NDI Porosity Allowables - Mechanical Tests

[Obsidian]

Hi All ,

We are currently preparing a justification to support an increase in the NDI porosity design allowable of monolithic and sandwich CFRP laminates .

Part of the justification revolves around a mechanical testing regime of samples that contain a higher level of porosity that seeks to gather relevant data that would support any such process .

We have already considered some matrix dependent tests such as ILSS and we envisage doing a hot / wet campaign as we envisage that these will contribute the greatest ' knock down ' component to any calcualtions . Additionally , we are looking at microscopy and mechanical and thermal fatigue cycles to gain an insight into any micro-cracking issues .

Does anyone else have experience of any further ( particulalrly ) mechanical tests that would be useful to support such a justification ?

Thanks In Advance

 

[RPstress]

The tests that govern the allowable stresses in your laminates matter most. You can use knock-downs based on matrix-dominated properties like ILSS as the worst case with some confidence, but the allowable laminate stresses are probably controlled by hot/wet open hole compression or CAI. Making laminates with a specific level of porosity is quite hard. Do you have a plan for that?

 

[Neubaten]

Just out of curiosity, is the porous material 919/3070P/45 prepreg?

 

[Obsidian]

Hi ,

In reverse - Neubaten - no its not a pre-preg application .

RPStress - Thanks for the response . We are aware of the challenge of creating laminates with defined levels of porosity in them . We are not trying to open up the allowable so that we can manufacture to a lesser standard . We are opening it up because we have been set the challenge of quantifying % porosity in our NDI Inspection process . We don't beleive this can be done in sandwich components in high volume production and therefore we are seeking to remove the NDI requirements for porosity by demonstrating that even ( relatively ) high levels of porosity still achieve the required RFs , therefore the NDI inspection issued used to locate FOD and delams only .

 

[SWComposites]

As RP said, you need to test the properties corresponding to your critical margins. ILSS is probably not one of those, and ILSS is not always sensitive to porosity. For sandwich structure, probably should test long beam flex specimens to assess facesheet compression strength (with hole, BVID, depending on what was done for your basic allowables); for laminates, test unnotched compression, OHC, CAI, again depending on what your current allowables are based on. To generate porosity in test panels, you can cure them under vacuum rather than autoclave pressure - this ofter results in panels with porosity. Or you can just cut specimens from production parts which have your typical porosity levels. High levels of porosity in sandwich panels will likely result in reduced allowables though you may still have acceptable margings (RFs), but you should make the appropriate reductions in the allowables based on the inspection method (or lack thereof), so that the documented RFs are correct.

 

[allcomposites]

As most post indicated, you should concentrate on matrix based properties. ILSS is probably the most appropriate, flexural and impact properties are also key. We have characterized mechanical properties of RTMed composites with various levels of porosity and we have found different behaviour for macro porosities (those between fabric tows) and micro voids inside tows. Some properties were more impacted by micro-cracking while in others no difference was observed for macro and micro. A 20% decrement of ILSS and flexural properties was measured for 3-4% voids. You can find published data on porosity/properties relationship, mainly in prepregs.

 

[RPstress]

SW: have you any data where porosity doesn't affect ILSS?

I have some test data showing a more or less linear degradation of ILSS with % porosity for a prepreg carbon epoxy. Porosity was between 0.5%–6%. It's not an enormous data set, but there were a reasonable number of specimens.

 

[allcomposites]

Check out this paper from Rezende et Al. They have related C-Scan inspection to void content and ILSS properties.

"Critical Void Content for Polymer Composite Laminates"
AIAA JOURNAL
Vol. 43, No. 6, June 2005

 

[SWComposites]

RP - data is proprietary; in some cases, porosity affects ILSS, in others the effect is minimal.

More importantly, many designs are not critical for interlaminar shear stress, so even if porosity has an effect on ILSS, it may not be important. The porosity evaluation should be focused on the properties which are critical for sizing the part.