Preliminary characterization of the comparative creep properties of unidirectional continuous and discontinuous carbon fiber laminates using a novel creep machine and method

Abstract

Over the last four decades, continuous carbon fiber (CF) reinforced composites have seen a rapid increase in use for aerospace applications. CF reinforced polymer composites can be seven times as strong and one and a half times lighter than aluminum. With these properties, they are an attractive material option when weight and strength optimization are paramount. Difficulties arise, however, when manufacturing complex shapes. Continuous CF materials have challenges forming to complex geometries due to the limited strain displacement of the stiff carbon fiber. Currently, methods used to manufacture many complex geometries are tedious, increasing both manufacturing cost and time. A potential solution is an aligned, discontinuous CF material, Stretch Broken Carbon Fiber (SBCF). In past material development efforts, SBCF has shown increased formability with similar mechanical properties to continuous carbon fiber. Past efforts did not characterize SBCF's long term behavior under creep loading. Breaking the fiber at its natural flaws creates inherent discontinuities in the material that are filled with the resin matrix. Creep in composite materials is a matrix dominated effect, therefore, discontinuities could potentially impact behavior. A major challenge of creep characterization is that testing methodologies require large machines capable of testing coupons at loads similar to those used for standard tensile tests. This size limits the general applicability of creep testing during rapid material development. For this study, creep testing of continuous CF and SBCF laminates in the fiber direction was conducted. To accomplish comparative testing, a novel creep testing method was developed. A simple lever arm creep testing machine was designed, fabricated, qualified, and implemented. A reduced geometry creep test coupon was developed alongside the creep machine machine. Additionally, a simplified data acquisition method was devised. Creep coupons were tested at approximately two thirds of their theoretical static ultimate strength for five hundred hours at 82.2 °C (180 °F). When loaded in the fiber direction, comparative preliminary creep tests of continuous and stretch broken CF found similar behavior. Both continuous and stretch broken CF materials did not exhibit measurable creep. Creep data collected support the idea that the mechanical properties of SBCF composites are comparable to those of continuous CF composites.