2-direction tension first ply failures... 1

[RPstress]

Typically for a 2-direction tension test on UD it pulls apart at a pretty low load, especially when hot/wet (maybe as low as 3500 psi/25 MPa B-basis). This translates to a strain of somewhere around 0.25% or 0.3%.

However, when sizing and analysing multi-directional laminates, a typical damage-tolerant allowable as used by Mr A. and Mr. B is maybe 0.5%.

This implies that 90-degree plies are going to get cracked well under limit load. A moderate fatigue load will do the trick.

Thus almost all structures designed to these methodologies will be flying around with extensive 90-degree ply cracking.

This may not be very structurally degrading, but you've got to wonder about the long-term matrix fatigue effects and moisture ingress.

Does anyone know if this really happens, or is there some greater vulnerability in the 2-direction tensile test that is effectively giving too low a value?

 

[SWComposites]

A tension test in the 2-direction of a uni-tape layup will fail at a very low load due to the extreme sensitivity to edge defects. However, the in-situ transverse tension strength of a tape ply tends to be significantly higher in a multidirectional laminate; up to a factor of 2-4 times higher, depending on the ply thickness and how many plies of like orientation are stacked adjacent to each other.

Typical aircraft CFRP laminates do NOT have significant matrix cracks at design ultimate load, and they are NOT flying around with any significant amounts of matrix cracks.

 

[RPstress]

Thanks SW.

When I reviewed that post I whould really have spotted that it was a little alarmist and replaced "will be flying around" with "might be" or similar.

We've got a secondary structure that we're sizing with ply-based CLT methods, and the low 2-direction allowable is killing us weight-wise if we want to keep it below limit. Some resins give high allowables (e.g., 8551-7 and 977-2), but some seem sensitive and give low ones (e.g., 8552 and M21). It's almost certainly a toughness thing.

In the past I've regarded this sort of FPF as irrelevant, but this time we've got an actual customer so satisfy and have to have our ducks in a row numbers-wise.

Does anyone know if there is any way to demonstrate that edge effects are pulling down the 2-direction allowable?

We do have testimony that 90-degree plies crack before ultimate laminate load is reached, but any matrix cracking above limit would be acceptable in this instance.

Is there any way to tell what stress or strain the onset of 90-degree internal ply cracking in a multi-directional laminate happens at?

 

[SWComposites]

To determine insitu matrix cracking you can either:
- incrementally load laminate specimens and x-ray after each load increment, or
- polish the edges of specimens and view the edges under a microscope (have seen this done both after unloading and removing the specimen from the test frame, and also while the specimen was under load).

 

[RPstress]

Thanks again SW. Hadn't thought of X-ray.