Multiaxial fatigue behaviour of composite materials: characterisation and modelling

Thanks to their lightness and versatility combined with excellent mechanical properties, composite materials underwent an increasing relevance in the last twenty years in many industrial fields, also for structural applications. This class of materials offer a wide range of advantages but it is still characterized by a quite high cost with respect to more traditional structural materials. One of the reasons is the lack of reliable design procedures, as well as of extensive experimental investigations in the literature providing clear and general information. This is particularly evident with reference to the behaviour of composite laminates and bonded joints under multiaxial fatigue loading, which often characterise in-service conditions. With the aim to acquire information on the multiaxial fatigue behaviour of unidirectional composites, a specimen configuration was first defined, suitable to characterise the matrix-dominated multiaxial fatigue behaviour of a composite lamina, which is of great importance for the damage evolution in laminates (chapter 2). Tubular specimens subjected to combined tension/torsion loading were identified as the best compromise between reliability of results, easiness of testing and possibility to obtain multiaxial stress states of interest. In the third chapter the results of an extensive experimental investigation on tubular specimens is presented. Tubes were fatigue tested with several values of the biaxiality (shear to transverse) ratio and load ratio (ratio between the minimum and the maximum fatigue loads). A strong influence of both parameters was found on the off-axis crack initiation and propagation phenomena, as well as on the damage mechanisms at the micro-scale. Uniaxial fatigue tests were then carried out on multidirectional flat laminates, designed to achieve local multiaxial stress states similar to those applied to the tubes by means of tension/torsion external loads. This activity, presented in the fourth chapter, revealed the equivalence between external (obtained by applying external loads in different directions) and internal (due to material anisotropy) multiaxial stress states. This represents a fundamental step for the extension of experimental results and predictive models to general loading conditions. The experimental activity on tubes provided information on the damage mechanisms at the micro-scale, responsible for fatigue failure of a unidirectional lamina. On the basis of these mechanisms a criterion for predicting crack initiation in a unidirectional lamina under multiaxial fatigue was developed by means of a multiscale approach (chapter 5). This criterion resulted in sound agreement with the new data on tubes, with data on flat unidirectional laminates from the literature and with crack initiation data on the off-axis layers of the laminates reported in chapter 4. A basic topic related to the analysis of multidirectional laminates is the stiffness degradation due to off-axis cracks in their plies. Dealing with such a topic, an analytical model was developed for predicting the stiffness of a laminate as a function of the crack density in its layers accounting for the interaction between cracks in different layers (chapter 6). In addition the model is capable of calculating the stress re-distribution due to the presence of cracks. This is fundamental for the development of a new procedure for predicting the fatigue crack density evolution in multidirectional laminates, presented in chapter 7. To this aim the experimental observations, the analytical models and criteria previously presented, combined with a statistical approach, have been used to predict the initiation and propagation of multiple cracks in a laminate. As a consequence, when this procedure is combined with the model presented in chapter 6, both the stiffness degradation and the stress re-distribution, useful for the estimation of the total fatigue life, can be predicted. When composite laminates are used as adherends in bonded joints the bonding surface represents a critical position for the onset of fatigue cracks. As a consequence an experimental investigation on the propagation of a bondline crack in composite bonded joints subjected to mixed mode I + II (opening + sliding) fatigue loading was carried out and presented in chapter 8. A criterion to predict the crack propagation rate under mixed mode loading was also developed, based on the damage mechanisms observed during the experimental campaign. Eventually, in Appendix A an analytical model for predicting the initiation of a fibre-matrix debond crack under biaxial static loads is presented. The model provides useful information on the influence of the main geometrical and interface parameters of the fibre-matrix interface strength