STATIC AND FATIGUE-CRACK GROWTH OF EPOXY ADHESIVES AND FRACTAL DIMENSIONS

Fractured surfaces of epoxy adhesives under mode I static and fatigue (cyclic) loading have fractal characteristics. The effects of rubber m odification, adhesive thickness and cross-head speed (static only) on both static and fatigue fracture surfaces of epoxy adhesives were exam ined using fractals. Under static loading, the fractal dimension becom es high due to rubber modification. It is related to the static crack growth properties for both unmodified and rubber-modified adhesives. T he following equation gives the relationship between the observed crac k extension resistance, G(I) and the fractal dimension, D: log G(I) = alpha(1) . D + beta(1), where alpha(1) and beta(1) are experimental co nstants. Regardless of whether adhesives contain rubber particles or n ot, the fractal dimension is not affected by adhesive thickness and cr oss-head speed. Under fatigue loading, the fractal dimension of the fr actured surfaces becomes high due to rubber modification under the sam e energy release rate range, Delta G(I) when Delta G(I) is higher than 100 J/m(2). The fractal dimension decreases with an increase in the f atigue crack growth rate, da/dN. A-relationship is given by the follow ing equation between a surface 'fatigue working' parameter for the cra ck, S(= da/dN . Delta G(I)) and the fractal dimension for unmodified a nd rubber modified adhesives: log S = log(da/dN . Delta G<INF>1</INF>) = alpha<INF>5</INF> .<INF></INF> D + beta<INF>5</INF>, where alpha(5) and beta(5) are experimental constants. Whether the adhesives contain rubber particles or not, the fractal dimension as well as the da/dN - Delta G(I), relation is little affected by adhesive thickness. (C) 19 98 Elsevier Science Ltd. All rights reserved.