Experimental and Numerical Analysis of Mode III Interlaminar Fracture in Composite Laminates using a modified Edge Crack Torsion Test

This research aims to contribute to the standardization of interlaminar fracture toughness tests for advanced composite laminates under mode III conditions. To this end, a modified Edge Crack Torsion (ECT) has been designed and evaluated through numerical simulations, using the Finite Element Method (FEM) in Abaqus software. The study focuses on assessing the resistance of the specimen to applied loads and quantifying the Energy Release Rate (ERR) in this particular fracture mode. To achieve this, the Virtual Crack Closure Technique (VCCT) was implemented in all numerical simulations to calculate the ERR at the onset of crack propagation along the interlaminar surfaces. To validate the adopted modeling in different tests, additional simulations were conducted for mode I (Double Cantilever Beam, DCB), mode II (End Notch Flexure, ENF), and Mixed Mode Bending (MMB, mode I and mode II dominant cases). The obtained numerical results were then compared with available experimental data from previous studies. In the mode III simulations, specimens with various initial crack lengths along their interface were modelled. While the ECT test is commonly used to determine the ERR in mode III fractures, no standardized procedure currently exists. In order to contribute to the mode III standardization, new characterizations are employed. To optimize material usage in laboratory samples and achieve a purer mode III ERR, width reduction was implemented in the final simulation models. Additionally, a Quasi-Trivial (QT) stacking sequence was independently applied to the ECT specimens to observe its effect on fracture behaviour. From an experimental perspective, a revised ECT fixture was designed based on previous studies using CAD/CAM software (CATIA). In addition, this fixture was modified in order to facilitate the application of the compliance calibration method using various specimens' widths. Overall, the numerical results indicate that specimens with shorter initial crack lengths exhibit a purer mode III ERR compared to those with longer initial cracks. However, specimens with longer initial cracks demonstrate a more uniform mode III ERR distribution along the crack front, which may be advantageous for data reduction. Lastly, the results obtained with QT stacking sequences show that ECT tests can be performed to assess the interlaminar fracture toughness of interfaces with different orientation angles without introducing spurious effects due to unwanted elastic couplings.