TECHNOLOGY AREA(S): Air Platform, Materials/Processes

ACQUISITION PROGRAM: PMA 261 H-53 Heavy Lift Helicopters

OBJECTIVE: Develop an innovative machining process that can effectively and precisely machine holes in composite structures while preventing induced damage.

DESCRIPTION: Fiber reinforced polymer (FRP) composites are a key enabling material in several U.S. military aircraft.  Composite materials are used in primary load bearing structures, as well as secondary non-load bearing structures and skins.  The size and complexity of composite components is constantly increasing as the desire for reduced weight drives the replacement of metallic components with low-density FRP.

FRP materials are currently being machined using techniques adapted from traditional metalworking, however the unique material properties of FRPs present several difficulties in the drilling of a simple fastener hole, of which there may be several dozen on a single aircraft component.  Additionally, fastener holes often require precision countersinks.  The highly abrasive nature of carbon, glass, and aramid fibers reduces tool life of traditional tungsten carbide drill bits, necessitating their frequent changing, and also affecting hole diameter as the drill bit is abraded by the material.  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 technique.

The Navy needs a tool to create a finished precision fastener hole with countersink using an innovative precision machining technique.  The technique should provide precise replication of high-quality holes with high placement accuracy while reducing the amount of consumable tooling required, such as drill bits, when compared to traditional machining techniques through the same composite material.  This precision fastener hole and countersink machining process should remove material without inducing damage to, or contamination of, the actual part. Precision within the specified hole diameter +0.006 in max, having a surface roughness height rating of 250 or less, and no breakout plies on the exit side.  For thickness greater than 0.100 in, no delamination 0.010 in deep from edge of hole or into the part from hole. No splintering allowed beyond 0.010 in deep at entrance/exit of hole.  For placement, the precision machining technique does not require a pilot hole be present to maintain dimensional accuracy. Automation can be leveraged to increase hole and countersink precision and placement.  Careful control of any applied or induced heat must be demonstrated to not cause damage to the composite material. The temperature limit at the location of the final diameter size should not exceed 50° F below the glass transition temperature for the composite material during the machining operation.

PHASE I: Develop an innovative approach for a precision machining tool to machine fastener holes 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 holes in selected composite material and model the temperature profile of the resulting fastener holes using commercially available analytical tools. The Phase I effort will include the development of prototype plans for Phase II.

PHASE II: Fully develop a prototype precision machining tool; demonstrate the precision fastener hole capability developed in Phase I; and expand the capability to include countersink in a laminate that contains both carbon and glass fibers.  Demonstrate the ability to machine a finished precision fastener hole in a 1” thick composite sandwich structure with relevant face sheet and core materials.  Validate the quality of the hole with traditional Non-Destructive Inspection (NDI) techniques and show that the quality is at least equivalent to that which is currently achievable with traditional drilling through similarly produced composite material.  Validate the predicted heat distribution of the material and the associated material properties around the hole experimentally.

PHASE III DUAL USE APPLICATIONS: Benchmark the precision machining system to machine and countersink fastener holes in composite structures for aircraft components.  Transition the technology to provide an efficient and effective tool to produce countersunk fastener holes in carbon and glass fiber laminate composite materials used for military air platforms, as well as civilian air vehicle components and other industrial applications.  The technology can be an effective and efficient machining and cutting tool for various components in both the military and commercial sectors such as aerospace, automobile, and marine.

REFERENCES:

1. El-Sonbaty, I., Khashaba, U. & Machaly, T.  “Factors affecting the machinability of GFR/epoxy composites.” Composite structures 2004, 63, no. 3: 329-338.  http://journals.sagepub.com/doi/abs/10.1177/0021998312451609

2. Li, Z. L. Zheng, H. Lim, G. Chu, P. & Li, L.  “Study on UV laser machining quality of carbon fibre reinforced composites.” Composites Part A: Applied Science and Manufacturing 2010, 41, no. 10: 1403-1408. http://laser.mace.manchester.ac.uk/uploads/tx_neofileshare/2011-10-20_15-46-59_UV_Laser_machining.pdf

3. Piquet, R. Ferret, B. Lachaud, F. & Swider, P. “Experimental analysis of drilling damage in thin carbon/epoxy plate using special drills.” Composites Part A: Applied Science and Manufacturing 2000, 31, no. 10: 1107-1115. http://www.sciencedirect.com/science/article/pii/S1359835X00000695

KEYWORDS: Composite Structure; Drilling; Temperature Profile; Precision Machining; Heat Distribution; Fastener Hole