Abstracts Category : Other

Interlayer toughening of carbon-fiber/benzoxazine composite laminates

by Ravinarayana Reddy Patlapati

Carbon-fiber composites are increasingly employed in the Aerospace and Automotive industries owing to their lightweight and excellent mechanical properties. However, this class of material, when subjected to out-of-plane loads, is often susceptible to an internal damage in the form of delamination that can severely reduce its load bearing capacity. Several toughening methods including the implementation of thermoplastic materials are used to increase the damage tolerance of the polymer-matrix composites. In particular, non-woven thermoplastic veils, when used as interleaving materials between the plies in a composite structure, is extremely efficient at improving the interlaminar (delamination) fracture toughness and impact-resistance of composites. In addition, the toughening of the polymer matrix, if not adversely affecting the manufacturing process, can result in an increase in the toughness-related properties of composite laminates such as the resistance to micro-cracking under thermal-cycling conditions. In this study, the effects of matrix toughening and interleaving of the composite with non-woven Polyamide (PA) veils on the Interlaminar Fracture Toughness (ILFT) of Carbon-fiber/Benzoxazine composites are investigated. Formulated Benzoxazine (BZ) resins in non-toughened and toughened variants along with several non-woven PA veils with different melt temperatures are used to manufacture composite laminates through the Vacuum Assisted Resin Transfer Molding (VARTM) process. The ILFT of composites is measured by obtaining the resistance to crack propagation in the interlayer under tensile forces (Mode-I ILFT) or shear forces (Mode-II ILFT). The critical strain energy release rate (Gc) recorded during interlaminar fracture gives a measure of the ILFT of a composite. The laminates interleaved with the PA veils show an increase of nearly 50% for the Mode-I crack initiation (GIc initiation), regardless of the melt temperature of the PA veils. The Mode-I crack propagation (GIc propagation) of the laminate increases by using the PA veils with melt temperatures lower than the cure temperature of the BZ resin. In the Mode-II ILFT (GIIc) tests, the laminates interleaved with the PA veils show a significant impact on the GIIc values, as increases of nearly 170% are observed. A strong correlation between PA melt temperatures and the GIIc values is noted. The greatest GIIc values are noted when the melt temperature of the PA veil is greater than the cure temperature of the BZ resin. The matrix toughness plays a significant role in affecting the GIc values. The laminates manufactured with the toughened BZ resin result in the greatest increase in the GIc values. In contrary, the use of the toughened BZ resin does not result in an improvement in the GIIc values.