A Puck-informed mode-resolved phase-field fatigue framework for unidirectional composites

Fatigue fracture in unidirectional fibre-reinforced composites is strongly mode dependent: transverse and off-axis cycling is governed by matrix and inter-fibre mechanisms, whereas fibre-aligned cycling activates a longitudinal channel with a higher fracture-energy scale and a different crack topology.

Single-damage-variable models can fit global stiffness loss but cannot identify the active mechanism.

This work proposes a Puck-informed, mode-resolved phase-field fatigue framework with separate channels for fibre-dominated and matrix/inter-fibre fatigue.

Each channel has its own fatigue history, threshold, and resistance-degradation law.

Fatigue does not directly degrade elastic stiffness; it lowers the fracture resistance of the active channel, while the corresponding phase field controls stiffness loss and crack-path evolution.

The formulation is implemented in Abaqus/Standard using a compact UMAT-UEL architecture with one orthotropic mechanical routine and two scalar phase-field layers.

Using one fixed IM7/8552 material and fatigue card, the model is verified through one-element tests, parameter sweeps, and centred-notch and open-hole tension cases at 0, 45, and 90 degrees under monotonic and cyclic loading.

Without orientation- or geometry-specific tuning, the framework reproduces transverse matrix/inter-fibre cracking at 90 degrees, off-axis cracking at 45 degrees, and longitudinal matrix splitting with delayed fibre activation at 0 degrees.

The fatigue lives follow the expected ordering: 45- and 90-degree cases fail within about 1,000 cycles, while 0-degree cases run out to 200,000 cycles without fibre cracking.

Additional load, hole-size, mesh, length-scale, and cycle-block studies confirm consistent crack modes and converged trends.

The study is a numerical verification and cross-geometry consistency assessment, not a calibrated experimental life-prediction claim.