Keywords: composite patch repair, stress intensity factor reduction, finite element analysis, crack analysis.

The generation of damage in service is an inevitable, natural characteristic of the aeronautical or maritime structures, which may significantly affect their integrity and safety. In many cases, full replacement of a structural element or part is not financially justified, thus leading to the necessity of repair. Consequently, design, analysis and repair technology constitute an ongoing challenge. The current work focuses on cracked metallic skin configurations, repaired by means of bonded composite patches. Baker [1,2] has summarized the benefits of these repairs. It is known that both the displacement and the stress field close to a crack tip are effected by the presence of finite boundaries of the domain consisting of the increase or the reduction of the Stress Intensity Factor (SIF). Many cases of circular holes in the close area of a crack have been already studied [3,4]. This paper examines the case of a thin metallic plate with a crack, repaired by means of a bonded composite patch Composite Patch Repair (CPR). The primary objective of this scientific effort is to quantify the decrease in K by implementing the defence hole technique into CPR, and then identify the parameters for optimum results for the given configuration.

Stress analysis of combined reinforcement techniques for the structural restoration of damaged plates is investigated in this paper. More specifically the drilling of stress relieving holes around the area of a crack before the application of CPR is considered. By utilizing the finite element (FE) method Stress Intensity Factors were calculated. Within the current work, a representative case of bonded composite repair of a cracked metallic structure is examined and a series of 3D elasticity linear static FE models are developed. These consist of centrally cracked rectangular plates, in which circular holes of various diameters are drilled above and below the crack line in several distances. This SIF reduction is found to be depended on both the holes' diameter but also their distance from the crack plane. When applying CPR the most common practice is to use a one sided patch. As determined, this causes the increase of SIF at the non supported side of the configuration. This problem can be overcome by introducing holes before repair, leading to a relative decrease of up to 16%. The analyses carried out have proven that as distance e increases, the effect of the defence holes weakens. As the node path leaves the vicinity of the patch this phenomenon subsides. In general, as the hole radii were raised, the provided decrease of SIF was in turn raised. For the specific configuration it is deduced that the maximum decrease (16%) was observed at a radius of 2.5 mm and a hole distance from the crack plane e (the distance between the plane of the crack and the end of the hole which is nearest to it) of 3mm.

1

Baker A.A., "Repair of cracked or defective metallic aircraft components with advanced fibre composites. An overview of Australian work", Comput Struct 2:153-81, 1984. doi:10.1016/0263-8223(84)90025-4

2

Baker A.A., Chester R.J. "Recent advances in bonded composite repair technology for metallic aircraft components", In: Advanced Composites 93. Chandra T., Dhingra A.K., editors. Proceedings of the International, Conference on Advanced Composite Materials, p. 45-9, 1993.

3

Bowie O.L., "Analysis of an Infinite Plate Containing Radial Cracks Originating at the Boundary of an Internal Circular Hole", J. Mathematics and Physics, 35, pp. 60-71, 1956.

4

Rooke D.P., "Stress Intensity Factors for Cracked Holes in the Presence of Other Boundaries", Fracture Mechanics in Engineering Practice Applied Science Publishers, Barking, pp. 149-163, 1977.

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