Study of rubber‐modified brittle epoxy systems. Part I: Fracture toughness measurements using the double‐notch four‐point‐bend method

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Abstract
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 (KIC ≤ 1.2 MPa. M1/2) polymeric materials.

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