Contact:
Dr. Richard Patton
Mississippi State University
Dept. of Mechanical Engineering
P.O. Box ME
MS State, MS 39763
patton@me.msstate.edu

Verification of the structural integrity of composite devices is hampered by the inability to detect flaws inherent in the laminate based, manufacturing process.  Ultrasonic detection methods have been developed, but cannot detect the small flaws prior to growth.  In addition, in-situ stresses arising during operation can result in microcracking with disastrous consequences, as in the X-33 composite cryogenic hydrogen tank.  For this reason, it is desirable to develop self-healing composites that do rely on manual intervention for their success.  The purpose of this proposal research is to develop self-healing composite materials suitable for use as cryogenic hydrogen fuel tanks.  Use of thermal cycling with self-healing composites  during the initial manufacturing process is proposed to reduce the occurrence of  hydrogen leaking caused by microcracking during cryogenic operation.

Carbon fiber epoxy resin matrix composite tanks undergo cracking when cooled to liquid hydrogen temperatures.  This microcracking results from a stress mismatch between carbon and the resin matrix due to differences in the coefficients of thermal expansion (CTEs).  These microcracks can develop during the manufacturing process due to stresses induced as the resin cures and contracts.  During cryogenic operation, additional stresses can cause the microcracks to grow in the matrix, resulting in hydrogen leak paths.  Such contraction occurs during cooling far below the resin's glass transition temperature where molecular relaxation of stresses is not a viable stress reduction mechanism.

It is proposed to develop self-healing resins for cryogenic applications to minimize the occurrence of catastrophic failure during service.  If the cracks induced during fabrication could be "healed" prior to putting the tank into cryogenic service, this would reduce the crack growth during cryogenic operation.  Thus, if a suitable self-healing system could be developed, a manufacturing process incorporating thermal cycling might produce a composite without the flaws that can develop into leakpaths during cryogenic operation.  In some respects, this would be analogous to stress relieving of metals to increase their cyclic life.