Composite Scarf Repairs Theory 3

[Tayyareci1]

Hi everybody,

I study on composite-composite scarf repairs and need theory of this repair type to understand better this application. Could you give me some references, please.

Thanks

 

[Compositepro]

This should help

http://www.tc.faa.gov/its/worldpac/techrpt/ar00-46.pdf

 

[Tayyareci1]

Thank you Compositepro

 

[SWComposites]

scholar.google.com

type: composite scarf repair

search returns lots of results

 

[blakmax]

To understnad scarf repairs, it is necessary to understand that the shear stress in a lap-joint is not unform. Way back in the 1930's Volkerson showed that the adhesive shear stresses peak at then ends of the joint and decay to zero in the middle of the joint if the overlap length is large enough. If the overlap length is too short, then the joint is susceptible to creep. In the early 70's Hart-Smith provided an elastic-plastic model which showed that for ductile adhesive systems, up to 80% of the bond strength was derived from plastic behaviour. Further, Hart-Smith provided design rules and equations for lap joints such that a designer could ensure that the potential strength of the adhesive was always greater than the design loads, or preferrably that the bond was stronger than the parent material, in which case the joint would never fail under any load case.

The theory behind scarf repairs is that if the scarf angle is sharp enough (about 1 in 30) the nasty "stress concentrations" represented by the shear stress peaks at the ends of a lap-joint could be virtually eliminated, giving a nearly uniform shear stress distribution. In practice, to avoid creep, the joint shear stress must be kept below the elastic limit. Given that such a large proportion of adhesive strength is derived from plastic behaviour, keeping the shear stress below the elastic limit for creep reasons means that the joint is not achieving full performance.

A further concern with scarf repairs is that the 1/30 scarf results in removal of substantial amounts of perfectly good structural material. If the structure was 0.5 inches thick, the repair would be 30 inches in diameter plus the size of the damage. You may want to grind out 30 inches of a structure but I certainly would not.

For thin composites, a lap repair can actually be stronger than the material. Hence the only reasons I would use a scarf repair would be 1. asthetics, 2. aerodynamic or clearance requirements on repair thickness or 3. stiffness restoration for stiffness critical structure.

 

[Compositepro]

The Air Force seems to be using mainly 20 to 1 scarf. In the literature I've seen 10 to 1 up to 50 to 1 investigated. I think above 20 to 1 the performance improvement is small.

 

[berkshire]

The European sailplane manufacturers have always advocated 30:1 to 50:1 scarf joints for thin skins. However on splices to main-spars they have allowed splice ratios on some parts of the wing to come down to 27:1 because of this worry about removing excessive good material.Also because there is a concern that an accurate 50:1 splice is very hard to do.
That a well done splice at 25:1 or 27:1 is going to be better than a poorly done 50:1 splice.
Lately on thin skin carbon fiber structures SR Ms have done away with scarf joints in favor of a technique loosely translated as (Hammer scarfing)where the foam core of a wing skin is deformed to allow a lap joint repair without creating a bulge in a laminar wing profile.
B.E.

 

[blakmax]

A better repair is a multiple-step repair where steps are cut using a plunge router. At every step there is a new shear stress trough generated in the adhesive, so load transfer is quite effective. You end up with a considerably smaller repair which is almost flush. There is still a degree of removal of perfectly good material, but far less than for a scarf repair.

 

[GBor]

Tayyareci1, What manufacturing process? Certain ones work better than others in scarf repairs. There are many repair procedures and some are dependent upon the manufacturing process so that you don't end up resin-rich...resin-poor...entrained air...