first-order zig-zag theory (FZZT),
[ref](https://www.sciencedirect.com/science/article/abs/pii/S026382239900063X)

allowing for discontinuities in displacement between layers.
 captures the "zig-zag" pattern of displacements through the thickness that occurs due to different layer stiffness.
 
The first-order zig-zag theory introduces additional terms to the displacement field assumptions used in classical laminate theory:

1. It adds a zig-zag function that allows for slope discontinuities at layer interfaces.
2. It maintains continuity of transverse stresses between layers.
3. It satisfies the traction-free boundary conditions on the top and bottom surfaces.