Forming properties of stretch broken carbon fiber for aircraft structures

Abstract

Continuous carbon fiber is known to be a superior material for its strength, stiffness, and high strength-to-weight ratio and is often incorporated in aerospace composites. A challenge, however, is that it's not versatile in forming deep drawn geometries, which require convoluted manufacturing techniques resulting in expensive components. To overcome this, a type of carbon fiber with a random discontinuous fiber alignment called stretch broken carbon fiber (SBCF) is proposed. SBCF has potential to form parts with complex geometries with comparable or better mechanical properties to that of continuous carbon fiber. Montana State University (MSU) developed its own version of SBCF manufacturing processes, and research is being conducted to understand how SBCF prepreg tows react to stretch drawing at elevated temperatures using aerospace-grade epoxy resin systems. Currently, several new methods have been proposed to rapidly test these materials. This research revealed that SBCF forms with greater ease than continuous carbon fiber and is expected to substantially reduce manufacturing times for aircraft structures. To comprehend the material's behavior, simple tensile tests were coursed to understand how gauge length and temperature affected the peak loads when compared to continuous carbon fiber. It was discovered that on average, SBCF experienced stresses that were ten times less than continuous fibers. Additional tensile tests were conducted at elevated temperature to determine the true stress versus true strain. These tests are particularly important because they represent the material's most accurate mechanical properties. The results led to the discovery that SBCF experienced strain softening behavior. Furthermore, a series of forming tests using a novel "forming fixture" revealed that increasing the gap lowered the peak forming loads while the plunger geometry had little to no effect on peak forces at both room and elevated temperatures.