Correlations Between Micromechanical Failure Processes and the Delamination Toughness of Graphite/Epoxy Systems

A combination of macroscopic delamination fracture toughness, scanning electron microscope (SEM) real-time fracture observations, and postfracture morphology were used to study the micromechanical processes of delamination failure in several graphite/epoxy systems.

Strain energy release rate G_Ic, G_IIc, and mixed mode G_I&IIc were obtained from unidirectional double cantilever beam specimens. Comparisons of these energy release rates with the resulting fracture surface mophology are used to clarify the relative importance of the formation of hackles and the fiber matrix interface adhesion to delamination toughness under Mode I, Mode II, and mixed Mode I & II loading conditions.

Real-time fracture observations of composite delamination in the SEM revealed the microprocesses of microcrack formation and coalescence. These observations coupled with G_Ic for the neat material, determined from compact tension specimens, provide insight on how resin toughness can be translated into composite delamination toughness.

The implications of hackle formation and their importance in the failure analysis of graphite/epoxy systems are discussed.