A series of epoxy networks with controlled molecular weight between crossli
nks (M-c) was constructed with a difunctional epoxy resin and a mixture of
aliphatic amines. The glass transition and yield strengths decreased as M-c
increased, while the elastic properties were independent of M-c. In determ
ining the effect of M-c on fracture toughness, the fracture behavior change
d from a brittle fracture to a ductile fracture as M-c increased. The tests
and analyses used to evaluate the fracture energy changed from a linear el
astic fracture mechanics approach for brittle failures to an elastic-plasti
c fracture mechanics approach for ductile failures. The ductile responses a
lso shaved an increasing fracture resistance with crack extension. Two popu
lar models were used to describe the fracture energy as a function of yield
strength. The analysis showed that a change in fracture energy by a therma
lly induced change in yield strength was equivalent to a change in fracture
energy by a chemically induced change in yield strength. In addition, comp
arisons between the two models allowed insightful relationships to be drawn
.