heat shield joint design represents a unique challenge for the insertion of the structural carbon fibers using conventional methods such as stitching or tufting. The high density of the woven carbon fiber preform of the heat shield prevents the insertion of a sewing needle without inflicting significant structural damage to the preform.
This invention defines a method for inserting dry carbon fibers through the thickness of high-density 3D woven carbon fiber material for the creation of a structural joint on a heat shield for extreme entry environments. The technique for holding the two separate pieces of material while creating the joint is defined, along with the approach for installing carbon fibers normal to the outer surface of the woven material using an industrial robot and tufting end effector. The method of securing the loops of the tufted carbon fibers is also defined.
A jig fixture is used to secure two separate pieces of 3D woven dry carbon fiber preform relative to one another. The fixture is a simple picture frame that captures the outer perimeter of the preforms. Pilot holes are drilled through the thickness of the preforms at the butt splice joint to minimize the penetration force of the needle during installation of the tufting seam. The holes are laid out in a predefined pattern using a paper template that is taped to the dry fiber preform top surface over the joint location. The hole pattern is matched to the robot path and pitch of the tufting seam. A drill motor is then used to drill pilot holes into the preform. The hole pattern template is then removed from the preform, and the preform is located inside a robotic stitching cell. An industrial robot arm with a tufting end effector is then used to insert carbon fiber sewing thread down into the predrilled pilot holes. Alignment of the preform to the robot cell is achieved by first rotating the crankshaft on the end effector to position the needle near bottom dead center. The robot is then moved to position the needle tip over the pilot hole at the beginning of each seam. The robot’s position is then “taught” to the corresponding line of instruction code software. The process is repeated for each seam end location.
The robot program is then inserting the tufting seams of carbon fiber sewing thread into the pilot holes. The preform is removed from the robot cell and rotated over, with the bottom surface facing up. A carbon fiber sewing thread is then inserted through the protruding loops of the tufting seam by hand. The thread is tied off, securing the tufting seam in place. The preform can then be removed from the jig fixture and safely handled for subsequent resin infusion processing without losing the integrity of the structural butt splice joint.