AFP composites

Mitigating out-of-plane fiber waviness in AFP laminates with tow-gaps

Selective PEI thermoplastic veil placement was used to control tow-gap morphology, reduce ply sinking, and stabilize damage evolution in cross-ply composite laminates.

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Thermoplastic veil placement within fiber tow-gaps and interlaminar regions — Tow-gap and veil placement concept for cross-ply laminates with a staggered-gap configuration.

Publication Fibers 2025, 13(11):145

DOI 10.3390/fib13110145

Methods Micro-CT, microscopy, DIC, PFA

Material system HexPly 8552 with PEI veils

Overview

Turning a tow-gap defect into a controlled morphology.

Fiber tow-gaps and overlaps from automated fiber placement can create resin-rich regions and local fiber waviness. During consolidation, deposited plies sink into gap regions, changing both external surface profile and internal load paths.

This study placed high-melting-temperature PEI thermoplastic veils directly into the tow-gaps. The veils preserved a more uniform laminate geometry and formed an interpenetrated network with the epoxy matrix, reducing resin-rich pockets while modifying failure behavior.

Main findings

Measured changes in morphology and response.

Surface waviness 0.47 mm to 0.23 mm

Peak-to-valley height variation was roughly halved by veil placement.

Resin-rich pockets 250-400 µm to below 50 µm

Microscopy and micro-CT showed smaller, less continuous resin-rich regions.

x-direction strength ~1300 MPa to ~1476 MPa

VEILx specimens recovered much of the strength lost in gap-only specimens.

x-direction stiffness ~81 GPa to ~100 GPa

Reduced waviness in load-bearing plies improved axial stiffness recovery.

Evidence

Figures extracted from the paper.

Surface profilometry height maps and profiles for staggered gaps with and without veils — Surface profilometry comparing staggered gaps with and without PEI veils.

Micro-CT fiber misalignment angle measurements with and without veil placement — Micro-CT based out-of-plane fiber misalignment measurements.

Box plot of maximum stress for pristine gap and veil samples — Tensile strength comparison across pristine, gap, and veil configurations.

Experimental and finite element strain distributions with matrix damage — DIC and progressive failure analysis used to compare strain and damage evolution.

Workflow

Experiment and simulation tied to the same defect geometry.

Fabrication: Cross-ply laminates were built with controlled staggered tow-gaps and selective PEI veil insertion.
Characterization: Surface profilometry, optical microscopy, and micro-CT quantified waviness, resin-rich regions, and internal morphology.
Testing: Quasi-static tensile tests and full-field DIC captured stiffness, strength, and strain localization.
Modeling: Compaction-driven geometry was passed into progressive failure analysis to study matrix damage and delamination.

Takeaway

Veils work best as morphology-control scaffolds.

The strength recovery was strongest when veil placement suppressed waviness in the load-bearing plies. In the transverse direction, veils changed damage propagation but did not replace missing 90° reinforcement. This makes the method most useful when veil placement is aligned with critical load paths in AFP structures.

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