Longitudinal tensile tests of single-fiber specimens containing AS-1 carbon fiber in epoxy and HMS-4

Longitudinal tensile tests of single-fiber specimens
containing AS-1 carbon fiber in epoxy and HMS-4 carbon fiber in epoxy produce
cleavage cracks (normal to the fiber direction) in the matrix adjacent to the
fiber rupture. However, the cleavage crack in the AS specimen is longer than in
the HMS specimen. Furthermore, the longitudinal tensile strength of the AS
specimen increases significantly with increasing matrix ductility, but that of
the HMS carbon specimen remains unaffected. Explain both phenomena in terms of
the energy released on fiber fracture.