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The onset and growth of interlaminar cracks (delamination) were experimentally studied in an IM7/MTM45 carbon-epoxy composite material under mixed mode I/II dynamic loading, with a mixity grade GII/G of 0.24, employing for this purpose an asymmetric specimen. Static tests were previously carried out to obtain the critical fracture properties of the material. The dynamic test strategy was defined using these data. This consisted in the application of different fixed percentages of the critical ERR, Gcr. The results enabled the determination of crack onset fatigue curves, ΔG -N (number of cycles before crack onset, for a given ERR). A fatigue limit value of 38% Gcr has been obtained, understanding as such the ERR necessary for the onset of delamination. Delamination growth rates were shown to fit a Paris law trend with average crack growth rates of 10−4 mm/cycle. The fractographic analysis carried out on the tested specimens confirmed the presence of mode I and II fracture typologies, as well as evidencing the progression of micro-cracks due to fatigue.

This work comprises a study of the reinforcement capacity provided by the addition of different types of nano-reinforcements of graphene oxide (GO) to epoxy matrices. A range of nanocomposites, res...

Composite materials manufactured by overlapping plies with certain specific geometries are likely to lose part of their strength due to the presence of internally delaminated regions. The aim of this paper is to experimentally evaluate the generation and propagation of these interlaminar cracks in a carbon-epoxy composite material subjected to fatigue loading under mixed mode I/II fracture. Two different test methods were used for this purpose: The standardized mixed-mode bending (MMB) test and the asymmetric double cantilever beam (ADCB) test, with the goal of exploring the viability of the ADCB test as a simpler alternative to perform than the MMB test, especially in fatigue testing. With this aim in mind and after prior static characterization of the material in which the critical values of the energy release rate were determined under both test methods, the levels of the energy release rate to be applied in fatigue tests were defined for two mode mixity ratios, GII/Gc = 0.2 and 0.4 (0.34 ADCB), and a fatigue loading ratio, R = Gmin/Gmax = 0.1. The G-N fatigue onset curves were subsequently obtained from these experimental data. The most relevant result of the study is that the fatigue limits obtained using the MMB method are generally more conservative than those obtained via the ADCB method.

In this paper, the phenomenon of delamination under static and dynamic loading of a composite made of an epoxy matrix and carbon fiber reinforcement has been studied, analyzing its fracture behavior under mixed mode I/II loading employing an asymmetric double cantilever beam test. Under static loading, some of the most representative formulations for calculating the energy release rate were analyzed, finding a good agreement between the results obtained by means of the different formulations. Under dynamic loading, the number of cycles necessary for the crack onset was determined (determination of ΔG − N fatigue curves and number of cycles necessary for the delamination onset for a given energy release rate). As regards the experimental results, apparent fatigue limits of the order of 38% of the critical fracture energy were obtained for an asymmetry coefficient of 0.1. Subsequent statistical analysis of the results enabled the fatigue limit to be defined more accurately. This was found to be 15% for this material, indicating the need to use these tools for the actual determination of the infinite fatigue life of the material. Finally, an optical study of the fracture surfaces was carried out which confirmed the presence of mixed mode fracture typologies.

This paper describes a method for characterizing composite materials subjected to mode III delamination fracture using a custom-designed testing device and test equipment which allows loads or displacements to be applied to the test specimen in two directions, one axial and the other torsional. To verify the method’s functionality experimentally, a composite material made up of an epoxy matrix and unidirectional carbon-fiber reinforcement was used in conjunction with an image analysis device for the purpose of determining the displacement field in the crack front of a double cantilever beam test specimen. According to the results, this test method permits almost pure mode III fracture tests to be carried out, as the mode II component is practically negligible. Another feature of the method is the improvement in the quality and ease of inserting the specimen in the device, thus permitting more repetitive results to be obtained with less dispersion.

The delamination phenomenon is one of the major failure mechanisms in composite materials formed by the stacking of successive layers. The fracture mode that has been studied the least within this phenomenon is mode III, due to the difficulty its simulation involves. A novel test device is presented in this paper that allows laminated composites to be subjected to this mode of loading. Characterization tests were carried out on two materials with different matrices in order to analyse the influence of the type of matrix in the fracture behaviour under mode III-fracture. Furthermore, two analytical models adapted to this test methodology have been used for the determination of the value of the energy release rate for this mode of loading in both materials. The load was applied at different distances from the front of the notch, this being another parameter that was analyzed. From the results obtained it follows that the device used in this work allows a dominant mode III and that this result holds for different points of load application studied.