The attention of the present paper is focused on the use of an infrared imaging device to monitor the thermal response of composite materials under cyclic bending. Three types of composites are considered including an epoxy matrix reinforced with either carbon fibres (CFRP) or glass fibres (GFRP) and a hybrid composite involving glass fibres and aluminium layers (FRML). The specimen surface, under bending, displays temperature variations pursuing the load variations with cooling down under tension and warming up under compression; such temperature variations are in agreement with the bending moment. It has been observed that the amplitude of temperature variations over the specimen surface depends on the material characteristics. In particular, the presence of a defect inside the material affects the temperature distribution with deviation from the usual bending moment trend. 1. Introduction Infrared thermography (IRT) has proved helpfulness in many industrial and research fields as stated by the proceedings of the four main international symposia [1–4]. Amongst its many applications, an infrared imaging device is helpful for thermoelastic stress analysis (TSA) purposes [5, 6] and to monitor the surface temperature change (thermoelastic effect) which is experienced by a body when subjected to stress variations under load [7]. The thermoelastic effect was first conceived by Lord Kelvin (Thomson) in 1978 [8]. Many years later, in 1956 [9], Biot performed a thermodynamic analysis and formulated the classical thermoelastic equation, which expresses the change in temperature of a solid in terms of the change in the sum of the principal stresses . The temperature variation, under reversible and adiabatic conditions (i.e., in the elastic regime and neglecting heat transfer within the body and to the environment), for isotropic materials can be written as where is the absolute body temperature, is the mean stress amplitude, and is the material thermoelastic constant. Equation (1) relates the temperature local variations to the stress variations. In particular, under adiabatic conditions, positive dilatation (tension) entails cooling of the material and vice versa. In metals, the thermoelastic limit is generally assumed [10] as an indication for the yielding point. In orthotropic materials as fibre-reinforced polymers (FRP) (1) is modified as with and being the thermal expansion coefficients along the principal material directions and the mean volumetric heat capacity. For complex composite materials a direct relationship between the mean stress and TSA data is
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