The tensile fatigue behavior of adhesive bonded joint of FRP was investigated under low cycle repeated stress, and the effect of the type of overlay plate in the adhesive bonded part and the effect of water environment on fatigue strength were examined. The specimen used was ortho phthalic acid unsaturated polyester reinforced with glass mat or roving cloth. The mother plate and overlay plate of the specimen were made of the same material. The fatigue testing machine was developed originally by the authors and it was driven by compressed air. The stress cycle was 100cycles/min. constant and the testing temperature was 40°C constant. The following results were obtained from the experiments:
(1) The fatigue strength was almost the same for all type joints, when the shearing strength of the adhesive surface area was adopted as the stress amplitude.
(2) The strength of adhesive bonded joint was ranked between 1/4 and 1/10 levels of that of the plain specimen.
(3) The fatigue strength of adhesive bonded joint was much influenced by water and it was decreased considerably.
(4) The formula useful to estimate the fiatgue life was proposed as Eq. (1).