Hygrothermal aging of composite single lap shear specimens comprised of AF-555M adhesive and T800H/3900-2 adherends

Fiber reinforced resin matrix composites and structural adhesives have found increased usage on commercial and military aircraft. These structural materials offer weight savings without sacrificing strength and mechanical performance. Since service history and long-term aging performance of these relatively new material systems are not well established, a long-term environmental aging study was undertaken. Adhesive bonds were prepared by secondary bonding of 3M™ Scotch-Weld™ AF-555M between pre-cured, unidirectional composite adherends comprised of Toray™ T800H/3900-2 prepreg. Single lap shear specimens (SLS) were fabricated and subsequently aged in an unstressed configuration at 82 °C (180 °F) and 85% relative humidity for up to 772 days. The aging conditions are more severe than those expected to occur in service and were selected in order to observe some changes within a reasonable exposure time. The apparent shear strengths were measured periodically at both room temperature and 82 °C (180 °F), and failure modes were determined and compared to control specimens that had been stored for equal periods of time at room temperature under low humidity. With hygrothermal aging, apparent shear strengths decreased and some changes in the failure mode were observed relative to those of the control specimens. However, in spite of the severe aging conditions, no adhesive failures indicative of interfacial failures were observed. SLS specimens that were hygrothermally aged, subsequently dried, and then tested did not exhibit complete recovery of unaged apparent shear strengths indicating some irreversible changes.