The localization effect is the first one.

A fracture mechanics study can answer your question very simply. 

Try to do a CCT (centre crack test) and capture the behavior visually and apply the fracture mechanics theory of infinitely high stresses at crack tip. 

Hope it helps.

Best Regards

Nasir

Hello Pouya,

regarding the interpretation of this phenomenon: The problem of delamination occurs when bulk and fibers are not really strongly chemically bound. In the open pores, bulk and bulk are brought together, which may lead to a much stronger binding. We have the same effect when 3D-printing several materials on diverse textiles - in most cases delamination is easy, but as soon as you use an open-pore textile through which the printed layers above and below the textile get in contact, the compound is very strong. The same procedure seems to happen here - the fiber layers are not fixed chemically in the open-pore composite, but mechanically, due to the strong links between upper and lower resin layers in the holes.

For modelling, I would suggest to try - as extremum - modelling the complete epoxy in the whole composite as a whole. However, if you create the composite layer by layer, I assume that the epoxy-epoxy connections in the holes, i.e. between two layers, may be weaker.

Best regards,

  Andrea

hello pouya

You should model the holes by decreasing the mechanical properties of interphase

We have seen a similar phenomena in plasma sprayed coatings. We have simulated a gradual interfacial pollution before deposition, inducing pre-existing cracks. For a low pre existing crack concentration, adhesion is weaker than without pre existing cracks. But for an increased crack concentration, when a crack is initiated, its propagation is stopped by the breaking out in a pre existing crack and thus total delamination does not occure. But it depends on the loading used for delaminate.

Hello,

 i think this is local phenomenon.Enhance the catalytic and wettability properties of fiber surfaces. it is from the strength of the

adhesion is  due to 1.van der Waals’ forces providing a physical adhesion,

                                    2.chemical and micromechanical interaction between a fiber-reinforced material and a composite.

 there is no need of practical set up as such we can go for FEM analysis. Also

The energy required to create a crack is larger than the energy to propagate the crack. The holes force the system to create a new crack; thus if you calculate the work to delaminate it should be higher proportional to the number of times the system has to initiate a new crack over propagating a crack.