Interlaminar Shear Strength testing questions / advice

[nickc88]

Hi all,

I have been doing a little bit of lab work lately as part of a screening / QA process for some of our products. Part of this includes short beam shear testing. I have a couple of questions and if anyone has any useful tips / observations from experience they would be much appreciated!

I'm working to ASTM D2344M.

1. I have been using this to calculate the ILSS (from the standard)
ILSS = 0.75 x P / (bxh) with P being the force obtained from
testing.

I could be wrong but wracking my brains back to mechanics of materials... is the max shear stress in a rectangular beam not given by 3P/2A? with this maximum being the peak of a parabolic distribution 0 at the top and bottom??? If this is the case, why does the standard specify 0.75 P/A?

2. The load / extension plots of most of the samples I test go in a somewhat linear fashion followed by a large drop in load. The graph ends up looking like a big tick.

Some however do not have such a nice failure. They might drop load slightly but not enough to trigger the 30% load drop failure critera. Some just gradually level out.

-Do samples that do not produce a "tick like" result mean that the sample is not failing in interlamina shear?

-Is it possible to infer an ILSS from a sample that did not have a clean failure?

3. Does anyone have any experience doing ILSS testing on filament wound tubes?

Thanks very much

 

[Compositepro]

1. Perhaps you are confusing vertical and horizontal shear. Interlaminar shear is horizontal shear and is zero at the center of the beam.

2. The large drop in load at failure is due to brittle failure. Ductile resins will not have a large sudden drop. Failure is when a crack starts. Fiber orientation will have a large effect on the test results. Uni will test significantly higher than fabric. Short beam shear (SBS)data is never used for part design. It is a very convenient and simple test for QC and comparing materials.

3. I believe that the SBS test was initially developed for filament wound structure like rocket motor casings. One of the specimen configurations specified in the test method is cut from NOL (Naval Ordinance Lab) rings which are made by filament winding.

 

[SWComposites]

re

1) shear is NOT zero at the midplane. The correct load to use is P/2, thus ILSS = (3/2) (P/2) / (bh) = 0.75 P / bh

2) probably the specimens that do not fail with a sharp load drop are not failing in shear. Non-shear (bending, crushing, etc) failures are common. Note that the title of the ASTM standard does not contain the word "shear" - this was very deliberate. The stress state is not pure uniform shear.

2A) SBS data is often used for part design, though many of us argue that it should not be and does not represent the interlaminar capability of an actual part. And also interlaminar failures are usually delaminations which are fracture processes, not stress based strength processes.

3) what do you need to know about SBS tests from filament wound tubes? lots of these tests have been run.

SW

 

[nickc88]

Thanks so much to both of you for the prompt replies.

I think the interlaminar shear exists due to the couple between verticle and horizontal shear...I seem to remember a square FBD with arrows all around it...

Duh! Yes of course its P/2 I really should have drawn the FBD. Thanks!

Yes, we dont really use these values for design. It is though (I hope) a good, fast and inexpensive way of QA testing the quality of cooks and laminates.

3) I guess we are just looking into some of our wound products...For instance since they're usually an ellipse type shape, the major axis walls end up thicker than the corners because the shrink tape is unable to pull as hard on the wall. We want to know what effect this has on the product etc.

a) are SBS samples permited to have any curvature in them? If so how much is too much before you need to go to the Curved Beam setup shown in ASTM2344.

Are there any QA tests you might recommend for filament wound products?
Can you think of a good book that covers the practical testing of composties that might be a good investment for me? I have plenty of theoretical knowledge and info but not as much of a testing background as I would like.

Nick

 

[SWComposites]

Yes, SBS specimens can have some curvature. If you can get shear failures with the roller fixture then use of it is ok. Don't think there is a hard rule on curvature where one has to use the other setup.

QA tests depend on the critical design conditions/loads, material, process - would have to have a lot more info in order to make recommendations.

Mil-Handbook-17 volume 1 has some info on testing.

Also, ASM Handbook 21, Composites has some info.

And the "Comprehensive Composite Materials" volume 5 has a chapter on test methods,

http://www.sciencedirect.com/science/referenceworks/9780080429939#ancs0065

NASA CR-4751 covers tests methods for textile composites.

SW