STUDY OF RUBBER-MODIFIED BRITTLE EPOXY SYSTEMS .1. FRACTURE-TOUGHNESS MEASUREMENTS USING THE DOUBLE-NOTCH 4-POINT-BEND METHOD

The double-notch four-point-bend (DN-4PB) technique is known to produce fruitful information revealing the toughening mechanisms around the sub-critically propagated crack tip. In this study, correlations among the single-edge-notch three-point-bend (SEN-3PB), the single-edge-notch four-point-bend (SEN-4PB), and the DN-4PB toughness measurement techniques have been conducted using various modified-epoxy systems. The toughnesses of both the neat and rubber-toughened epoxy systems are found to be independent of the testing techniques used. Specifically, when the peak load is plotted against B.W1/2/Y (B: thickness, W: width, and Y: correction factor), the data obtained from SEN-3PB, SEN-4PB, and DN-4PB all fall on the same line. The slope of this line is defined as the stress intensity factor. These results imply that with a single DN-4PB test, it is possible to gain the information needed to describe both the toughening mechanisms and the fracture toughness value of relatively brittle (K(IC) less-than-or-equal-to 1.2 MPa.m1/2) polymeric materials.