Investigation on the effect of drilling process on mechanical and fatigue behaviour in carbon fiber reinforced composite pipes

Fiber reinforced polymer composites are widely used in many fields such as aviation, automotive, maritime and transportation thanks to their excellent properties such as high strength and damping ability, lightness and high corrosion resistance. Composites can be produced in different geometries with various methods depending on the purpose of use. Especially in the production of composite pipes with biaxial fiber orientation, filament winding and roll (prepreg) wrapping methods are widely used. In addition to the mechanical behavior, machinability characteristics of composites produced by different methods will also be different. Although composites are often produced with a geometry close to a net shape, the conventional drilling process is widely used for making holes on the composite for assembly operations. On the other hand, while a sample is being prepared for the ring tensile (split-disk) test method, which is widely used in determining the mechanical properties of composite pipes, holes are drilled on the composite pipe in order to create a reduced section. When the composites, which have an anisotropic structure, are drilled, drilling-induced delamination occurs in the first and last layer, and micro-damages also occur on the borehole surfaces. Due to these machinability-related damages, many parts are rejected, especially in the aerospace industry. In this study, firstly, the drilling process in composites with different surface curvatures by drilling from the inner surface (concave curved surface) and outer surface (convex curved surface) of the composite pipe produced by the roll (prepreg) wrapping method with fiber orientation of +45°/-45° was examined and compared with flat surfaced (plate) composite. Then, the drilling characteristics of carbon fiber reinforced composite pipes with +45°/-45° fiber orientation produced by filament winding and roll wrapping methods were examined and compared with the composite plate is of the same fiber orientation. Drilling machinability characteristics were investigated with different cutting parameters and with or without using support. The machinability was evaluated by considering thrust forces, drilling-induced delamination, borehole surface damages and quality. After the machinability tests, the mechanical behavior of filament wound and roll wrapped composite pipes drilled in three different conditions, taking into account the maximum thrust force and damage, the least thrust force and damage, and the use of support in the range of the tested parameters were investigated. The mechanical behavior and damage development of the composite pipes drilled in different conditions were analyzed by hoop (ring) tensile, hoop tensile-tensile fatigue and radial compression tests. The test results showed that when drilled from the inner surface of the composite pipe, less force and damage occurred compared to drilling from the outer surface, and larger force and damage occurred in the composite plate compared to the composite pipes under the same cutting conditions. With the use of support, the hole exit delamination damage caused by drilling in the composite plate and filament wound pipe was significantly reduced. While the behavior of the filament wound composite pipe under hoop tensile load was better than the roll wrapped composite pipe, the roll wrapped composite pipe performed better than filament wound pipe under hoop tensile-tensile dynamic load and radial compression load. In filament wound composite pipe, it is observed that different drilling conditions affect the mechanical behaviors, whereas in roll wrapped composite pipe, it is determined that drilling conditions do not have a significant effect on the mechanical behavior due to the fact that manufacturing defects are more dominant.