Forming parameters and quantification of continous and stretch broken carbon fibers

Abstract

Continuous carbon fibers are premium reinforcing material for aerospace composites. Carbon fiber reinforced polymers are five times stronger than steel and twice as stiff, making it an ideal candidate for structural aircraft components where weight is an important factor. The challenge with continuous carbon fibers is their difficulty to form deep drawn parts requiring intricate manufacturing techniques that increase manufacturing time, cost, and material waste. An alternative to continuous carbon fibers is stretch broken carbon fiber (SBCF). SBCF is a form of aligned discontinuous fiber, it has been proposed as an alternative to overcome this formability challenge. SBCF provides flexibility to form complex shapes while maintaining comparable strength and stiffness. A variety of testing methods have been developed to study both the ability of SBCF to form over traditional continuous carbon fiber and how different iterations of SBCF perform against each other. These include testing carbon fiber tows in tension on a universal test stand as well as designing and creating a forming tool that tests resin impregnated tows under different geometry conditions and temperatures. Tensile properties of both a continuous tow and a SBCF tow were evaluated at different gauge lengths and temperatures. It shows that SBCF tow maximum load increases as the gauge length decreases as well as elevated temperature has a clear effect on the tensile properties when fiber continuity is considered. Cross-sectional areas of continuous and SBCF tows were calculated using both areal weight and scanning electron microscopy showing that in general continuous fiber tows have a larger cross-section than SBCF. Using a forming fixture to test samples, results were statistically analyzed in order to display the significance of geometry and temperature on the maximum forming load of different fibers. The suite of testing and results indicate that in general SBCF maintains superior formability to that of continuous fibers. Overall lower maximum force is required for SBCF to form into deep drawn shapes. This supports their ability to be used more readily in complex aircraft structure while minimizing the disadvantages posed by traditional carbon composites.