Failure Analysis and Resin Evaluation of IUS SRM-1 92-Inch Prototype Motor Case.

Abstract : The IUS SRM-1 92-in. diameter prototype Kevlar/epoxy motor case failed catastrophically at approximately 50 percent design load during hydrostatic burst testing on 18 October 1978. The Aerospace Materials Sciences Laboratory has carried out a failure analysis of the SRM-1 prototype motor case, briefly looked at two 18-in. subscale development vessels corresponding to SRM-1 and SRM-2, and carried out a preliminary evaluation of the epoxy resin system used for winding the motor case. Examination of the three motor cases and preliminary examination of the resin system suggest that poor adhesion is obtained between the Kevlar fiber and this epoxy resin and further that this resin system gels in three to four days at room temperature. The result of poor adhesion is not known but is expected to be detrimental to case performance in the dome regions. However, excessively long winding times (on the order of six days), coupled with the short gel times of an extremely brittle resin system plus a lack of temperature control during winding, result in an extremely nonuniform Kevlar/epoxy composite. Since the local strains incurred through both premature gellation and thermal excursions during manufacturing cannot be relieved in a brittle matrix, premature failure can occur in a stepwise fashion at loadings well below design expectations, especially if the composite layers are not uniformly loaded. (Author)