Hot/Wet CSAI

[TheBigM]

I have to perform some hot/wet testing of compression strength after impact specimens and would appreciate opinions on when the damage should take place. Should it be before or after conditioning? What is the most common choice? Its something that is hard to find answers to.
This is pretty critical and so I want to make sure that I make the correct choice.

 

[RPstress]

In the past we have always done the impact RT/dry and then conditioned the specimens. When impacting structural parts for cert testing it has had to be RT/AR (as received) if they're too big to oven-dry.

Most of the damage (cracks) in the resin is propagated by mode I and mode II (and of course mixed mode I/II). If the critical fracture toughness is increased when hot/wet this would be probably be suffucient justification for the procedure, along with the fact that the vast majority of field damage won't be done in extreme conditions (often -50°C and +100°C).

Most matrix-type epoxies show increasing toughness with temperature. (Googling toughness temperature epoxy will give a few hits.)

Ideally, the material system would be checked for modes I and II toughness at different temperatures (including ones <RT) both dry and wet. However, testing this thorough is rare (I don't know of any, myself). I guess you could check the actual CAI strength for all different conditions to find the most critical, but that would be a tad expensive.

 

[SWComposites]

If this a normal aircraft part (no extreme temperature conditions, no extreme damage threat situations) and you are testing a typical epoxy material, then impacting before conditioning should be acceptable, and a lot easier for the test lab. If in doubt, ask your customer or cert agency.

 

[TheBigM]

That's great, thanks guys. I've just managed to locate the method in ASTM and they concur with yourselves.

Thanks for answering so promptly.

M

 

[RPstress]

The latest version of the Airbus CAI test spec (AITM 1-0010) also says to impact before conditioning.