The fatigue tests have been carried out with glassfibre reinforced plastics composed of laminae of various types, such as chopped strand mat, cloth, and roving. The criterion of the fatigue failure is found to be the strain amplitude in the outmost layer. Provided that the rigidity of the FRP of combined laminae and the allowable strain amplitude of the outmost layer are known, the fatigue strength of the combined FRP is to be obtained, though the second layer seems to have a little effect. When the inner layer has considerably lower strength than the outmost layer, the fatigue failure is initiated in the inner layer even under bending fatigue. In this case, the criterion is the strength of the inner weak layer, but the adjoining layers have some effecct on the fatigue strength of the combined FRP. When cloths are used for the outmost layer and their warps have a large angle with the load direction, the layers are hardly expected to have strengthening effect.
