Interlayer toughening of composite materials

This article describes the efforts that led to the development of low‐cost interlayer materials that may be used to significantly improve the compression‐after‐impact strength of composite laminates and to, correspondingly, dramatically reduce the area of damage because of impact. Moreover, with the proper selection of the interlayer material, other mechanical properties such as open‐hole compression and shear strength are unaffected or improved over laminates made from identical materials, but without the interlayer. These interlayer materials comprise very lightweight nonwoven veils that may either be melt‐bonded or knitted into noncrimped unidirectional and multiaxial fabric forms using commercial, production equipment. Other than some nominal set‐up costs, these materials add nothing to fabric manufacturing costs than the cost of the materials themselves. Fabrics made using the interlayers may be handled and infused with resin in exactly the same manner as fabrics without the interlayer veils. Proper selection of the veil material is essential, however. An improperly selected interlayer veil material can significantly reduce a composite material's performance. This technology represents a key enabler for the use of low‐cost vacuum‐assisted resin transfer molding processes, low‐cost preforms, and liquid‐molding resins to supplant prepreg as the building‐block material of choice for composite structures. Details of the test methods, results, and conclusions are contained herein. POLYM. COMPOS., 2009.