Static and Fatigue Crack Growth in Epoxy Adhesives and Fractal Dimension

Professor Toru Fujii
Department of Mechanical Engineering and Systems
Doshisha University
Kyoto, Japan
 
Sponsored by the Department of Engineering Mechanics and the Center for Materials Research and Analysis

Date:  Monday, May 19, 1997
Time:  3:00 p.m.
Place: 306 Bancroft Hall


Fractured surfaces of epoxy adhesives under Mode I static and fatigue (cyclic) loadings have fractal characteristics.  The effects of rubber modification, adhesive thickness and cross-head speed on static and fatigue fracture surfaces of epoxy adhesives were examined using fractal analysis.

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:

logG_I = a₁ X D + b₁ where a₁ and b₁ 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 fractured surfaces becomes high due to rubber modification in the same energy release rate range, DG_I when DG_I is higher than 100 J/m².  The fractal dimension decreases with an increase in fatigue crack growth rate.  da/dN.  The relationship is given by the following equation:

where a₅ and b₅ are experimental constants.  Whether the adhesives contain rubber particles or not, the fractal dimension as well as the da/dN - DG_I relation is little affected by adhesive thickness.