N151-072 TITLE: Resin Infusible Carbon Fiber Unidirectional Broadgoods for Fatigue Dominated Applications

TECHNOLOGY AREAS: Air Platform, Ground/Sea Vehicles, Materials/Processes

OBJECTIVE: Develop unstitched, uncrimped, dry carbon fiber unidirectional broadgoods intended for manufacturing of composite structures by resin infusion with the purpose of increased fatigue performance.

DESCRIPTION: Infused carbon epoxy composite laminate fatigue specimens were exhibiting lower than expected fatigue runout strains (at 10 million cycles, R=-1), and the observed fatigue degradation was initiating as microcracks at the stitching in the 90 degree plies of a laminate containing 0, +/-45, and 90 degree plies. Follow on screening tests of variations on the unidirectional fabric, such as thinner or different stitching threads, did not show any improvements. Stitching is the usual method of holding unidirectional fibers together forming a dry fabric used for wet resin fabrication methods, such as the infusion process used here. Autoclave cured unidirectional prepreg, which is held together by the B-staged epoxy resin rather than stitching, exhibits higher fatigue runout strains, but when compared to infusion the autoclave process significantly increases the cost of manufacturing.

This SBIR focuses on developing dry carbon unidirectional broadgoods consisting of straight fibers with no features that can create stress concentrations or resin rich areas, while maintaining a 55% fiber volume fraction when infused. The technology can be developed using standard modulus carbon fibers but should be extendable to intermediate and higher modulus fibers. The technology should be applicable to unidirectional cure ply thickness ranging from 0.005" to 0.025" - the researchers feel that thinner plies will improve fatigue performance, but there will be a tradeoff since thicker plies reduces manufacturing costs. The fatigue performance goal for a quasi-isotropic laminate (layup [0/45/90/-45]ns ) using the unidirectional broadgoods is runout at 10 million cycles, R=-1, at 3000 microstrain, with the specimens exhibiting no microcracking.

PHASE I: Develop concept(s) for dry fabric and demonstrate feasibility at lab scale using the specifications cited in the Description. Feasibility includes: demonstration of the technique used to form broadgoods from fiber tows; show that the dry fabric can maintain its shape when handled and draped dry; and show that it can be resin infused (epoxy resin TBD).

Phase I Option, if awarded, would be initial set up for Phase II, including planning and purchasing any long lead items.

PHASE II: Refine the concept to represent production material and show that good quality laminates can be infused. Quality would be assessed by conducting material testing to measure volume fractions, microscopy inspection, and 90 degree tensile performance (or another test to show good fiber matrix bond). Fabricate enough broadgoods for a panel 24" x 24" x 0.25" thick. Fabricate a laminate by resin infusion (resin TBD), and cut and test specimens. Conduct mechanical tests of composite specimens, including fatigue screening (+/- 5000 microstrain), and fatigue threshold (goal is at least +/- 3000 microstrain for 10 million cycles).

PHASE III: Coordinate with follow on programs of the FNC to incorporate material into next generation composite structural designs. Tasks will include developing the material database, conducting long term structural evaluation of large scale beams (6" thick), and manufacturing a full scale part with complex curvature as part of certifying and qualifying the material for Navy use. When appropriate the small business will focus on scaling up manufacturing capabilities and commercialization plans.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: Composite structures with design envelopes controlled by cost and fatigue would benefit from this technology. These structures exist extensively in the energy and transportation industries. Composites are used in parts and structures such as shafting, wind turbine blade, trailers, bridges, equipment foundations, and springs.

REFERENCES:
1. Mandell, J. F., Samborsky, D. D. & Cairns, D. S. "Fatigue of Composite Materials and Substructures for Wind Turbine Blades," Contractor Report SAND2002-0771, Sandia National Laboratories, Albuquerque, NM, 2002.

2. Griffin, D., Roberts, D. & Laird, D. "Alternative Materials for Megawatt-Scale Wind Turbine Blades: Coupon and Subscale Testing of Carbon Fiber Composites," 44th AIAA Aerospace Sciences Meeting and Exhibit, AIAA 2006-1197, Jan 2006, Nevada.

KEYWORDS: Composite; Carbon Fiber; Epoxy; Fatigue; Resin Infusion; VARTM

** TOPIC AUTHOR (TPOC) **

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