POLYMER REINFORCEMENTS FOR RETARDING FATIGUE-CRACK GROWTH IN METALS

Pure rolled, annealed copper and copper-polyimide (Kapton) laminates w ere tested under constant amplitude cyclic loading to determine the fa tigue crack growth rate. It was found that the laminated samples could sustain a much higher load for the same fatigue life, or had a longer fatigue life for the same load. This result is due to the polyimide b ridging across cracks in the copper. The reduction of the crack tip st resses due to bridging is quantified by an analytical approximation an d by layered finite element analyses. Despite the low elastic modulus of polyimide relative to copper, the stress reduction is significant d ue to the high effective stiffness of the bridging layer which results from the thinness of the adhesive layer between the copper and polyim ide. When the experimental data from the laminated samples are analyze d using the results of a layered finite element analysis good correlat ion is obtained between the crack growth rate in pure copper and in th e copper-polyimide laminates.