Carbon Fiber CTW Properties

[GBIRLESPCH]

I'm writing up the test plan for a carbon fiber composites aileron. The Cert Engineer is asking why I don't do cold temperature wet coupon tests. My only reply right now is that it's not critical compared to cold temperature dry, but I have no concrete evidence for that. Looking back on various test programs I've been involved over the last 40 years, there are no CTW tests, so it's been understood for a long time. Anyone got a reference I can use to put this to bed?

 

[rb1957]

I've never heard of "cold temp wet" as a design case for composite. Sure it's a box (in hot/cold, wet/dry) but does it Need to be filled ?? I've only heard of RT dry and hot wet as design allowables.

another day in paradise, or is paradise one day closer ?

 

[Compositepro]

This is an interesting question, in that I have never seen it asked or discussed before. I started to put down some thoughts in reply and realized I need to think some more. Nylon is one polymer where one can clearly see the difference between dry and wet polymer at room temperature, particularly in the case of nylon bagging films.

 

[ESPcomposites]

Some statements from "Practical Analysis of Aircraft Composites"

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Section 21.4. At a minimum, three conditions are usually considered:

- CTD (cold temperature, dry) also known as “cold/dry”
- RTD (room temperature, dry), RTA (room temperature, ambient), or RTW (room temperature, wet)
- ETW (elevated temperature, wet) also known as “hot/wet”

The CTW (cold temperature, wet) and ETD (elevated temperature, dry) conditions are typically subcritical compared to the CTD and ETW conditions. Therefore, the CTW and ETD conditions are not tested as often.

Section 21.4.3. ETW. At the ETW condition, the strength and stiffness of a polymer matrix is reduced. This combination (elevated temperature and wet environment) is considered because there is a synergistic detrimental effect.

At the ply level, the ETW condition reduces the compression strength and the shear modulus. At the laminate level, the compression and bearing strengths are reduced (See also Section 21.10.6). This is because the softened matrix has a reduced ability to support the fibers. The laminate properties from Table 21.5 are in agreement with these expectations. Interlaminar properties, which are dominated by the matrix material, are especially sensitive to the ETW condition (See Section 21.13).

For carbon fiber laminates, the reduction of in-plane strength is usually most severe for the compression strengths (OHC and FHC) in the ETW condition (maximum reduction of approximately 25–35%).[R1, R2, R17, R18, R19] This is because the epoxy matrix is sensitive to the ETW environments, and the matrix significantly affects the laminate’s compressive strength. Tensile strengths (OHT and FHT) are less affected and have a maximum strength reduction of approximately 15–25%.

Section 21.4.4 CTD. At the cold temperature and dry condition, the fibers and epoxy become more brittle and stiff. Tension strength may be reduced because of embrittlement,[R16] and there may be a reduced ability to soften the effect of stress concentrations. Compression strength increases because of the added stiffness of the matrix, which stabilizes the fibers.

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[R1] National Center for Advanced Materials Performance (NCAMP), Hexcel 8552 IM7 Unidirectional Prepreg 190 gsm & 35% RC Qualification Material Property Data Report, Test Report Number: CAM-RP-2009-015 Rev A, April 22, 2011.

[R2] National Center for Advanced Materials Performance (NCAMP), Hexcel 8552 IM7 Unidirectional Prepreg 190 gsm & 35% RC Qualification Statistical Analysis Report, Report Number: NCP-RP-2009-028 Rev A, November 16, 2012.

[R16] Baker, A., Dutton, S., and Kelly, D., Composite Materials for Aircraft Structures, Second Edition, AIAA, Reston, Virginia, 2004.

[R17] National Center for Advanced Materials Performance (NCAMP), Hexcel 8552S AS4 Plain Weave Fabric Qualification Statistical Analysis Report, NIAR Document NCP-RP-2010-011 Rev B, November 15, 2012.

[R18] National Center for Advanced Materials Performance (NCAMP), Hexcel 8552 AS4 Unidirectional Prepreg Qualification Statistical Analysis Report, Report Number: NCP-RP-2010-008 Rev D, October 26, 2011.

[R19] National Center for Advanced Materials Performance (NCAMP), Newport NCT4708 MR60H 300gsm 38%RC Unidirectional Material Allowables Statistical Analysis Report, Report Number: NCP-RP-2010-074 Rev N/C, November 22, 2011.

Brian

http://www.espcomposites.com

 

[SWComposites]

Go to sae.org, purchase a copy of CMH-17 Volume 1, and have a read thru Chapter 2.

 

[SWComposites]

And, FYI, tension properties and some Interlaminar properties are critical at cold conditions, so CTA tests are needed. Also, drying specimens is a complete waste of time and $.

 

[GBIRLESPCH]

Thanks all for your answers guys,

CMH-17 Volume 1, Chapter 2 does indeed have evidence that CTD is more critical than CTW, particularly for compressive strength with graphite/epoxy composites. Since this strength is partially matrix dependent, this must mean that something happens to the matrix - possibly softening due to the moisture. That's enough evidence for my purposes.

SWComposites - you are right. I guess we basically interchange CTD and CTA when we really mean CTA.

I'm performing level 2/ level 3 tests, and have screened for CTA, RTA, and ETW. For the CAI, crippling. etc. tests, ETW is always critical.

thanks again

 

[WKTaylor]

Curious...

I have seen effects of moisture intrusion within composite laminates, that are NOT fully 'pressure-sealed'. Moisture intrusion turns to water accumulation that can then 'freeze/thaw' as acft ascends/descends/moves thru various atmospheric/thermal conditions. This freezing/thawing effect can damage core and laminates with 'open pockets' since liquid moisture expands when frozen... and will steam/out-gas when re-warmed/heated.

NOTE.
On the Space shuttle external tank [ET]: the 'orange' thermal-protective coating... applied primarily for launch aerodynamic heating friction resistance that would otherwise affect the liquefied propellant performance... had a nasty tendency of accumulating air-voids in the coating during application, that were difficult to detect. When the ET was filled with cryogenic fuel [LO2 and LH2] the aluminum tank circumference-diameter contracted stressing the [chilled] silicone-rubber/fiber-reinforced coating and would create aggravated micro-liquefied air [O2/N2] pockets within the voids. During launch, the coating heated rapidly... well past the temperature of boiling water... which cause the 'micro-void-pockets of liquefied air' to explosively expand which often blew-off 'chunks' of thermal coating.

It was evident, early-on, that the thermal coating had to be inspected for voids... and that repair-filling was mandatory before flight. The critical Orbiter thermal protective 'black tiles' [around the fuselage-nose/belly and underside of the wing] were subject to damage from these liberated chunks of Rubber.

Regards, Wil Taylor

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