Composite fastener hole damage

Figure 15.3 Composite damage due to fastener holes, (a) Tjfpical bolted junction between a composite wing skin and metallic connection (b) failure modes in a bolted joint under tension [6] and (c) compressive failure modes in a plate with an open hole.

Fastener holes should be straight and round within the limits specified. Normal hole tolerance is 0.075 mm (0.003 in). Interference fits may cause delamination of the composite. Holes should be drilled perpendicular to the sheet within one degree. Special sleeved fasteners can limit the chances of damage in the clearance hole and still provide an interference fit. Fasteners can also be bonded in place with adhesives to reduce fretting. 

Drilling and machining can damage composites. Several techniques exist for producing quality holes in composites. Carbon, aramid, and boron fiber reinforced materials each require different drilling methods and tools. When composites are cut, fibers are exposed. These fibers can absorb water, which weakens the material. Sealants can be used to prevent moisture absorption in the clearance hole. Sleeved fasteners can also provide fits that reduce water absorption as well as provide tightness. 

Bonded joints are extensively employed in the construction of composite structures in aerospace applications, maritime structures, lifting equipment, wind mills as well as automotive industries [3, 4, 5], Unlike the bolt hole in mechanical fastening that causes a stress concentration in the composite joint plates, adhesively bonded joints minimize the potential for stress concentration within the joint. Besides, applications where lower structural weight, improved damage tolerance design philosophy are required, adhesively bonded joints provides a potential solution. Bonded joints are an efficent fastening solution also for hybrid structures, i.e., structures where composite parts are connected to metal parts. 

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