Cracks and microcracks in stitched structural composites manufactured with resin film infusion process

High performance stitched structural composites were produced to identify cracking in matrix resin-rich areas as well as microcracking in the intralaminar regions. In the past, there have been several mechanisms suggested to explain phenomena prevalent in stitched composites such as cracks in matrix resin-rich areas, microcracks in intralaminar regions when thermally cycled, preventing the utilization of stitched composites for airplane structural applications. Recently, material and technology developments have provided the possibility of reducing serious cracking and microcracking. These include homogeneous toughened resin systems and new bendable stitching fibers. Accordingly, this study focused on examining different epoxy resin matrices introduced by resin film infusion process to different types ofstitched unidirectional carbon fiber or woven reinforcements. These included non-stitched and stitched quasi-isotropic preforms with Kevlar® or carbon stitching fibers, as well as woven stitched preforms. Crack formation in resin-rich areas during processing was investigated for the stitched systems. Microcracking mechanisms were identified and related to the stitching fibers used. All laminates were examined using optical microscopy after exposure to hot/cold environmental changes. Overall, this research focused on understanding cracking and microcracking mechanisms of matrix resins in stitched composites and their limitations in load bearing structural applications.