The Gap Test: Effects of Crack Parallel Compression on Fracture in Carbon Fiber Composites

TL;DR

This study demonstrates that crack parallel compression significantly reduces fracture energy in carbon fiber composites, revealing a weakening effect that challenges the traditional view of fracture energy as a constant property.

Contribution

It introduces the first experimental evidence that crack parallel compression weakens composites and develops a tensorial damage model for accurate fracture prediction.

Findings

Fracture energy decreases up to 37% with increased crack parallel compression.

Crack tip splitting is induced by parallel compression, weakening the structure.

Tensorial damage models are essential for accurate fracture predictions.

Abstract

This paper explores the global Mode I fracture energy of a carbon fiber composite subject to a biaxial stress state at a crack tip, specifically in which one stress component is compressive and parallel to the crack. Based on an experimental technique previously coined as The Gap Test and Bazant's Type II Size Effect Law, it is found that there is a monotonic decrease in the Mode I fracture energy as the crack parallel compressive stress increases. Compared to the nominal value of fracture energy, where no crack parallel compression is applied, the fracture energy is observed to decrease by up to 37% for a compressive stress equal to 44% of the compressive failure limit of the composite. This weakening effect is attributed to splitting cracks that are induced at the crack tip due to the crack parallel compression, which are identified via crack tip photomicroscopy. This is a novel result that challenges the century old hypothesis of fracture energy being a constant material property and further, shows for the first time that crack parallel compression leads to a composite structure being dangerously weaker than expected. The experimental campaign is also buttressed with a computational campaign that provides a framework capable of capturing the effects of crack parallel compression. Through the use of the crack band model, which correctly characterizes the fracture process zone tensorially, coupled with a fully tensorial damage law, the simulated results provide satisfactory agreement with the experimental data. Conversely, when a reduced tensorial damage law defines the crack band it is shown that the structural strength and fracture energy are dangerously overpredicted. This emphasizes the importance of using a crack band model coupled with a fully tensorial damage law to accurately predict fracture in composites.