Combating Amine Blush: Root Cause and Corrective Action of a Compromised Bond on Europa Clipper

A structural debond of a mechanical fitting occurred during vehicle-level static test of the Europa Clipper spacecraft. This event was identified by both an anomalous reading in a test strain gage and an audible noise. Testing was immediately halted and a thorough investigation commenced. Finite element analysis provides good evidence that a debond of a reinforcement fitting would produce the strain signature observed in test. Structural analysis also indicated that a properly-bonded fitting should have substantial capability beyond the loads applied in test. Furthermore, a proof test of the bonded fitting immediately following completion of the bond curing cycle was successfully performed with no detrimental observation. Root cause analysis and investigation ultimately uncovered two causes behind the debond observed in spacecraft test. The primary cause was identified as a phenomenon known as amine blush, compromising the integrity of the adhesive. Amine blush is the formation of carbamates on the exposed surface of an epoxy due to unfavorable conditions during the bonding operation. Surface carbamates present a waxy quality which substantially degrades the adhesive capability of the epoxy. Thus, the epoxy itself was predisposed to not adhere properly to the spacecraft. The secondary cause identified was the nature of the local proof testing of the bonded fittings immediately following the bond cure. The proof test was designed to be quick and simple, using a vertical pull on the fitting to a load level which reproduced flight-like stresses in the adhesive. Although an appropriate overall stress magnitude was achieved in the adhesive, the stress peaked on the opposite side of the adhesive bond line in the proof test than it does during the spacecraft test and as in flight. These differences were known and accepted at the time. However, the formation of amine blush is not symmetric on either side of a bond line. Therefore, by exercising the incorrect side of the bond line during the local proof test, the degraded epoxy was not identified in the local proof test. The local proof test resulted in a false-positive indication of the bonded joint's capability. Corrective action for the primary root case, amine blush, involved enhancements to the bonding procedure to mitigate the formation of carbamates. Corrective action for the secondary cause was the design of a more flight-like local proof test for the bonded fitting. Implementation of the two corrective actions was successful and the bonded fitting has been demonstrated to be soundly installed on the Europa Clipper spacecraft.