Friction Drilling Fasteners for Composite Structures

OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Advanced Materials OBJECTIVE: Develop an innovative friction drilling process that can effectively and precisely fasten together composite structures while preventing induced damage. DESCRIPTION: This SBIR topic seeks development of an innovative process to fasten composite materials without destroying the integrity of the material. A process such as friction drilling has the potential both to fasten and bond carbon fiber- or glass-based composite material with polymer reinforcement. The Navy uses fiber reinforced polymer (FRP) composites in many military aircraft. Composite materials are in primary load-bearing structures and secondary nonload-bearing structures and skins. The size and complexity of composite components are constantly increasing as the desire for reduced weight drives the replacement of metallic components with low-density FRP. Fatigue and overload conditions require thorough tests and analyses to qualify connections to composite parts for airworthiness. Additionally, nondestructive inspections (NDI) are a crucial requirement for using these connections. There may be several dozen fastener locations on a single aircraft component requiring a robust and rapid connection process for composites. Hole drilling techniques for FRP material originated from traditional metalworking; however, the unique material properties of FRPs present difficulties in the drilling of a simple fastener hole. Additionally, fastener holes often require precision countersinks. The highly abrasive nature of carbon, glass, and aramid fibers reduces the tool life of traditional tungsten carbide drill bits. This problem necessitates their frequent changing and affects the hole diameter as the material abrades the drill bit. The frictional heat generated by the drill bit can cause severe damage to the polymer matrix, resulting in a loss of strength that can be extremely difficult to detect. Lastly, FRP materials are prone to delamination in several situations due to improper drilling techniques. A friction drilling process may produce a more robust connection. An alternative method to secure composite parts to other parts involves the use of adhesives. The adhesives require high levels of cleanliness, fixturing tools, curing/wait times, and multiple personnel for assembling non-rigid, large parts. Production must wait for the adhesive to cure and for the removal of the fixturing tools. The shear, and out-of-plane, loads that adhesives transfer from component to component are complex. Adhesives are generally much weaker than fasteners. A process such as friction drilling appears to offer an alternative to adhesives in many applications. The Navy has a need to address the following technical challenges to qualify a process such as friction drilling: (a) precision fastener locking with robust bushing collars, (b) no breakout plies on the exit side, (c) no delamination from edge of hole or into the part, (d) no splintering allowed at entrance/exit of hole, (e) no fatigue or ultimate strength damage from pilot holes, (f) applied or induced heat must not damage the composite material, (g) automate the process for bushing collar formation consistency and resilience, (h) modeling and simulation of the process including temperature profile, (i) modeling and simulation of the progressive damage for fatigue and overload analyses, (j) demonstration of equivalent or better fatigue properties than the current processes, (k) demonstration of ultimate load capabilities equivalent or better than the current processes. PHASE I: Develop an innovative approach for a friction drilling fastener of relevant diameter and depth in either a carbon fiber- or glass-based composite material with polymer reinforcement representative of those materials used in military aircraft today. Demonstrate feasibility of the developed approach for producing bonded composite materials. The Phase I effort will include the development of prototype plans for Phase II. PHASE II: Show that the strength quality can be at least equal to what is currently achievable with traditional drilling or adhesive bonding with similarly produced composite materials. Validate the associated material strength properties around the friction drill bonded region through fatigue and overload testing. Fully develop a prototype friction machining tool, demonstrate the precision fastener capability developed in Phase I, and expand the capability for rapid assembly. PHASE III DUAL USE APPLICATIONS: Demonstrate fatigue properties and ultimate load capabilities equal to or better than the current processes to transition this technology to applicable platforms. This SBIR topic will greatly assist the recreational marine industry, the aerospace industry, the wind turbine industry, and any other industry that uses composite materials. REFERENCES: 1. Nagel, P., & Meschut, G. (2017). Flow drill screwing of fibre-reinforced plastic-metal composites without a pilot hole. Welding in the World, 61(5), 1057-1067. https://doi.org/10.1007/s40194-017-0493-2 2. Alphonse, M., Raja, V. B., Logesh, K., & Nachippan, N. M. (2017, May). Evolution and recent trends in friction drilling technique and the application of thermography. In IOP Conference Series: Materials Science and Engineering (Vol. 197, No. 1, p. 012058). IOP Publishing. https://iopscience.iop.org/article/10.1088/1757-899X/197/1/012058/meta 3. Kumar, B. S., Baskar, N., & Rajaguru, K. (2020). Drilling operation: A review. Materials Today: Proceedings, 21, 926-933. https://doi.org/10.1016/j.matpr.2019.08.160 KEYWORDS: Friction drilling; flow drilling; composites; thermoset; thermoplastic; fatigue