As regards temperature, for the untoughened resin E, [[delta].sub.c] is constant and values are in agreement with those present in literature for similar materials [21, 22] for which fracture occurs by the propagation of a single craze ahead of the crack tip.
The two materials show different trends, fracture toughness increases as crack speed increases for the untoughened resin E while the opposite is observed for the toughened resin EI.
The fracture surfaces of the toughened and untoughened epoxies were examined with scanning electron microscope.
The SEM micrographs of the fracture surfaces of untoughened and toughened epoxy resin are shown in Figs.
Figure 7 also shows that for untoughened
composites (i.e., composites having 0 wt.% rubber) the transition in impact strength occurs at temperature above 70[degrees]C while, when rubber is introduced, the transition occurs at a temperature below 70[degrees]C.
LEFM tests were conducted on the [TABULAR DATA FOR TABLE 1 OMITTED] untoughened
nylons and the phenolic/burlap system according to ASTM D5045 at a rate of 0.25 cm/min on a screw-driven mechanical tester.
 noted, for systems based on the DGEBA cured with a 4,4'-diamine-3,3'dimethyldicyclohexylmethane, an increase of the Gc from 0.15 kJ/[m.sup.2], for the untoughened
resin, to 0.43 kJ/[m.sup.2] for the modified resin with 25 wt% of CTBN.
 found that the temperature of maximum toughness was shifted some 50-120[degrees]C lower in rubber-toughened thermoset systems, compared to untoughened
systems, with the actual toughness of the former systems being 4X higher.
Resin 9, an untoughened
formulation with silica, showed a unique reproducible behavior where the minimum viscosity temperature actually was significantly below the reaction initiation temperature.
This influence of the particle size, however, is not general in untoughened
polymer blends; this is because similar ductilities in blends with different miscibility levels were observed among others in PMMA/phenoxy (33) and in PEI/PAr blends (46).
However, there is an average 3 [degrees] C decrease in [T.sub.g] for the rubber-toughened epoxy resins compared with the [T.sub.g]s for the untoughened
epoxy resins, indicating that very little rubber still remains in the matrix phase.
Using a toughened carbon fiber composite as a model system and a conventional untoughened
composite as a base for comparison, impact damages and residual compressive strengths of the composites were compared.