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F.A. Díaz et alii, Frattura ed Integrità Strutturale, 25 (2013) 109-116; DOI: 10.3221/IGF-ESIS.25.16 110 method provides a direct measurement of the effective Δ K that is usually measured indirectly by compliance techniques. This arises from the fact that with TSA the near crack tip stress distribution is obtained from the temperature variations at the specimen surface as a result of the thermoelastic effect, providing a direct assessment of the cyclic strains in the field around the crack tip. Hence, the observed crack tip stress pattern is the result of the specimen’s response to the applied loading cycle. To support the idea that TSA can provide accurate information about the real fatigue crack driving force, a set experiments using aluminium 2024 CT specimens have been conducted. As a result, Δ K results obtained using thermoelastic images have been employed to infer an equivalent value for the opening/closing load for increasing R - ratios. Results have been compared with those obtained using the strain offset technique, showing in all cases a good level of agreement, highlighting the value of TSA for fatigue damage assessment. P HYSICAL PRINCIPLES OF THERMOELASTIC EFFECT he thermoelastic effect was first reported by Lord Kelvin [4] in 1853. It states that any substance in nature experiences changes in its temperature when its volume is changed due to the application of a force: compressive loads cause an increase in temperature while tensile load produces a decrease in temperature. Consequently, if a cyclic load is applied to a component there will be a cyclic change in temperature. Under elastic conditions, these temperature variations are normally quite small (tens of mK) and they are normally ignored in the classical theory of elasticity. However, with the use of high precision infrared detectors these temperature changes can be measured. The thermoelastic effect is a reversible conversion between the mechanical and thermal forms of energy, since the temperature variation will reverse when the load is withdrawn. However, this energy conversion is reversible only if the elastic range of the material is not exceeded and there is no significant transport of heat during loading and unloading of the structure. Moreover, thermoelastic theory states that under adiabatic and reversible conditions, the temperature variations experienced by the cyclically loaded material are proportional to the sum of principal stresses. The relation between the change in temperature due to the application of a cyclic loading and the stress range of a linear elastic and homogeneous material can be written as:   1 2 p T T C          (1) Where  is the coefficient of thermal expansion, T is the absolute temperature of the material,  is the density, C p is the specific heat at constant pressure and  1 and  2 are the principal stresses. To ensure that the experimentally recorded variation is linear, the load cycle must be fast enough to prevent heat transport and thus achieve adiabatic conditions. Truly adiabatic conditions may be achieved only if the thermal conductivity of the material is zero or no stress gradients are present in the specimen. However, if the load frequency is high enough the thermal diffusion length is reduced and the presence of non-adiabatic effects is minimized. In modern TSA, an infrared camera based on a staring array of photon detectors is used to measure the temperature changes at the surface of the component as a consequence of an applied cyclic load (figure 1A). The technique measures load-correlated temperature signals in a cyclically loaded body using infrared detectors. Thus, the analysis of thermoelastic response has to be done under dynamic conditions at an adequate frequency to ensure adiabatic conditions in order to prevent heat transfer through the test piece. When adiabatic conditions are achieved and maintained during the test, the relation between the induced temperature change and the change in the sum of principal stresses is assumed to be linear, and thus the variation in the sum of principal stresses can be experimentally inferred by processing the thermoelastic signal according to the following equation:       1 2 x y x y E A S 1                  (2) Where, E, is the Young’s modulus, ν , is the Poisson’s ratio, ε x and ε y , are the strains in two orthogonal directions, S , is the thermoelastic signal and A is a calibration constant. To translate the thermal units into stresses a calibration process needs to be performed [5]. This process consists essentially in defining the stress at a point in the images for a given load on the structure. A common method used for calibration consists of generating an independent measure of stress using strain gauges as illustrated in figure 1.B. T