Characterization of stretch broken carbon fiber materials for automated forming processes

Automated forming technologies have been implemented in production by Northrop Grumman, to significant economic benefit. Labor intensive hand lay up processes have been replaced by efficient flat ply collimation and double diaphragm forming. Limitations of this latter fabrication method are normally defined by part complexity, and present in the form of laminate wrinkling. Large values of compound curvature are associated with required shear deformations beyond the capability of continuous fiber materials. Expansion of the formable part spectrum may be possible with materials that allow deformation modes that eliminate the peaks in the spatially varying shear spectrum, and this underpins the promise of the new Stretch Broken Carbon Fiber (SBCF) material forms recently developed by Hexcel Corporation. In this study a number of characterization tests were developed that were devised to be representative of the diaphragm forming process. These methods were applied to the SBCF materials in various forms (unidirectional, woven) as well as continuous fiber analogues. Uniaxial and biaxial stretch tests were used to assess the formability of the materials in order to identify process technologies that will allow the potential benefits to be realized.