In-situ evaluation of corrosion at insulation/aluminum bondline

Abstract

A motor aging assessment program for an in-service solid rocket motor discovered a bondline separation at the interface between the aluminum polar fitting and internal insulation. Subsequent dissection of the hardware revealed the cause of the separation to be surface corrosion of the polar fitting. Coincident with the observed separation, in-situ tension and shear testing identified associated bond strength reductions. As a result of these findings, analyses and testing were performed to assess the loading conditions on the bondline for the purpose of establishing a repair criteria, if necessary, or developing a test that could be used to demonstrate that adequate margins of safety exist for known separations under flight loading.Finite element analysis methods were used to determine bondline stresses and deflections (crack tip opening) for motor operating and non-operating loads. These analytical results were then used to calculate stress intensity factors using a procedure developed by R. E. Smelser. Motor-like fracture mechanics specimens were fabricated and tested to provide data for in-situ critical stress intensity factors determined from analytical representations of test specimen results. Flight hardware load tests and associated analyses were used to verify bondline capability by comparison with laboratory test results and minimum demonstrated strengths. 4Specimen and flight hardware bondline capabilities were found to be consistent with "good bondline strengths. Fracture mechanics testing of the subject bondline showed pressure and load rate effects, consistent with motor ignition loading, produced an enhanced bond capability factor of 1.4. To demonstrate the in-situ bondline capability of flight hardware, a load test was devised for demonstrating bondline "toughness". The successful load testing of flight hardware bondlines resulted in verification of minimum bondline capability. demonstrating the flightworthiness of the motors.