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II.1.10.27 Inter-laminar shear criterion

The inter-laminar shear criterion is based on the comparison of inter-laminar shear stress with resin shear allowable:

FIIlss = τinter-laminar τinter-laminarall,

In which the inter-laminar shear stress is a scalar stress calculated as follows:

τinter-laminar = τxz 2 + τyz 2. (II.1.133)

An interface between two plies passes the failure criterion if the following condition is satisfied

τxz2 + τyz2 τinter-laminarall.

The reserve factor is of course:

RFIlss = 1 FoS FIIlss,

and the strength ratio is given by:

SRIlss = FoS FIIlss.

This criterion is different from the other criteria in this that it does not use ply material allowables. Instead, the ply ilss allowable or the laminate ilss allowable is used.

Note also that FeResPost calculates the inter-laminar shear criterion on the lower face of plies only. This criterion is referred to as “Ilss” in FeResPost.

A second version of the inter-laminar shear stress criterion, referred to as “Ilss_b” criterion in FeResPost, is defined in FeResPost. This criterion differs from the more usual “Ilss” criterion by the fact that its calculation is not limited to the lower face of laminate plies. It can also be calculated at mid-ply or ply upper face. This can be handy when one wishes to evaluate the ILSS criterion on finite element results that are extracted at mid ply thickness only. (Actually, the calculation of ILSS failure criterion using stresses extracted from FE results, or calculated from FE shell forces and moments, is the reason that has justified the introduction of this second version of ILSS criterion.)