Fatigue damage analysis of composite materials using thermography-based techniques

Composite materials are nowadays used in many fields of industry, especially for producing large structural components in many applications ranging from naval to aerospace. Beside to the capability and versatility of uses, the study of damage in composites is not easy due to the different failure mechanisms that can occur simultaneously or in different conditions. The characterisation of composites represents then a critical stage of assessing mechanical properties and resistance and a careful attention has to be put in the study and damage analysis. Due to this, the fatigue performances imposed by Standards have to be verified by means of experimental techniques involving experimental campaign in laboratory on samples or directly on large components. However, classical procedures for evaluating the fatigue resistance of materials present two issues: the expensive and time-consuming tests because of the high number of specimens being tested, and the totally absence of information on occurring damage. In the last few years, great efforts have been made to develop a number of methods aimed at reducing testing time and, subsequently, the cost of the experimental campaign. Among the different techniques, for instance, thermographic methods are considered as a useful tool for the rapid evaluation of fatigue damage and fatigue resistance at specific cycles number (endurance limit). The capability of thermography, is not only, related to the experimental procedure providing specific tests capable of assessing fatigue resistance in accelerated way, but also to study the energy involved in the fatigue processes. As previously said, damage mechanisms in composite materials are difficult to be understood and even a small scale anomaly can lead the failure of the material without visible damage or visible signs of the onset of failure phenomena. For this reason, energy intrinsically dissipated can be another point of view to face up to a sudden failure. In this way, energy-related parameters assessed by the analysis of thermographic signal can be useful for assessing information related to the onset of irreversible damage. The focus of the present research is to provide an innovative method for process thermal signal from innovative composites obtained by Automated Fiber Placement process in order to understand the fatigue behaviour qualitatively and quantitatively.