Abstract

For testing and analysing crack propagation under fatigue loading, crack onset precedes any crack growth. In this work, crack onset during the testing of Double Cantilever Beam (DCB) specimens is studied. The insert cycle experiments provide an experimental reference for our analysis using finite element methods. The insert cycle is analysed using three different methods, which are the Virtual Crack Closure Technique (VCCT), Cohesive Zone Modelling (CZM) and a combined method. In the combined method, the crack onset is modelled using CZM and propagation using VCCT. In other words, we strive to model the crack nucleation phenomenon in the adhesive bond of DCB specimen.

Abstract

The purpose of the present study is the evaluation of the bending-twisting coupling effect in the analysis of the DCB test by means of the analytical determination of the energy release rate of angle-ply laminated composites. The aim is to extend a previous work done by the same authors to new laminate configurations. Angle-ply laminates with symmetrical arms are analyzed, since in this case the application of a bending moment by means of the piano hinges causes a torsional rotation in each of the arms. That torsion is prevented by the hinges, due to the configuration of the test. Therefore, there is a non-uniform load distribution applied by the hinges, whose resultant is the applied load P and the resultant moment is an unknown twisting moment mt, which prevents rotation at the point of application. The resolution of the resulting system of forces and moments allows obtaining analytically the energy release rate, and analyzing the terms due exclusively to the bending-twisting coupling.

Abstract

The accurate determination of interlaminar fracture toughness at cryogenic temperatures is critical for understanding the performance of composite materials in extreme environments, such as those encountered for liquid hydrogen storage applications. However, traditional measurement techniques, such as the Double Cantilever Beam (DCB) test, face significant challenges at cryogenic temperatures. One of the primary difficulties is the inability to directly measure the crack length during the test due to the immersion of specimens in cryogenic fluids, which is necessary to maintain the desired temperature.

In this study, we propose and validate an alternative methodology for estimating the crack length indirectly based on compliance calibration. Using a pre-characterized relationship between specimen compliance and crack length at ambient conditions, we develop a compliance-based approach that eliminates the need for direct crack length optical measurement during the cryogenic test. This method is applied to carbon fiber-reinforced epoxy composites, and its accuracy is evaluated by comparing results obtained at room temperature using both the standard method and the compliance-based approach.

Results demonstrate that the compliance-based method provides reliable fracture toughness measurements at room temperature, with good agreement with the standard approach. Extending the method to cryogenic conditions, we present interlaminar fracture toughness measurements at 77 K.

Our findings establish the compliance-based approach as a robust and practical solution for characterizing fracture toughness in cryogenic environments, overcoming experimental limitations associated with direct crack length measurements. This work contributes to the development of advanced testing protocols and enhances our understanding of composite material performance under cryogenic conditions, paving the way for their reliable use in liquid hydrogen tanks for the next generation of hydrogen-powered aircraft.