Composite materials are widely used in various fields including the aerospace, automotive, vessels. Such structures, especially in aircraft, are merged by adhesives instead of the fastener to lighten their weight. However, these adhesive joints are week in through-thickness-direction with several typical failure modes: delamination(failure between layers), intra-laminar(failure in a laminar), cohesive(failure in adhesive) and interfacial(failure at interface between composite adherend and adhesive) failure. Damaged aircraft structures due to these failure can’t maintain the integrity of the initial design, then, decrease its strength and stiffness and endanger the safety of the aircraft. Thus, maintenance and repair technology for damaged part is necessary to maintain the integrity of structures.
In this study, an experimental research for maintenance and repair technology was conducted to identify effects of scarf patch repair on the damaged single-lap joints. In the test, effects of scarf ratio, defect size and stacking sequence were investigated and test results compared with undamaged specimens to evaluate strength recovery of repaired specimen.
In the case of scarf ratio and defect size, maximum strength recovery rate exhibited with the specimen of scarf ratio 1/10 and defect size 6 mm as 101% and minimum strength recovery rate exhibited with the specimen of scarf ratio 1/30 and defect size 6 mm as 69.3%. In the effect of stacking sequence, strength recovery rate vary from 93.1% to 123%. In fatigue tests, repaired specimens show reduction of fatigue strength, which reaches 106 cycles, as 7% compared to undamaged specimen. The reason of these differences of static and fatigue results between repaired and undamaged specimen are due to surface treatment of fabric layer in lower laminate regardless of effect with respect to scarf ratio, defect size and stacking sequence. In all of the specimens, the failure occurred at tip of lower laminate, not at the repaired region. This phenomenon is a characteristic of single-lap joint that crack usually occurred at the tip of overlap region was propagated rapidly until final failure. In conclusion, there is no problem in structural integrity if the surface treatment at the overlap region and the method of scarf repair were proper. Furthermore, when fabric layer need to be applied at the surface attached with adhesive, it is recommended to avoid the application or to do significant attention with respect to the surface treatment.