Correlation between rheological and mechanical properties in a low-temperature cure prepreg composite

Abstract

With an ever growing fleet of commercial airplanes utilizing composite structures, it is increasingly important to develop cost-effective and robust repair procedures. Issues invariably occur during on-site repair which casts doubt on the structural suitability of the part. This makes the ability to quantify the relationship between dwell temperature, cure state, and mechanical properties extremely important. Curing temperatures play a vital role in the difficulty of repairs. Lower cure temperatures mean lower cost, less possibility of damage to surrounding material, and therefore, the potential for a more robust repair scheme. As such, low temperature cure materials are of great interest to the composite repair community. Current state-of-the-art says that cure must be very precise to ensure structural integrity. As such, any repair with thermocouple readings outside a very narrow band must be reworked at substantial cost. Often, when rejected repair materials are tested, they are found to be structurally sound. The search for a solution to this problem has been the motivation for this study. Rheometer testing quantifies the viscoelastic properties of the composite material as it cures. This ex-situ approach allows a very detailed and accurate view of cure state properties. These properties include glass transition temperature (Tg), gel time, vitrification time, G’ (storage modulus), G” (loss modulus), and Tan_ (G”/G’). Glass transition temperature, gel time, and G’ were used to correlate viscoelastic properties with mechanical properties. The mechanical properties of interest were short beam shear strength, compression strength, compression modulus, compressive Poisson’s ratio, tension strength, and tension modulus. Correlations were attempted for all mentioned mechanical properties. It was found that the highly resin-dominated compression and short beam shear strengths showed a strong correlation with viscoelastic properties. Both of these mechanical properties showed a very strong relationship to the nearly constant portion of the storage modulus G’ after curing is complete. Compression and short beam shear strengths showed a weaker relationship with Tg, and none of the properties considered showed good correlation with gel time. All other mechanical properties showed little or no change based on dwell temperature and, therefore, no acceptable correlation to viscoelastic properties was achieved. Mechanical properties which correlated well with the viscoelastic properties showed no statistical difference for dwell temperatures from 200°F to 260°F, and the nonresin-dominated properties showed little difference from 180°F to 260°F. This significant finding indicates that fiber-dominated properties remain constant for a wide range of cure temperatures. Resin-dominated properties remain statistically constant over a much larger cure temperature range than currently utilized by industry.