Resin properties following a cure

[midsidenode]

Does anyone know of an acceptable test for evaluating resin properties of a laminate after a cure? I was thinking of probably using ASTM D2344 (short beam, interlaminar shear) but my problem is that the laminate has a cured ply thickness of only .025 inches. This is well below the parameters required for this type of test.

 

[SWComposites]

What is your reason for needing “resin properties”?
To check the cure? Tests such as Tg, DSC are probably better.

How many plies in your layup?

 

[midsidenode]

SW, I should have clarified my intentions a little better. I'm looking at this from a strength perspective. I have a repair that must be carried out on some skin. The repair was performed remotely using a portable hot bonder unit. The person that performed the repair assures me that the proper cure cycle was performed but the area around the repaired zone appears to be a little "darker" than what I would have expected. I'm concerned that the resin properties around the repaired zone have been degraded beyond what I'm comfortable with. I would normally ask someone to obtain some short beam shear data in these "dark" areas and compare the results to pristine areas in an effort to see how much degradation occurred. Nevertheless, because these skins are so thin, I cannot have them prepare sufficient size samples the conduct an ASTM D2344 test. This test requires a minimum length to thickness ratio that cannot be met. My thought is that since transverse shear is parabolic across the thickness with the max shear at the midplane, i would bond equally thick aluminum strips on both sides of the thin sample and "hope" for an interlaminar failure in the sample. What do you think? Or would there would be a better test?

 

[SWComposites]

You could try that approach to get a usable SBS test specimen, but you would need to bond on the doublers with a lower temp cure adhesive, and would need to "hope" that the failure surface is in the composite laminate and not in the bonding adhesive.

I would try to get a Tg measurement of the laminate. From CMH-17 Volume 1 Chapter 6 Rev H (can be purchased at sae.org):
Several different thermal analysis techniques are commonly used for extent of cure measurements in fiber-reinforced thermoset matrix composites. These include differential scanning calorimetry (DSC) or dynamic thermal analysis (DTA) to measure the extent of the residual curing exotherm and dynamic mechanical analysis (DMA) or thermomechanical analysis (TMA) to measure the glass transition temperature (Tg). Polymers which are only partially cured will have lower Tg than those with advanced cure, which will approach a maximum value if fully cured.
ASTM D7028 is the DMA Tg test method for composites.

But either test type requires removal of specimens from the part? is this feasible for your situation?

And didn't the hot bonder have a temperature recording unit connected to the TCs in the repair area?

What is the material and cure temp of the parent skin? what is the material and cure temp for the repair material?

 

[midsidenode]

I agree that the act of bonding doublers to the specimens and thinking that the skin will experience interlaminar failure during a test is kind of a pipe dream. I'm not sure it's even work attempting. I do have access to every type of Tg testing nevertheless, I do not see how I can use Tg to characterize how much of damage (e.g. structural degradation) has been experienced by the resin assuming that the laminate was at a complete state of cure prior to the repair which subsequently charred it. Fortunately (or unfortuately - however you want to look at it) this entire part is scrapped due to other reasons, so i have the opportunity to sample it in whatever way i want. I want to understand what is happening here with the charring and if it is resulting in some un-acceptable damage to the matrix. I've reviewed the thermocouple data and it all appears that the heat/vacuum during the repair was applied properly.

 

[SWComposites]

Ok, that clarifies a bit.
This is a somewhat difficult problem to sort out; have dealt with similar ones before (one of the classics: browned areas due to halogen heat lamps being placed way too close to a part to speed up the cure of liquid shim because some idiot shop manager wanted to reduce cycle time; ugh, what a nightmare)
So is the part really "charred" or just slightly discolored?
It would help to know the part cure temp and repair cure temp. I'm assuming these are epoxy resins.
Some epoxy resins will change color even when exposed to temperatures below the original cure temp.
If the part saw a temperature above the original cure temp (essentially a "post-cure") then the Tg value should increase relative to baseline.
If the part is thin, then getting a interlaminar shear or compression test is difficult if not impossible; tension tests won't be useful to detect a resin change. What is the part thickness?
Is this a carbon fiber part or fiberglass part?

 

[midsidenode]

SW,
The resin is cyanate ester so you can imagine what kind of an item i'm dealing with here.
The production cure temp is 350F with a 400F post cure.
The repair cure temp was 350F with no post cure - only 90 minutes.
The area around the repair looks to be on the verge of charring. I've worked with this material (slightly different flavor here) for quite a long time and with similar skin thicknesses, but i have never seen it "darken" like this before. It makes me wonder if there is some sort of operator/equipment error when using the portable hot-bonder.
The more more i think about this, the less confident i am about pulling off a successful short beam shear test. Nevertheless, i'm really interested developing an objective measure for quantifying % resin degradation between the production cure skin vs. the "darkened/charred" production cured skin surrounding the repair area caused by the repair blanket. Perhaps one could make a set of spectometer standards and correlate those to % degradation using short beam shear. I'm just spit-balling it, but i'm thinking i would be uncomfortable with anything that indicates a 10% or greater reduction in short beam shear capability. What do you think?

 

[Compositepro]

Darkening of resin after exposure to elevated temperature during post-cure is normal and does not necessarily mean that any degradation has occurred. But, it is also quite possible for a heating blanket to grossly overheat, if the thermocouple is not located properly to sense any hot spots. Several thermocouples should always be used to monitor expected cool or hot spots. You can expose composite samples to various times and temperatures to make standards for color change. This will tell you if there was over-heating.

 

[SWComposites]

Agree - heat blankets are notorious for uneven and over temperatures.

I think I would try to replicate the conditions on some separate flat laminate test panels, with thickness sufficient to get good test specimens.
Make and test a baseline panel.
Then expose another panel to an extra 350F cure for 90 minutes, check color and test.
Then expose another panel to a cure at 400F for 90 minutes, check color and test.
Repeat increasing extra cure temp by 50 degrees until you get the color to sort of match your part.

 

[SWComposites]

And as far as acceptance criteria if you determine there resin strength reduction, it all depends on the strength margins in the part and whether this is a one off use or will see long term fatigue.

 

[midsidenode]

Yes, you guys are correct. There is always usually some darkening. It just seems a little excessive this time. I want to be able to prove to my customer (and to some extent- myself) that all is still well. I think the idea of just creating some panels with proper thicknesses for SBS and varying repair cure cycles is my best bet. This cyanate ester prepreg is just so d@amn expensive. I’ll just have to do it to do what is right.