Approving blend repairs beyond SRM limits

[Bower7]

Hi All,

lately, my co-workers and I have been discussing approving blend repairs beyond SRM limits. We get a lot of requests to fly on till next C check with blend repairs and it would save a lot of time/money not to have to go to the OEM all the time. I was wondering if any of you guys could please shed some light on a couple of tricks and tips on how to do this?

One method that has been discussed, mainly for fuse and wing skin, is to base the justification on how much material the SRM allows you to blend. For example, if the SRM allows a max 2"x2" blend of max 0.01" deep, then a 1"x1" blend that is 0.02" deep will be acceptable for strength. Is this reasonable and a generally accepted method? What about applying this to longerons and frames?

Thanks for your help!

 

[rb1957]

Without hard data, we're making a rationalisation (or a "story"). For me, the story you've presented isn't very convincing; in particular the aspect ratio of the blend is important (0.02" deep by 1" dia is 4x 0.01" deep by 2" dia). Where the blend is is important (mid-panel would be less critical than near an edge, rivet line, splice.

With data, the answer is easy (what's the B/P MS ? how much strength is being removed ?).

Also consider how the OEM prepares the SRM. I doubt the blend allowance is precisely determined from the strength calcs. I suspect that it is a typically accepted value (10% thickness?) with stay-out zones (splices, rivet lines, edges).

another day in paradise, or is paradise one day closer ?

 

[LiftDivergence]

I agree with rb, you have to be very careful when making those kinds of justifications. One pretty solid methodology that might work depending on the structure you are dealing with is to reverse engineer a maximum load in the material you are dealing with based on a local attachment or joint.

Usually, in the absence of OEM loads, static substantiation relies on reverse engineering from material capability, so obviously if you have a blend this wont work. But if there is a structural joint nearby, the limit of the load in your part is based on the joint strength. This is not always cut and dried either though. It's all about the LOAD PATH.

There are other ways in which the OEM might allow you to reduce net section area - allowable fastener oversizing (although blend limits can be different at fasteners), drawing tolerances, etc. Also, look at nearby design details like production fastener rows. If there is a row of fasteners in the same load path that creates a net loss greater than your blend, that might be a comparison.

Basically, you must be able to state that for material in the same load path, on a cross section in the same direction, the minimum allowable net cross sectional area is XXXXX. Over the same width and in the same direction, at some other point in the same load path, there is still a greater net cross sectional area at my blendout. Since this would be a comparative analysis, make sure you are comparing apples to apples so to speak. Wing skins are typically tapered or padded up, etc. The base thickness needs to be the same if you are comparing to some other point.

I would also like to point out you should think ahead to the DT analysis if this is to be permanent. Also, although there are exceptions for certain blends, generally the best course of action is to simply repair the damage. If you end up having a hard time substantiating the blend, don't be afraid to tell the customer that it requires a repair.

Keep em' Flying
//Fight Corrosion!

 

[SWComposites]

Bower7 - absolutely not. At least not without a lot of detail analysis/justification, as mentioned above. A deeper, shorter blend will have a different Kt, which will affect fatigue/DT. It is nowhere near as simple as amount of material removed.