Test MethodImprovement of adhesive toughness measurement

The double cantilever beam (DCB) method for adhesive toughness measurement was improved by incorporating a sufficiently sharp crack made by a wedge-tapping method. A known route to producing cracks via loading-unloading cycles was proved unreliable because the cycles produced plastic deformation in the adhesive where new cracks propagated. Abnormally high toughness values with large standard deviations were obtained with cracks made by embedding a non-sticky insert. Only instantly propagated cracks made by tapping were sufficiently sharp to produce reproducible, accurate toughness measurements. However, toughened resin was insensitive to crack sharpness. A crack length to adherend length ratio of 0.2-0.5 is recommended.