Static and fatigue crack growth of epoxy adhesives and fractal dimensions

Abstract

Fractured surfaces of epoxy adhesives under mode I static and fatigue (cyclic) loading have fractal characteristics. The effects of rubber modification, adhesive thickness and cross-head speed (static only) on both static and fatigue fracture surfaces of epoxy adhesives were examined using fractals. Under static loading, the fractal dimension becomes high due to rubber modification. It is related to the static crack growth properties for both unmodified and rubber-modified adhesives. The following equation gives the relationship between the observed crack extension resistance, G I and the fractal dimension, D: log G I =α 1 · D+β 1, where α 1 and β 1 are experimental constants. Regardless of whether adhesives contain rubber particles or not, the fractal dimension is not affected by adhesive thickness and cross-head speed. Under fatigue loading, the fractal dimension of the fractured surfaces becomes high due to rubber modification under the same energy release rate range, Δ G I when Δ G I is higher than 100 J/m 2. The fractal dimension decreases with an increase in the fatigue crack growth rate, d a/d N. A relationship is given by the following equation between a surface ‘fatigue working’ parameter for the crack, S(= da/ dN · ΔG I ) and the fractal dimension for unmodified and rubber modified adhesives: log S= log da dN · ΔG 1 =α 5 · D+β 5, where α 5 and β 5 are experimental constants. Whether the adhesives contain rubber particles or not, the fractal dimension as well as the d a/d N−Δ G I relation is little affected by adhesive thickness.