Abstract

The evaluation of the quality of composite-composite bonded joints is a major problem for the aeronautical sector given by the fact that a defective joint not only could paralyze the productive process, but it could also involve very high restoration costs. Currently, the quality of the bond is quantified by an interlaminar fracture toughness test. This test measures GIC in a test piece that is bonded theoretically under the same conditions as the actual piece. The measurement of GIC is performed by the DCB test or by the Drum Peel test. Both tests cannot be carried out on the actual part or "in situ". The present article shows a device that carries out the peeling test on the real part allowing to obtain results comparable to those that could be obtained with the traditional procedures of DCB and Drum Peel. In order to verify the validity of the new device that has been designed, an experimental campaign has been carried out to test non-symmetrical composite-composite laminates with different techniques, by comparing the different GIC values obtained. A numerical study carried out in another paper confirms that the evaluation of GIC obtained with the experimental results is adequate.

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.