Why the thickness direction matters
Many laminated composite systems derive most reinforcement from in-plane fibres. Interlaminar load transfer depends substantially on the matrix and interfaces between layers. Geometry transitions, free edges, impact damage and out-of-plane loading can therefore require special attention.
Adding fibres through thickness can change crack growth and load transfer, but the result depends on architecture, material, density and process quality. It is more accurate to say that a route is designed to improve thickness linkage than to claim that delamination is eliminated.
Three different routes
Stitching
Stitching inserts a defined thread through a layered reinforcement or preform. It can improve layer binding but may introduce local fibre distortion, needle damage or resin-rich zones depending on process and material.
Needling / 2.5D
Needling uses barbed needles to carry fibre segments between layers. It is valuable for thick felts, carbon-fibre precursors, insulation structures and shaped forms. The Z linkage is entangled and discontinuous rather than a deliberately woven continuous yarn path.
3D orthogonal weaving
An orthogonal architecture plans X, Y and Z yarn systems during textile formation. This can integrate thickness reinforcement with the section itself, while also requiring specialised machinery and control of crimp, density, compaction and downstream processing.
How to select a route
Selection should consider fibre type, architecture, preform thickness, geometry, permeability, achievable fibre volume, damage tolerance, cost, scale and qualification route. A useful development programme compares specimens made with controlled material and process conditions.
NASA impact studies illustrate that results vary with textile architecture; in one comparison, the tested stitched materials performed better than the tested 2D braids and 3D weaves in some measures. See NASA CR-198265.