Carbon Fiber Reinforced Plastic (CFRP) laminate composites have superior characteristics such as high mechanical strength and lightness. However, CFRP laminate composites are not homogeneity material like metals, therefore, these show unique damage and fracture form, that is, various damages, for example cracks in base material (plastic), these in fiber bundle and interface separations in fiber bundle, affect mutually, then CFRP laminate composites lead to fracture in the end.
In this study, the fatigue strength of CFRP laminate composite with circular hole are examined with different temperatures, high temperature (82℃), room temperature (25℃) and low temperature (－54℃). In addition, by using the specimens with a circular hole of the CFRP laminate composite with cumulative fatigue damage, the observation of the fatigue damage progression by the soft X-Ray inspection and the optical microscope observation of the cross-section of the specimens in which the layer interface separations and the separations in the layer generate were carried out. The results are following,
⑴ When the maximum stress of cyclic stress is large, the case of low temperature (－54℃) showed short fatigue life compared with the cases of 82℃ and 25℃. ⑵ In case of small maximum stress, the fatigue life is longer the temperature is lower in order of 82℃, 25℃ and －54℃. ⑶ When temperature is low, although many cracks in base metal or fiber bundle generate and exist in CFRP laminate composite, these cracks are prevented from developing to the layer interface separation or the separation in layer.

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Recently, Carbon Fiber Reinforced Plastic (CFRP) composites are applied to pressure equipment such as pressure vessels and pressure pipes, etc., not only aircraft structure. When pressure equipment or aircraft structure like that made of CFRP laminate composite material are damaged, one of the repair treatments is the bonded repair.
In this study, focusing on the repair treatment with scarf joint which is a bonded repair method and used also for the repair of the aircraft structure damage, the tensile strength and fatigue strength of the bonded scarf joints were examined by using the two-dimensional scarf joint specimens which simulate the cross-section of an actual repaired part with scarf joint with flat conical shape. Furthermore, the residual tensile strength and the residual fatigue strength after impact loading were evaluated by using the bonded scarf joint specimen with impact loading damage. The following results are obtained.
⑴ Larger scarf angle of the bonded CFRP scarf joint has lower tensile strength. ⑵ The doubler patches are effective in the tensile strength improvement of bonded CFRP scarf joint, the tensile strength of the scarf joint is reinforced about 7 to 10 % by attaching doubler patches. ⑶ σ_R−N_f curves for base material of CFRP laminate composite, the bonded CFRP scarf joints with scarf angle 2 degrees and 6 degrees, and the scarf joints of scarf angle 2 degrees attached doubler patches were obtained. ⑷ The tensile strength of the bonded CFRP scarf joint with scarf angle 2 degrees decreases about 6 to 9% after impact loading with 30.5 J. ⑸ The effect of impact loading with 30.5 J on the fatigue strength of the bonded CFRP scarf joints, which scarf angle is 2 degrees, with and without doubler patches was within the variability of the measured data. ⑹ In case of the bonded repair treatment with the scarf joint, the scarf angle as small as possible is preferred from comparison between the scarf angle 2 degrees and 6 degrees.

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The occurrence and progress of fracture in a cracked body is evaluated by fracture mechanics. When the small-scale yielding condition is satisfied, stress intensity factor is effective as a fracture mechanics parameter. However, in structures made of steels, large plastic deformation often appears before fracture occurs, and in this case, J-integral is used as a fracture mechanics parameter. Reference stress method is one of the representative evaluation methods for J-integral. It is a simplified evaluation method without any re-strictions on the stress-strain relation, and J-integral can be evaluated as long as appropriate stress intensity factor solution and reference stress solution are provided. This paper explains the basic concept of the reference stress method, the meaning of the reference stress solution, and the specific procedure of the evaluation.

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