COMPOSITE PANEL, COMPRESSION AFTER IMPACT

[Caztech]

Fellow aeronautical stress engineers:

I am currently processing a concession in which there is loss of thickness. The standard to which I am working yields a knock-down factor on compression after impact capacity. Applying the knock-down factor to the tabulated damage tolerance reserve factor will lead to rejection of the panel, but that tabulated reserve factor is a tensile case. Given that I am investigating compression after impact, I suspect that I should be looking at the minimum compressive damage tolerance reserve factor. Can any of you please provide confirmation (or otherwise)? If you can confirm my supposition, can you please provide a quotable reference?

Thanks in anticipation.

Louis

 

[SWComposites]

Yes you are correct. It is common senses.

 

[Caztech]

Thanks, SWComposites.

 

[WKTaylor]

Caztech... I have a few documents relating to composite structures impact damage.

Searching the web for appropriate/related documents should already have been Your step #1.

It is important to note that [ballistic] impacts comes in many flavors [wide variety of velocities] and from many sources: to name a few... Taxiway/runway debris, engine/system/component 'bursts', animals, crash/runway excursions, battle. Some might also include shock/blast-waves as a form of impact. BTW impact damage to helicopter rotors is a way-different 'beast'.

NOTE. RE this topic area... You must ALSO become intimately familiar with NDE/NDI of composite parts to detect/characterize damage.

NOTE. I had a short list of reference documents ready for this post... but I prefer hearing what You found... first.


Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[IRstuff]

Is this panel ONLY supposed to deal with impacts?

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[Caztech]

Hi Guys:

The subject panels are in the underside of a wing leading edge. As such they are loaded primarily by sympathetic bending. They are also checked out for burst duct pressure. Accordingly, tension loading is higher than compression, reflected in the low tension damage tolerance RF. The concession case is loss of thickness due to resin bleed. The standard with which we have to comply yields a knock-down factor for compression after impact. The damage tolerance analysis reported in the dossier which I am using is based upon an impact of given magnitude and nature, coupled with service loading. Since tension is greater, the reported RF is a tension case. Compression RF's are listed in supporting files.

As a conservative first step, we apply the knock-down to the damage tolerance RF and if the result is greater than 1, job done. In the present case this first shot yielded concession RF < 1. I am aware of the meaning of compression after impact so I feel confident that it is appropriate to factor the lowest compression damage tolerance RF by the knock-down factor. However, I do not consider myself to be a composites expert, so I sought confirmation from others.

Stay safe!

 

[Compositepro]

If resin bleed is the cause of thinning the compression strength does not reduce because the strength is in the fibers. The column buckling strength would be lower.

 

[SWComposites]

Sounds like a secondary structure part, so really strange that it would be analyzed to a damaged tension allowable. Though maybe it is actually sized to open hole tension allowables. Either way you still should have a knockdown factor to apply to the tension RF though it should be a lot less severe than compression factor. This must be an Airbus aircraft.

 

[SWComposites]

Rubbish. Lower resin content leads to reduced compression strength as there is less resin to support the fibers. I have seen lots of test data supporting this.

 

[Caztech]

Hi SWComposites:

The damage tolerance analysis included tension as well as compression cases; it was a tension case which produced the lowest RF hence its appearance in the RF table in the dossier. Compression cases are listed in the supporting documentation.

The standard to which I am working does not yield a tension after impact knock-down, but it does give one for compression after impact in line with your comments.

Your responses have given me the confidence to address the compression after impact knock-down by applying it to the lowest compression damage tolerance RF.

Stay safe!

 

[WKTaylor]

CT...

"The standard to which I am working..."

Perhaps if we knew what 'document/rule/standard' You are working to...?


Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]