Hydraulic bulge testing to compare formability of continuous and stretch broken carbon fiber reinforced polymer composites

The use of carbon fiber reinforced polymer composites has increased with the increased need for high-strength, low-density materials, particularly in the aerospace industry. Stretch broken carbon fiber (SBCF) is a form of carbon fiber created by statistically distributed breakage of aligned fibers in a tow at inherent flaw points, resulting in a material constituted of collimated short fibers with an average length larger than chopped fibers. While continuous carbon fiber composites have desirable material properties, the limited ability to form in complex geometries prevents their wide adoption. SBCF composites exhibit pseudo-plastic deformation that can potentially enable the use of traditional metal forming techniques like stamping and press forming, widely used for mass production applications. To investigate the formability of carbon fiber reinforced polymer composites prepared with either continuous or stretch broken Hexcel IM-7 12 K fibers and impregnated with Huntsman RDM 2019–053 resin, hydraulic bulge testing was performed at atmospheric pressure and elevated temperature to explore the strain behavior under biaxial stress conditions for the material system. Results based on deformation of surface patterning, bulge apex displacement and measurement of the bulge internal surface and volume, support the enhanced formability of the SBCF material when compared to its continuous counterpart. The SBCF enhanced formability is characterized by an axisymmetric stress response and a failure mechanism similar to the one observed for sheet metal.