The FMLs, which is made of stacked aluminum alloys, adhesive films and the SRPP as a recyclable thermoplastic composite material, is well known for their improved fatigue, impact resistance, superior damage tolerance and slow crack growth rate compared to the traditional metallic materials. Due to the outstanding properties of the FMLs, it has been applied to the upper fuselage of the Airbus A380 aircraft and this can save up to 25% weight. However, the joining problems occur by appling and designing the FMLs instead of the conventional metallic materials. Therefore, new joining technologies are required to use the FMLs effectively.
Generally, the bolted joining method and adhesive bonding method are used for joining with FMLs. The bolted joint can derive the high joint strength but the weight and production cost are increased by the additional joining element. Adhesive bonding can be used to join any type of materials but the joint strength is weak when subjected to peel loads and the pre-surface treatment and long curing time are required. To overcome these problems, the clinching has been developed into a new branch of mechanical joining techniques. The clinching is a high-speed mechanical fastening techniques which is suitable for point joining advanced lightweight materials that are dissimilar, coated and hard to weld.
In this study, the tapered-hole clinching, which enables the joining of dissimilar materials, was alternatively proposed. In the tapered-hole clinching process, a lower sheet with hole of taper shape is placed above a die and the lower sheet replace the role of die in the conventional clinching process. The tapered-hole clinching does not require the additional joining element and it is not necessary to design a die having a complicated shape because flat die is used.
Through Taguchi method, it can be seen that contribution ratio of the punch corner radius has the greatest effect as 39.52% and the tapered length is the factor with the lowest effect in the tensile-shear force. In the S/N ratio distribution, the largest values of each level are determined as the optimal combination that can improve the tensile-shear force. Hence, the combination of R1C1H1S3 can improve the joint strength in the vertical direction of the joint surface. To verify the results of the numerical simulations on the THC joint process, the actual experiments on the joining and tensile-shear test were carried out. In the cinclusion, the optimal combination of the design parameters can improve the joining strength about 10% and the cross-section of the THC process formed in the actual experiments were in good agreement with the results of the numerical simulation.