Infrared thermography to monitoring mechanical tests on composite materials: Experimental procedure and data analysis

The success of a new composite material depends strongly on its ability to fulfil the users requirements. This can be achieved through proper conduct of the three phases: design, fabrication and validation. To get the best, the design phase needs availability of information about the material's properties and its intended in-service performance; in particular, understanding the impact damaging of composites represents one of the major design concerns especially for the aerospace sector. In this context, the Ph.D's activities are carried out with the intention to help broaden the exploitation of infrared thermography (IRT) to either get information useful for the designer, or to validate the performance of a new material. Thus, the attention is devised towards monitoring of cyclic bending and impact tests on different types of composites involving either a thermoset, or a thermoplastic matrix and different types of fibres as reinforcement. This involves visualization of temperature variations, which are coupled with either thermo-elastic, or thermoplastic effects and which may be very small, at the edge of the instrument sensitivity, therefore strongly affected by the instrument temporal random noise. A solution is proposed, which involves the use of an unloaded reference specimen and implementation of a correction Reference Area Method; this method is validated through specific tests. As a main finding, this method is effective to restore the harmonic trend coupled with cyclic bending tests, which, at low bending frequency, is completely disrupted by the temporal noise random jumps. Cyclic bending tests are carried out with a prototype machine designed and realized during the PhD course. The Reference Area Method is also used coupled with a post-processing procedure specifically developed to evaluate the extension of the impact damage. The thermal images are recorded during impact tests with a modified Charpy pendulum, specifically to detect and outline the extension of the thin delamination at the boundary of the sound material. It is also demonstrated that information, such as deformation of the surface and duration of the impactor pushing force, as well as initiation of the impact damage, can be derived which are mostly important to assess the impact resistance of a new composite material.