Effects of Thermal Cycling on the Mechanical and Physical Properties of a Space Qualified Epoxy Adhesive

Structural adhesives used for space applications must be able to survive large tempera ture fluctuations while maintaining their mechanical strength. This paper describes a series of experiments performed to document property degradation of Hysol Corps EA 956 epoxy due to temperature extremes experienced in the orbital environment. Testing in cluded tension, lap shear, coefficient of thermal expansion (CTE) and differential scanning calorimetry (DSC). Specimens were tested in the "as cast" condition or after 5 to 25 ther mal cycles between -300 and +300°F. Lap shear data were obtained at —300, —65, 0, 150, 225, 300, 375, and 450 ° F. CTE measurements were made from - 300 to + 400 ° F us ing a quartz dilatometer. Changes in the glass transition temperature of the epoxy were measured using DSC. Test data indicate that thermal cycling can produce 5 to 30 percent decrease in mechanical strength depending on the exact test temperature. In both the ther mally cycled and as cast samples the greatest change in mechanical strength occurred above 150 °F. The decrease in strength was especially sharp in the thermally cycled coupons. CTE data indicate that the exact value depends strongly on temperature; three distinct regions are observed when sample strain is plotted as a function of temperature with the CTE increasing with increasing temperature. DSC traces on as cast and thermally cycled samples show that thermal cycling produces a change in the physical structure of the resin resulting in a significant increase in the glass transition temperature of the network.