With the wide application of fiber-reinforced composite material in aircraft, space structures and robot arms, the design and manufacture of composite joints have become a very important research area because they are often the weakest areas in composite structures. In this study, the torque capabilities of the tubular adhesive joint and the co-cure joint were studied. Also, the expert cure system for composite materials, which was used for the manufacturing of the co-cure joint, was developed to have better composite products in shorter curing time.
The effects of the adhesive thickness, residual thermal stress on the torque capability of the tubular single lap joints were studied. The torque capability of the adhesive joints were experimentally determined and found to be inversely proportional to the adhesive thickness. In order to match the experimental results to the theoretical analyses, the elastic-perfectly plastic material properties of the adhesive were used for the closed form solution. Also, the residual thermal stresses of the joints were calculated by the finite element method and it was found that the residual thermal stress played an important role when the adhesive thickness was large.
In order to compare the experimental results with the calculated results, the stress and torque capabilities were analyzed by the 3-dimensional finite element method taking into consideration of the nonlinear properties of the adhesive. From the experiment, it was found that the torque capabilities of the adhesively bonded double lap joint was 2.7 times larger than that of the single lap joint. Also, it was found that the fatigue limit of the double lap joint was 16 times larger than that of the single lap joint.
The excess resin in the co-cure joint was found to act as an adhesive when the adherends were not knurled, hence, it should not be extracted. Also, it was found that the properties of the composite materials had a great effect on the strength of the co-cure joint when the adherend was knurled and the fiber volume fraction of the composite materials should be lower than 61\% to increase the shear properties of the composite materials because the co-cure joints were fractured by the intralaminar or interlaminar shear modes.
The expert cure system for carbon fiber epoxy composite materials, which controls the temperature and pressure of the autoclave according to the several rules, was developed to manufacture better composite products in shorter curing time. The rules were based on the on-line measured quantities such as the dielectric properties and temperature of the composites and the pressure of the autoclave. The expert cure system completed the cure of 8-ply carbon fiber composite materials for 40 minutes. The mechanical properties of the composite materials were not inferior to those of the composite materials manufactured by the conventional 150 minutes cure cycle. Also, the expert cure system completed the cure of 52-ply thick carbon fiber composite materials for 90 minutes and the consolidation of the composite materials was fulfilled more effectively.
