Dear Ditho,

If I correctly understood, you mean to ask how to measure the maximum normal strength between fiber and interface. In a way its pretty not easy.  However one of the easiest ways to do is the transverse tensile test of a unidirectional laminate. In that case, try to keep the samples' width as less as possible and also assure that the failure is adhesive.

I wouldn't derive the normal strength using formula. Though the interface/phase seems to be isotropic, in reality it is not.

Hope this answers your question. Good luck!

Dear Satis,

In fact, it will give different result. Because when you pull UD laminate transverse, we cannot guarantee whether the failure is due to fiber matrix debonding of due to matrix cracking.

It is well known now that people use microbond test or pull out test to measure the interfacial strength. But most of them report the IFSS. That is why i wonder there must be a simple educated guess on how to convert the IFSS to maximum normal strength.

Thanks by the way for the response.

Dear Ditho,

I accept your view on failure. I think I did mentioned about this on my previous answer. Of course one can derive a formula (with 110% correction factor ;) ). But to verify it one has to however prove it. In this case it is too difficult to do so unless you measure a clean adhesive failure between interface and fiber normal to fiber's axis. As of now the pull-out or push-out gives only IFSS, if performed with extreme care. Because the matrix crack can also appear here which sometimes is extremely difficult to observe.

Good luck!