# Compressive Failure Mechanisms of NCF Laminates with Double-Hole Defects

## TL;DR

This study examines how hole spacing and orientation affect the compressive strength of NCF laminates, finding that increasing spacing beyond a threshold provides diminishing returns.

## Contribution

The novel contribution is identifying a spacing threshold for compressive strength and validating a combined intralaminar/interlaminar failure model in NCF laminates.

## Key findings

- OHC strength changes minimally when spacing exceeds s/D ≥ 3.
- Transverse alignment is weakest at s/D = 2, with reduced directional sensitivity at larger spacing.
- Apparent elastic stiffness remains nearly constant across all configurations.

## Abstract

What are the main findings?
OHC strength shows a spacing threshold: changes beyond s/D ≥ 3 are small.At s/D = 2, transverse alignment is the weakest; direction sensitivity diminishes at larger spacing.Apparent elastic stiffness stays nearly constant across all nine configurations (≤~1% variation).

OHC strength shows a spacing threshold: changes beyond s/D ≥ 3 are small.

At s/D = 2, transverse alignment is the weakest; direction sensitivity diminishes at larger spacing.

Apparent elastic stiffness stays nearly constant across all nine configurations (≤~1% variation).

What are the implication of the main findings?
Design-wise, increasing spacing from s/D = 2 → 3 is the key gain; further spacing gives diminishing returns.NCF stitching/drilling defects can promote ligament damage coalescence at small spacing, so narrow ligaments should be avoided.The FE framework (3D Hashin + shear-coupled matrix compression + cohesive delamination) reproduces response and failure mechanisms.

Design-wise, increasing spacing from s/D = 2 → 3 is the key gain; further spacing gives diminishing returns.

NCF stitching/drilling defects can promote ligament damage coalescence at small spacing, so narrow ligaments should be avoided.

The FE framework (3D Hashin + shear-coupled matrix compression + cohesive delamination) reproduces response and failure mechanisms.

Open-hole compression (OHC) tests were carried out on non-crimp fabric (NCF) laminates with varied open-hole orientation (angle to the loading direction) and inter-hole spacing. Failure modes were documented by scanning electron microscopy (SEM), and the compressive strength was quantified. Finite element simulations in Abaqus were developed to replicate the tests, establishing a progressive-damage model for open-hole laminates under compression. Intralaminar failure was described using the three-dimensional Hashin failure criterion and a modified matrix compression criterion incorporating shear coupling effects, while interlaminar delamination was modeled with cohesive elements, enabling the simulation of damage initiation, growth, delamination, and final collapse. The results show that hole orientation and spacing have a pronounced effect on open-hole compression (OHC) strength. A spacing threshold is observed, beyond which further increases in spacing provide little additional benefit. In contrast, the apparent elastic stiffness is essentially insensitive to hole spacing and orientation. The combined intralaminar and interlaminar model successfully reproduces the characteristic mechanical response—linear elasticity followed by catastrophic failure—in good agreement with the experiments.