Rework allowance for fatigue analysis 4

[feajob]

Hello,

As per our stress methodology, we consider rework allowance for stress analysis of lugs and pins, ...

But, I didn't find any reference that recommends rework allowance for fatigue analysis. I don't think that using rework allowance is recommended for fatigue analysis. Because, we already consider many penalization factors (e.g., surface finish factor, processing factor (anodizing), chrome plating factor, stress concentration factor, ...). A scatter factor (Ref. AC 23-13A) is also used in a fatigue analysis to address the uncertainties inherent in a fatigue analysis.

I appreciate to know your feedback about the following questions:
1) Is it required to consider rework allowance for fatigue analysis?
2) If so, is there any open published reference for this purpose?

Thank you,
A.A.Y.

 

[rb1957]

to coin a phrase, "do or don't do".

there are pretty reasonable arguments not to consider it. a couple more to add to your list would be the repair would be incorporated late (presumably) in the life of the part. so ...
1) the life in the repaired state would be less than the original design,
2) the repair would reasonably "zero time" the hole, removing material close to the hole, installing an interference fit bush, ...

if it was repaired in production that would be a different matter, and presumably you'd analyze the repair for the life of the plane.

 

[feajob]

Thank you for your reply rb1957. I am completely agree with your arguments.

During the last 10 years, I worked with various customers in this field and I have never seen such request (including rework allowance for fatigue analysis).

A.A.Y.

 

[SWComposites]

along rb lines,
- if the issue is for in-factory rework, then you probably should include a rework allowance in the fatigue analysis
- if the issue is for in-service rework, and having a reduced fatigue life for the part will not be a significant economic burden or does not cause a significant inspection problem, then a rework allowance in the fatigue analysis is probably not necessary

also,
- there is no regulatory requirement for rework allowances - it is a customer/economic issues
- if your customer wants a rework allowance in the fatigue analysis, why are you arguing with them? they probably have a good reason for it.

SW

 

[FastMouse]

Hi feajob,

An interesting topic! Apologies for turning up late to the party.

From experience, this subject can generate a lot of debate, but ultimately it comes back to the FARs and also how much you want to do for the customer.

In the past, I performed rework allowances based on fatigue criteria for lug bore oversizing, and blendouts on wing skins spar flanges & spar webs. This was on FAR 25 aircraft and was done to give the OEM and the aircraft users coverage to repair up to the blendout limits without any impact on the MRB Structures program. Doing this helped the shop floor and the end-users to keep the repairs within limits that avoided any additional inspections, but maybe even more importantly allowed the in-service repairs to be put into the SRM reducing the paperwork & admin overhead.

The work was not *strictly* required. Not doing the work would not have impacted safety, grounded aircraft or stopped production; we could have just done concession analyses as required and created a one-off RAS for each in-service repair. It simply saved all stakeholders time, money & hassle by giving general coverage up-front, so the decision to do the work was a managerial decision, not a technical or an airworthiness decision.

However, if you are looking for reasons to justify not doing the analysis, I do not think that you can say "...we already consider many penalization factors e.g., surface finish factor, processing factor..." In my view, these are not "penalizing factors". They are parameters that describe the fatigue performance of the design, and are not in some way penalizing. Just because you have included, say, a parameter for anodizing, it does not mean that you have performed a penalizing calculation. It means that the design is anodized, and you have included the effect of anodizing in the analysis. No more, no less.

Similarly, I do not think that it can be argued that a rework analysis is not required because a scatter factor has been used to cater for the uncertainties inherent in a fatigue analysis. There is no uncertainty in the geometry. You know the hole diameters and the material thicknesses (+- some tolerance) and you do your calculations for that geometry. If you are aware that you have some other diameter or thickness due to a repair, omitting that different data from the analysis and stating that the safety factor accounts for the fact that you are applying the results to a repaired geometry but analyzing a different geometry with better fatigue performance should lead to a difficult discussion with your DER/CVE/regulator/whoever at some point.

So, returning to your questions.

> Is it required to consider rework allowance for fatigue analysis?

As far as I am aware, no.

> If so, is there any open published reference for this purpose?

Refer to the relevant FARs. If they don't say that you need to do it for certification, then you probably don't need to do it for certification. (...but always remember that relying on the FARs is no defense in court!)

If I had no enthusiasm for doing an up-front rework fatigue analyses, my bias, after confirming that the FARs don’t say otherwise, would be to say that there is no over-riding technical requirement to the work, and that it is a managerial decision affecting the quality of the data delivered to the users and the downstream costs (concessions, RASes, admin, etc). I would ask my DER to agree to that point of view first!

Best wishes,

FastMouse

 

[feajob]

Hello FastMouse,

Thank you for your comprehensive response. I am glad you shared your experience with us.

Regards,
A.A.Y.

 

[WKTaylor]

feajob... my 2-cents worth...

Never forget a lesson that I learned early on...

A well defined and analyzed repair, poorly executed by techs, isn't worth the paper it's drawn on... especially when durability and damage tolerance is crucial.

A marginal repair [or a repair at the max limts], executed by the techs with precision and care, [attention to fatigue details such as fastener/hole fit/quality, deburring/radiusing edges, fine surface finishes, proper NDI, good corrosion protection, etc.] can produce amazing results.

OH yeah... one other trusim: double or triple the expected service life of "temporary repairs"; they have a way of living on-and-on-and-on until some one realizes they "way over-flew" the repair service life. Then... it becomes another engineering problem.


The fatigue-devil is in the details!

Regards, Wil Taylor

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