Qualification of aircraft structures

[rcoe]

I am trying to learn about qualification of load carrying aircraft structures. Especially I am interested in understanding the requirements for full scale testing when changes are made to the structure or component. What kind of variations are tolerated without new tests have to be made? And, how does modelling tools reduce the requirements for qualifications?

Please let me know if you have any supplying information if we talk about composite structures.

Can you provide a reference for a code that contains the information that I try to find?

Best regards,
Rasmus

 

[rb1957]

it all depends !

what piece of structure ? primary, secondary ??

detailled design ... single loadpath, multi-loadpath ??

how significant a change ? imapct of change on the loadpaths ??

criticality ... birdstrike ???

analyzable ... can FEA show that the difference is small ?

and finally, what will you local certification people buy into ?

 

[YoungTurk]

rb hit it on the head; but since it sounds like you're starting at the beginning I'd move his last point to first on the list. There are no cut and dry rules regarding what is acceptable under what conditions, and with few exceptions it will be the certification authority who determines exactly how stringent the engineering/testing validation will be.

 

[rcoe]

I get your point. Some years back I worked with qualificatin of flexible riser. For these structures a general qualification rule was that design models are valid for pipes 2'' (ID) outside the full scale test used to validate the model. This variation is probably motivated by the very good understanding of the materials and the various failure modes.

Recently, I moved into the composite bussiness. Each time we deliver a product we have to make to make a full-scale test because small variations are needed from costumer to costumer.

My thought then was to investigate what are the practice in aeronautical engineering and qualification. Which recommendations do the composite qualificaiton codes give when it comes to variations in the structual components.

Hope that you can help me on the rigth track.

best regards,
Rasmus

 

[rb1957]

ok, a little more general ...

how good is your corelation between test (strain gauges, deflections, ...) and analysis (FEA, calcs, ...)?

if yo have a good analytical uderstanding of the structure and it's response to loading then you "should" be able to qualify small changes with analysis.

next question, is it cheaper to test or to analyze ?

the veto maybe with the customer ... he may specify a test is required. still you may be able to convince him that you know what's happening and your analysis is good.

 

[GBor]

Rasmus,

We need to back up a bit because it sounds like most of our initial responses would come from dealing with the FAA (US). Rb asked who your local authorities were, and your response to that may change the responses that you receive back.

 

[rcoe]

Thanks Gbor and RB. Today I have been studying AC20-107B which is an advisory circular from FAA. That brought me one step closer. Thanks for that.

What remains for me is to get a bit closer to understand how large changes can be tolerated before new mid- or full scale tests have to be conducted. Are there any criteria stating when new tests must be made?

Lets assume that the center wing box of the airbus A380 has been fully certified but new requirements to the fueltanks arise. Then, say, a hole and a few other minor structural changes must be made to the wing box. If the structure was made from aluminium these minor changes just needed to be recalculated by FEA - this is because the material failure mode is well understood. As the wing box is made from composite materials the failuremode may suddently change and probebly a new test would be required.

Regards
Rasmus

 

[rb1957]

it sounds like there is a detailed FEM, and you've got good characterisation of the material. personally i'd got with an analytical substantiation of the change. i'm sure you're not going to do a static test of every small change (that adds a couple of holes, presumably suitably reinforced). you Might do a component test of a big enough panel to capture the difference ... but IMHO that's a 1/2 way choice (not much better than your analytical solution).

 

[YoungTurk]

If the question is driven by use of composite materials rather than aluminum, then the answer depends on how well you understand the behavior of the composites.

How do you determine a suitable reinforcement if you don't understand what failure modes you may introduce? Delamination comes to mind as a failure mode you're likely to introduce that wouldn't be adressed by typical aluminum reinforcement techniques. It may be a question of working to understand the composite material behavior better before you can determine whether a test is required.

 

[gwolf2]

Small changes usually involve holes, more or larger than originally present, or holes where none were present before. On Airbuses, it's fatigue which dominates the design of most components not ultimate strength.

 

[rcoe]

Thank you everybody,

your responses have been quite helpfull. However, I still have a few unsnswered questions that I need to solve. I think these questions are better suited for a composite forum.



Best regards,
Rasmus

 

[SvT75]

rcoe,

Check out the CMH-17 workgroup (

http://www.cmh-17.org).

Additionally, the MIL-HDBK-17 can help you further.

Bottomline is that small changes in composites may result in major changes in load distribution in your structure. Therefore it may be possible that you have to start from the bottom and work your way up in the building block pyramid. Possibly you will have to start with design-value development.

By the way, the CMH-17 workgroup has very good courses for composite qualification held every 6 months. Courses are lead by FAA and EASA lead employees (Simon Waite and Larry Ilcewicz).

Hope this helps.

Best regards,
Sergius