Issue 39

A. Risitano et alii, Frattura ed Integrità Strutturale, 39 (2017) 202-215; DOI: 10.3221/IGF-ESIS.39.20 203 I NTRODUCTION n the last thirty years, the methodologies adopted to estimate conventional fatigue limit for steel ductile materials has implemented with infrared sensors. Indeed, the surface temperature of the specimen has been correlated to stress applied during the fatigue test. In 1986 [1], Curti et al. were the first researchers that developed and used thermography to explore the thermal map over the surface of a specimen and thereby determine the fatigue limit. In 1988 [2], Luong illustrated three advantages of infrared thermography technique: observation of the physical processes of damage and failure in metals; detection of the occurrence of intrinsic dissipation; evaluation of the fatigue strength in a very short time. In 2000 [3], La Rosa et al. presented a new methodology (Risitano Method) for the determination of the conventional fatigue limit of materials or mechanical components. In 2002 [4], Fargione et al. presented a procedure for the definition of the whole fatigue curve by the thermographic method. In 2005 [5], Curà et al. presented a new thermographic method based on an iteration procedure for the determination of the fatigue limit of materials and components. The thermographic results have been compared to the corresponding obtained by means of a different thermographic method [3]. In 2007 [6], local 1D and 2D expressions of the heat diffusion equation were used to separately estimate the coupling and dissipative sources accompanying the fatigue test of an aluminium alloy. In 2007 [7], Meneghetti defined a theoretical model in order to derive the specific heat loss per cycle from temperature measurements performed during the fatigue test. In 2008 [8], Crupi developed a theoretical model, introducing new relationships able to correlate the damping with other mechanical properties defining the different failure modes. In 2010 [9], Amiri et al. showed that the slope of the temperature plotted as a function of time at the beginning of the test can be effectively utilized as an index for fatigue life prediction. In 2013 [10], Fargione et al. applied thermographic method on mechanical components to individuate crack paths and damage evaluation. In 2014 [11], Crupi et al. were the first researchers that detected the radiometric surface temperature during ultrasonic fatigue tests in order to extend the thermographic method in very high cycle fatigue regime. Chrysochoos et al. in [12] have determined experimentally the energy balances of pseudo-elastic behaviour of a shape memory alloy using IR techniques and in [13], using infrared image processing, have determined the evolution of the heat source’s distribution on steel samples during monotone tension tests. The same authors in [14] presented an infrared data processing developed to analyse the calorific manifestations accompanying elastoplastic transformation during tensile tests. And in [15] have investigated experimentally on thermal and calorimetric effects induced by Luders band propagation during monotonic and quasi-static tensile tests. In previous papers, authors investigated the variation of temperature during static tensile tests of plastic [16] and metallic [17] materials to connect the variation of the slope of the stress strain versus temperature curve with the conventional fatigue limit of the material. In [18], authors suggested that during quasi-static tensile tests the area, where first irreversible plasticization occurred, is detectable on T vs   curve by the variation of temperature (slope). This variation identifies the transition zone between thermoelastic and thermoplastic behaviour, or in other words, the beginning of irreversible micro-plasticisation. Figure 1: Specimen dimensions [mm]. The method proposed in this paper is based on hypothesis that high cycle fatigue failures occur when local stresses generate irreversible plastic deformation amplified by micro-defects. For some static tensile test parameters, such as load speed [N/s], the change of the slope is not easy to define; in these cases the evaluation of the dissipation energy can help I