# Mode I and II Interlaminar Fracture in Laminated Composites: A Size Effect Study

## TL;DR

This study examines how the size of laminated composite specimens influences their interlaminar fracture behavior under Mode I and II loading, highlighting the importance of considering nonlinear effects of the Fracture Process Zone for accurate fracture energy estimation.

## Contribution

It provides a detailed analysis of size effects on interlaminar fracture in composites, demonstrating the applicability of Bažant's Size Effect Law to both Mode I and II fractures.

## Key findings

- Large specimens behave more brittle, aligning with LEFM predictions.
- Small specimens show pseudo-ductility and deviate from LEFM.
- Neglecting FPZ effects can underestimate fracture energy by up to 55.

## Abstract

This work investigates the mode I and II interlaminar fracturing behavior of laminated composites and the related size effects. Fracture tests on geometrically scaled Double Cantilever Beam (DCB) and End Notch Flexure (ENF) specimens were conducted to understand the nonlinear effects of the cohesive stresses in the Fracture Process Zone (FPZ). The results show a significant difference between the mode I and mode II fracturing behaviors. It is shown that, while the strength of the DCB specimens scales according to the Linear Elastic Fracture Mechanics (LEFM), this is not the case for the ENF specimens. Small specimens exhibit a pronounced pseudo-ductility with limited size effect and a significant deviation from LEFM, whereas larger specimens behave in a more brittle way, with the size effect on nominal strength closer to that predicted by LEFM. This behavior, due to the significant size of the Fracture Process Zone (FPZ) compared to the specimen size, needs to be taken into serious consideration. It is shown that, for the specimen sizes investigated in this work, neglecting the non-linear effects of the FPZ can lead to an underestimation of the fracture energy by as much as 55%, with an error decreasing for increasing specimen sizes. Both the mode I and II test data can be captured very accurately by Ba\v{z}ant's type II Size Effect Law (SEL).