I assume you are using solid elements to mesh the two materials.
Let's say you have a material called fiber, which has a rod shape that exists in a material called epoxy.
A model of the composite would have a solid block of epoxy with rod-shaped holes in it, and rods would fill those rod-shaped holes with fiber material.
For that model to have no crack, you would use shared topology to cause the mesh of one material to be connected by shared nodes to the next material.
But maybe you have a material called coating on the surface of a material called substrate.
You do the same thing with shared topology to make a composite material with no cracks.
What you need to do to introduce a crack at the interface is to split the faces on each material at the interface. When you do shared topology, you exclude the faces where you want the crack and include the faces where you don't want a crack.
If you want more detailed instructions, you will need to provide a far more detailed description of your geometry with a lot of images inserted into your reply to show what you are trying to do.