Improved Operating Limit Temperature Determination For Aerospace Adhesives

Aerospace adhesives are assigned a material operating limit (MOL) temperature. According to the Composite Materials Handbook - 17 (CMH-17), MOLs are set so that "materials are not operated in service under conditions where a slight increase in temperature might cause a significant loss in strength or stiffness, and to avoid irreversible property changes." However, there are no fixed criteria for establishment of a MOL. This ambiguity increases risk and impedes innovation in the design of aerospace structures. The goal of this work is to provide more specific and useful guidance on how to reliably determine the working temperature range for aerospace adhesives. In this work, differential scanning calorimetry (DSC) is used to identify the glass transition, which is often the basis for MOL determination, but which lacks a direct correlation to mechanical behavior. Lap shear specimens are scaled-down for future dynamic mechanical analysis. A quadruple lap shear (QLS) geometry is found to be more easily scaled down than single lap shear (SLS) specimens. Future work will use the QLS specimen geometry to establish time-temperature-property relationships for a variety of aerospace adhesives. Establishing these relationships for a variety of epoxy, modified epoxy, and alternative chemistries will inform the development of more specific MOL determination procedures. Over time, standardization of bonding processes can decreased reliance on fasteners, thereby decreasing airframe weight and associated costs.