Influence of void content on strength of laminate

[Onda]

Dear All,

We asked a supplier to produce a carbon fiber/epoxy component with thick laminate of UD carbon up to 30mm, build in prepreg and vacuum bagged (no autoclave).
Due to an internal misunderstanding we did not agreed an acceptance procedure.
Once the component done we asked an independent to verify the component with ultrasound.
The report shown an average void content of 4%. Section made from the component show poor consolidation, probably due to lack of debulking during the construction.
I would like to better understand the influence of void content on compression and interlaminar shear strength of CF.
I've read various paper on the subject (and also looked previous post in this forum), and I've found that there are two main issue with this laminate.
1) Reduction of static property of the laminate.
2) Reduction of fatigue property of the composite.

Various papers reports different influence of voids (also because the percentage isn't the only characteristic of voids, also shape and dimension play a role), ranging from a linear law to some kind of exponential one.

I would like to know if somebody have experience on the subject and can point me on papers or documents to let me improve my knowledge.

My task is to understand if we can accept the component or (better) well motivate the non conformity of it. The supplier states that the strength of the component isn't influenced by the void content, but obviously this isn't the reality.

thanks very much for your help

Onda

 

[SWComposites]

Porosity above ~2% definitely reduces static strength and fatigue performance; more so compression, in-plane shear and interlaminar shear than tension. There isn't much data in the open literature; most of it is proprietary test data. You will likely need to run some tests to determine the strength properties with the type and amount of voids in your part. Whether the part is acceptable depends on the design loads, environments, etc. And the supplier does not know what they are talking about and are just trying to avoid dealing with the issue. Vacuum bag curing is known for producing parts with much higher porosity than autoclave cured parts; porosity can be reduced in vacuum cured parts, but it takes the right combination of material, tooling and cure cycle; and some process trials and material screening is likely needed to fine an appropriate combination.

 

[RPstress]

Using our in-house data I get a reduction in static interlaminar shear strength of 25% (a factor of 0.75). Sorry, but I can't share the details except to say that was for 8552 autoclave resin and HS woven carbon; that's our IP gained at quite some expense!

As SW says, the data tends to be proprietary. Data on fatigue is hard to come by as it is, complicating it with porosity makes it pretty rare.

Interlaminar shear from a short beam shear test is usually the property most affected by porosity, so using the knockdown from it for the allowable compression strength is usually safe (even if the compression strength is based on open hole compression). It's a nice cheap test and reasonably repeatable.

As you've got a porous laminate it would pay to use some of it to develop a preliminary effect for that material. Make sure you section some of it to find the porosity by measuring the void area on a cross section. Just NDT or acid digestion are not very reliable ways of measuring porosity. A lot of the work that goes into allowing for its effects is correlating the attenuation of the ultrsonic signal with the percentage porosity.

I have to say that for a laminate that thick an out-of-autoclave epoxy resin will quite possibly give 4% or so, so it might not be a bad result—I wouldn't count on doing much better.

 

[RPstress]

PS: Here is some independent info on porosity and strength just in case you need proof:

http://www.iccm-central.org/Proceedings/ICCM12proceedings/site/papers/pap592.pdf

 

[tmvo]

Under room/dry condition, usually 4% porosity is not a big issue on mechanical strength. However, if your part is going to use in high moisture or hygrothermal condition, it is definitely a very critical issue. As RPstress said, cure a thick part in out-of-autoclave usually have higher porosity than autoclave part. I am not sure what type of prepreg you used for your part. As far as I know, some oven cured thick parts even have up to 5% of porosity and still have not show that much reduction in mechanical properties under room/dry condition.

 

[Onda]

RPstress, SWComposites, tmvo,
I appreciate the comments, we will make ILSS test on specimens toke from the component. Usually good composite parts made under vacuum that we use are under 2.5% voids.
Looking on different papers found on net I've made the following picture:
Up to 1% voids mechanical property are considered without any degradation.
From 1 to 5% there is a sort of linear (or quasi linear) degradation of mechanical property. Slope of degradation depend on test, fiber orientation, type and average dimension of void etc.
As RPstress stated ILSS is the most affected property, followed by compression strength, shear strength and tensile strength.
The amount of degradation can vary, upon test and property, from 2% to 10% for each point of void added. The picture I've is obviously quite loose, but i don't think is easy to have a better estimate without numerous tests.
Thanks very much.
Onda

 

[allcomposites]

Dear Onda, the issue of voids contents and their impact on mechanical properties has been widely studied by researchers and manufactures during last 3 decades. Porosities are identified in two categories, MACRO and MICRO porosities. Their impact on mechanical properties is quite different.

Macro pososities are those appearing between fiber tows and can be visible. Micro porosities in the other hand, appear within the fiber tows and are hard to find. These last are of course the worse since they are in contact with fiber filaments creating a discontinuity that leads to filament breakage.

In the general case, porosities are accepted below 1% (which es the measure error +/-1%). Above this value properties will be highly impacted by porosities. 3% porosites can reduce up to 20% ultimate strength of the laminate.

You can find detailed information on this paper (and many others)

"Porosity reduction using optimized flow velocity in Resin Transfer Molding"

http://www.scopus.com/record/displa...origin=inward&txGid=tVjuYGW1xd2FHy4BDmkCfdq:2

You should take a look on the porosities (Macro or Micro) to have better idea on how these will impact the performance of the pieces.

AllComposites