Compositepro, thank you for your reply.
I was reviewing a composite design allowables document and came up with F33t, F33c & F13 values for “Unfolding”. Are these values then the max allowed for interlaminar tension, compression & shear respectively to prevent “delamination”?
The F33t, F33c & F13 values might cover for delamination depending on how they were determined and from what type of test and whether the test included intentional flaws in the peak stress area. Ideally the test specimen would be very similar to the structural detail you are analyzing. If it is not, then the values very likely do not provide a good prediction of delamination onset. Fracture mechanics based methods are much more accurate for delamination prediction compared to stress based methods
What was the 'composite design allowables' document? A bit more context might make a difference.
I also have not come across 'unfolding' in any sort of technical sense.
F33t, F33c are the through thickness direct stress directions, F13 is through thickness shear. The 33 and 13 nomenclature probably indicates that the 'F's are 'material' values, so that they are for a laminate with all its plies at 0 degrees (Z and X would usually indicate 'laminate' directions as opposed to material).
F33c is somewhat controversial. If it's for unidirectional plies the failure mechanism is by what I've heard some people call 'log-rolling,' where the fibres can roll over one another as the resin fails. A real laminate (or one made of fabric) is quite a lot stronger than F33c for a UD.
As SW says, F33t is also somewhat controversial. If it's for a laminate failing in 33 tension up due to a corner radius opening up then there is a quarter corner bend test standard. However, it's quite variable in its values and when damage is introduced *very* variable.
From the test data that I have seen, F33t is all over the place, even without flaws. That being said, what are your thoughts on the Chang-Springer interaction? With the interlaminar shear playing a relatively small role (generally), can you confidently determine an interaction between interlaminar shear and tension (when the tension allowable varies considerably)? I suppose it is a relatively conservative form so it should be OK. If nothing else, it is a typical type of interaction, but I just wonder if the physics really hold up and if you can really prove it.
I will have another go at the paper by Chang and Springer, but I wonder if they addressed some of the papers which reported significant scatter. This is largely an academic question I suppose, but interesting to me nonetheless.
Google gives no hits for "Chang-Springer interaction"! (It goes on to give non-useful hits for chang springer interaction without the quotes.) I have no record of it in any of my own references. Is this simultaneous through-thickness compression with interlaminar shear? I have some stuff on that but not by a Chang or Springer. What's their interaction?
The C-S failure criteria is a simple elliptical interaction between interlaminar tension and interlaminar shear. Was developed for predicting the strength of a curve laminate in bending. Published in a paper in the late '80's IIRC. If I recall where I put a copy I'll post the reference.
It works ok only if the "allowable" F33 and F13 values are backed out of curved laminate test data using the criteria and the allowable values are only used in analysis of similar structure (same thickness, radius, etc). And the curved laminate test specimens should have intentional flaws (delam's) in the radius to avoid high scatter and unconservative results.
It is also used in an ESDU (don't have the number off hand). If memory serves me correctly, the original paper by Chang-Springer assumed that F22t = F33t, which seems to be a poor assumption even without a flaw (and a very poor assumption as an engineering solution). Actual implementations don't assume this, as you have stated.
I suppose it was a guess at an interaction, like many other failure criteria, but there is nothing particularly special about the physics of it. It seems most of these sorts of failure criteria are just guesses and then they try to prove them later. Some of them just so happen to fit test data better than others and hence become used more often.
What I have wondered is if just using the interlaminar tension component would be good enough (neglecting the interlaminar shear). If the F33 allowable is then backed out using that assumption, I suspect it would work pretty well (i.e. non interactive failure criterion).
Thanks SW and ESP. My personal interest was in how the shear strength is increased by through-thickness compression. Oddly my current work may well include an aspect of how through-thickness tension and shear can conspire to cause early failure, so it's relevant!
To the OP gkstress: still quite curious as to what 'unfolding' may be.
I am debating whether or not to just address a pure moment or include the shear component. I don't think the shear component is a very big driver, but it makes solving the stress state more challenging. Anyway, if you have any other good info, let me know.
Thank you all for your feedback.
From my understanding the term 'unfolding'(not frequently used) describes composite curved angle delamination onset as a result of through thickness stresses. I am currently researching in order to get a better understanding of this phenomenon.
ESPComposites thanks for the software link, very useful tools.
