Scarf repairs

Kawakami H, Feraboh P. Lightning strike damage resistance and tolerance of scarf-repaired mesh-protected carbon fiber composites. Compos Part A Appl Sci Manuf 2011 42 1247-62. http //dx.doi.Org/10.1016/j.compositesa.2011.05.007. 

The bonded repair can take the form of either an external patch, internal patch or a flush scarf or stepped repair as described in M1L-HDBK-17-3F 3. The internal patch usually is not an option due to accessibility. For simplicity the external lap is commonly used on internal component repairs such as bulkheads and inner skins. To maintain aerodynamic cleanliness and to minimise moment-induced failure modes, however, the flush scarf repair is preferred [1]. Furthermore, on composite control surfaces (flaps, ailerons etc.) which have critical mass balance limitations, the lighter weight flush scarf repair is often the only acceptable means of repair, but a high skill level and longer time is required to prepare the damaged area for repair. 

Figure 14.2 Typical flush stepped and scarf repair.

An industry standard scarf repair first involves removal of topcoat and primer and then using a hand-held pneumatic router or grinder to machine the damaged portion of the laminate into a circular shape. The sides of the circular cutaway must then be machined to a taper at the required angle. Two approaches can be employed, either stepped or scarfed. The scarfed joint is theoretically superior (better stress distribution) but little difference is seen in practice. The joint face is sanded to a constant... 

The design analysis of a scarf may be considered similar to a single lap bonded joint, detailed analysis of which can be found in MIL-HDBK 17-3E 3. The analysis of a bonded joint is made complex, however, by the modulus difference of the adhesive compared to the adherends and the relative thicknesses of both which causes a non-linear distribution of the shear forces in a lap joint with peak stresses at the ends [1]. Scarf repairs provide a more uniform stress distribution however, to achieve this an adequate scarf angle is required [24] shown in Eigure 14.6. 

Scarf repairs can be modelled to a first approximation as a simple scarf joint. If the patch matches the parent material in stifliiess and expansion coefficient, simple theory... 

In practice, a general rule of thumb for scarfing is to remove 12.7 mm per ply. Ply thickness varies depending on the materials used in the original structure, but for carbon fibre unidirectional tape, it can be as thin as 0.127 mm. Clearly a scarf repair on a structure of 6.35 mm thickness (50 plies thick) would approximately translate to a scarf of 635 mm. 

Baker AA, Chester RJ, Hugo GR, Radtke TC. Scarf repairs to highly strained graphite/ epoxy structure. Int J Adhes Adhes 1999 19 161-71. 

Baker AA. Development of a hard-patch approach for scarf repair of composite structure 2006. Defence science and technology organisation report TR-1892, 19. 

Whittingham B, Baker AA, Harman A, Bitton D. Micrographic studies on adhesively bonded scarf repairs to thick composite aircraft structure. Compos Part A 2009 40(9) 1419-32. 

Wang CH, Gunnion AJ. Optimum shapes of scarf repairs. Comp A 2009 40(9) 1407—18. Soutis C, Hu FZ. Failure analysis of scarf-patch-repaired composite laminates loaded in compression. AIAA J 2000 38(4) 734—40. 

The problem with scarf repairs is that they need extensive facilities and often require the structure to be sent to a major repair depot. To overcome this requirement, a new simplified repair methodology is currently under development. This methodology mimics that used in the composite repair of cracked metallic structures in that it only uses an external patch, or doubler, and is aptly illustrated in the work of Paul and Jones [1], Jones et al. [2] and Chalkley and Chester [5]. 

White C, Whittingham B, Li HC, Herszberg I, Mouritz AP. 2007. ACAM 2007, Eng Aus, 1, 198-203. Vibration based structural health monitoring of adhesively bonded composite scarf repairs. 

Campilho RDSG, de Moura MFSF, Domingues JJMS (2007) Stress and failure analyses of scarf repaired CFRP laminates using a cohesive damage model. 

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