Scarf Repairs for Aircraft Structures
In the first of a series of technical articles, the challenges of primary structure repair of composites is discussed, leading to the development of a “hard patch” scarf repair system.
The design and certification of repairs is a major challenge to the further exploitation of composite materials in aerospace structures. Composite materials are already extensively used in tertiary structures (eg. fairings) and secondary structures (eg. flaps). In these applications if a repaired structure were to fail, at worst the aircraft operation would be affected, but a major catastrophe is highly unlikely.
However, as new aircraft programs drive the use of composites into safety-critical primary structure, the design and certification of repairs becomes evermore challenging. Common repair techniques such as patch repairs are suitable for only relatively thin structures, while bolted repairs have limited capacity for strength restoration. For such heavily-loaded structures, scarf repairs may be the only viable solution to avoid costly component replacement. The challenge is further intensified if the structure is highly optimised and/or experiences high service temperatures.
To be certified, a repair must demonstrate :
- Restoration of residual strength
- Prevention or slowing of residual damage growth (if present)
- Minimum change in local stiffness or stress distribution
- Durability in the airframe operating environment
- Damage tolerance
- Proof of satisfactory design and implementation (quality)
- No unforseen consequences: aerodynamic, flutter or clearance
Design & Analysis
Properties of an ideal scarf repair are summarised in Table 1 . The conventional wisdom is that the ply orientations should match the parent structure (item 4), however it has been shown by analysis and test that mismatch of plies may reduce the peak bondlines stresses and result in a stronger repair [3, 4, 5]. It has also been shown that optimising the planform geometry of a scarf repair (i.e. elliptical versus circular in shape) for orthotropic structures can reduce peak stresses in the adhesive and minimise the amount of pristine material that must be removed in the process . Similarly, for the case of dissimilar materials, optimising the scarf angle profile can reduce peak adhesive stresses .
Table 1: Properties of an “ideal” scarf repair, after Ref. 
1. Ply ends stay aligned and intact (moulded and soft-patch)
2. Uniform and consistently uniform bond-line thickness
3. Maintain integrity of feather edge region of a patch (machined patch shown)
4. Continuity of ply orientation between patch and parent
5. Free from voids in adhesive
6. Consolidation of fibres in patch and parent
Initial design of a scarf repair can be based on the design of a scarf joint, where the bondline stress can be related to the applied tensile load. The shear stress distribution along a 5º scarf joint in a quasi-isotropic laminate is compared to that of a unidirectional laminate in Figure 1. It can be seen that the peak stresses, which coincide with the termination of 0º plies in the quasi-isotropic laminate, are well above the average shear stress. Consideration of the adhesive plasticity (either analytically, or using nonlinear finite element models) provides a mechanism to cope with such local stress concentrations in the design of a scarf repair. It has also been shown that as the laminate thickness (and total number of plies) increases, the significance of these peaks reduces (Figure 1b).
(Figure 1a) (Figure 1b)
Figure 1: (a) comparison of peel and shear stresses in a quasi-isotropic laminate to a unidirectional laminate, and (b) with increasing number of plies in a quasi-isotropic laminate, where the red lines indicate theoretical values based on isotropic adherends, after Ref. 
Among other properties listed above, a repair must demonstrate damage tolerance. Testing of scarf joints under impact energies approaching barely visible impact damage (BVID) levels have resulted in damage to the composite adherends, particularly near the scarf tip regions which are critical for joint strength . The presence of a doubler has been shown to improve damage tolerance slightly at higher impact energy levels . Development of a validated design method that considers damage tolerance, as well as fatigue and durability, is an important step towards the certification of bonded repairs.
Because of the low scarf angles required, a significant amount of pristine material is removed in the implementation of a scarf repair. This can potentially be reduced by further optimising the design, or adding a structural doubler in addition to the scarf repair [8, 9]. Although robotic solutions are under development, present techniques heavily rely on the skill and experience of training operators to machine out the scarf region by hand. So-called “hard” patch techniques, where the patch is either moulded or machined from a blank laminate and cured off the aircraft, can potentially provide better repair quality than “soft” patches, which are cured in-situ, because of the challenges of providing adequate temperature and pressure. However, locating the hard patch and controlling the bondline thickness are just two of the challenges associated with hard patch techniques.
Figure 2: (a) hand machining of scarf region and (b) example hard patch machined from laminate, after Ref.  (Click to enlarge.)
Under Project P3.1, “Robust Composite Repairs”, CRC-ACS will continue to progress repair technology through:
- greater understanding of all damage modes (not just adhesive failure)
- improved design and analysis methods, that consider damage tolerance, fatigue, durability, etc. as well as static strength
- improved inspection processes
- robust, repeatable, reliable repair processes
The CRC-ACS participants in Robust Composite Repairs are ACS Australia Pty Ltd, Bishop Aeronautical Engineers (Germany), DSTO (Australia), DLR (Germany), EADS Australia Pacific on behalf of Cassidian Air Systems (Germany), Monash University (Australia), MSC.Software Australia Pty Ltd, Pacific ESI (Australia), RMIT University, University of Bordeaux (France) and The University of Queensland (Australia).
- Baker, A.A., A Proposed Approach for Certification of Bonded Composite Repairs to Flight-Critical Airframe Structure, Applied Composite Materials, DOI 10.1007/s10443-010-9161-z
- Whittingham, B., Baker, A.A., Harman, A. and Bitton, D., Micrographic studies on adhesively bonded scarf repairs to thick composite aircraft structure, Composites: Part A 40 (2009), pp. 1419–1432
- Gunnion, A.J. and Herszberg, I., Parametric study of scarf joints in composite structures, Composite Structures, Volume 75, Issues 1-4, September 2006, pp. 364-376
- Wang, C.H. and Gunnion, A.J., On the design methodology of scarf repairs to composite laminates, Composites Science and Technology, Volume 68, Issue 1, January 2008, pp. 35-46
- Breitzman, T.D., Iarve, E.V., Cook, B.M. Cook, Schoeppner, G.A., and Lipton, R.P., Optimization of a composite scarf repair patch under tensile loading, Composites Part A: Applied Science and Manufacturing, Volume 40, Issue 12, December 2009, Pages 1921-1930
- Wang, C.H. and Gunnion, A.J., Optimum Shapes for Minimising Bond Stress in Scarf Repairs. Proc. 5th Australasian Congress on Applied Mechanics (ACAM 2007), Brisbane, Australia, (717-722). 10-12 December, 2007.
- Harman, A.B., and Wang, C.H., Improved design methods for scarf repairs to highly strained composite aircraft structure, Composite Structures 75 (2006), pp. 132–144.
- Baker, A.A., Development of a Hard-Patch Approach for Scarf Repair of Composite Structure, DSTO-TR-1892, 2006.
- Rider, A.N., Wang, C.H. and Chang, P., Bonded repairs for carbon/BMI composite at high operating temperatures, Composites: Part A 41 (2010) 902–912
- Harman, A.B., Wang, C.H., Damage Tolerance and Impact Resistance of Composite Scarf Joints, ICCM-16, 2007