Time- and Temperature-Dependent Failures of a Bonded Joint

This report covers time- and temperature-dependent behavior of a tubular lap bonded joint to provide a design methodology for windmill blade structures. The tubular joint bonds a cast-iron rod and a GFRP composite pipe together with an epoxy type of an adhesive material containing chopped glass fibers. We proposed a new fabrication method to make concentric and void-less specimens of the tubularlap joint with a thick adhesive bondline in order to simulate the root bond of a blade. For a better understanding of the behavior of the bonded joint, we studied viscoelastic behavior of the adhesive materials by measuring creep compliance of this adhesive material at several temperatures during the loading period. We also performed constant-rate of monotonically increased uniaxial tensiletests to measure static strength of the tubular lap joint at several temperatures at different strain-rates. We also performed fatigue tests by applying uniaxial tensile-tensile cyclic loadings to measure fatigue strength of the bonded joint at several temperatures, frequencies, and stress ratios, and to show their dependency. We can estimate the fatigue life of the bonded joint at any loadlevels at any frequency and temperature with a certain probability. A numerical example shows how to apply the life estimation method to a structure subjected to an arbitrary load history by using rainflow cycle counting.