What is the applicable Code/std for fracture assessment for Metals in Aricraft & Aerospace engg? 2

[Jason8zhu]

Dear all，

For metals in Aricraft & Aerospace, if we have fond some small flaws and we need to assess if the flaw will develop to crack that is not acceptable.
What is the applicable Code/std to conduct fracture/fatigue assessment?

Thanks.

 

[Sparweb]

The simple answer is FAR 25, paragraphs 571. But it's not really as simple as that. Once you touch the subject you discover a very deep body of knowledge on material properties, test data, cyclic load specra, and then there's the progress that these requirements made through time, which means that some aircraft comply with different rules, depending on when they were designed. I would not recommend trying to figure this out from scratch without tapping an experienced analyst's shoulder.

Let me step back a bit...

If your member status shows you in the "Marine/Ocean" industry, then may I guess that you are looking for guidance in the aviation industry because you are not finding suitable answers from design guidelines in your own industry? Which is odd because the monocoque structure of steel ocean vessels has a lot of similarities to aircraft monocoque fuselages, with the exception of scale and environment, and that design requirements existed for them, just like there are for aircraft.

So, where is this crack? In what kind of vessel? Why have you not posted in a forum appropriate to ocean vessels where codes specific to your problem can be addressed?

STF

 

[rb1957]

finding flaws is usually the first step of either rejection (scrapping) or repairing. Whilst it is possible to accept them, it is highly unusual. Sparweb has directed you to the standard (25.571), but you also need to understand the guidence material (AC) and the subject matter ... talk to someone who does this work, you won't get much satisfaction from a message board on the internet. There are just way too many questions before we can offer sensible help ...

are you an OEM or a supplier ? if OEM, there should be people closer to you than the internet who can help; if a supplier, talk first to the OEM.

what sort of component are we talking about ? rather than structure, as Sparweb is talking about, I think this is a heavy fitting, with flaws found by ultrasonics ?

what size flaws are you talking about ? another bigger than 0.005" is unlikely to be acceptable.

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

 

[Jason8zhu]

Thanks Sparweb & rb1957 both your guidANce. I am a FEA guy for stress analysis and fatigue, sometimes, fracture assessment.

For Ocen industry, I know BS 7910 was widely applied to assess Fatigue and Fracture. And I already had experience on this assessment.
For Nuclear industry, I know R6 is utilised.

That is why I am wondering what code to comply with for Aircraft & Aerospace industry. Now I went through FAR 25.571. Thanks to both your reply that I got a clear understanding that the case in Aircraft & Aerospace is quite different from that in Ocean industry.

Thanks.

 

[Sparweb]

Hi Jason,
I see now that you have a deeper background in the subject than I could assume from your first post. Don't be afraid to ask more specific questions - the subject interests us as much as it interests you.
More broadly, sudying the solution to similar problems in different industries can be a trip through history as much as an exploration of the applied sciences. I hope your investigations prove fruitful.

STF

 

[rb1957]

the nuke, and I guess the ship, design people are governed by codes, like the civil architects and their buildings. The aircraft business is different in that our "code", FAR25 as an example, specifies a standard to be demonstrated, and does not define the means to show that the standard has been achieved. There is supporting material, FAA ACs (Advisory Circular), which outlines an acceptable approach. Sometimes these AC outline quite detailed analysis steps, an example is the FAA's Chicago Office AC for damage tolerance. It is proposed for modifiers of aircraft structures and not for OEMs, who are expected to have their own methodologies. personally I don't like this method. OEMs generally outline their methodology in general terms ('cause they invest a lot of money in deriving them) and these approaches are typically approved by their local airworthiness governing body.

So you see it seems we have very different approaches. There are several online resources that can guide you through the general steps of damage tolerance.

Is this for general knowledge or a future job ?

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

 

[tbuelna]

Jason8zhu- You got some great input above. I would just add that fracture control in metal structures of FAA certified aircraft is a very complex subject that involves design/analysis, procurement of raw materials, manufacturing processes, extensive QA procedures, and specific maintenance/inspections over the aircraft's operating life. Most certified aircraft models also have Structural Repair Manuals (SRMs) that provide guidelines for evaluating damage & performing most types of repairs.

It's good that you read FAR part 25.571 and the related AC 25.571-1D, but much of this material is oriented towards certifying new designs. Since you seem more interested in evaluating damage in existing structures, then I'd also recommend reading AC 25.1529-1A and AC 120-93.

I read a summary of BS7910 which seems to be oriented towards fusion welded structures such as pipelines, platforms, pressure vessels, etc. There is not much fusion welding used to produce modern metal aircraft structures, so this is one area where BS7910 will likely differ from FAA FARs. However, the area where BS7910 and FAA FARs seem to differ most is the approach to ensuring a sound structure during operating life. BS7910 includes a Fit For Service approach that evaluates things on a case-by-case basis. With safety and reliability of metal aircraft structures, the approach used by FAA FARs is strict adherence to controlled/validated/approved processes for everything from design/analysis, throughout manufacturing, and until the component is removed from service.