The bending fatigue behavior of adhesive bonded joints of FRP was investigated under relatively low cycle repeated stress, and the effect of type of over-laid plate in adhesive bonded part and the effect of atmospheric temperature on fatigue strength were examined. The specimens used were orthophthalic acid unsaturated polyester reinforced with glass mat or glass mat combined with roving cloth. The mother plate and the over-laid plate of the specimens were made of the same material. The fatigue testing machine was developed originally by the authors and the bending method of 4 point supporting system was adopted. And the bending moment was applied by an unbalanced rotating weight. The stress cycle was 400cycles/min constant and the testing air temperatures were 23°C, 50°C and 80°C. The following results were obtained from the experiments.
(1) The observed fatigue strength of the adhesive bonded joint was increased more as the over-laid plate was strengthened, if the bending stress imposed on the adhesive surface area was adopted as the stress amplitude.
(2) The fatigue strength of the both-side adhesive bonded specimen was twice that of the plain specimen, and the increase of fatigue strength for the one-side adhesive bonded specimen was between 1.3 and 1.6 times.
(3) The fatigue strength was considerably decreased at above 50°C.
(4) Dynamic creep properties were much influenced by the atmospheric temperature.
(5) The formula proposed previously by the present authors for the estimation of fatigue life in the tensile fatigue test, which is given as Eq. (1), was found to be applicable also for the bending fatigue test.
