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A hysteresis cycle can define the variation in mechanical characteristics of fatigue life and failure response. Traditional analysis is performed using forcedisplacement signals derived from the testing equipment, but this approach is prone to large errors due to clearances and slips in the measuring chain. For this reason, the authors here propose a novel setup able to assess accurately the strain status of specimens by digital image correlation (DIC) and a procedure to calculate the hysteresis energy by thermographic analysis in order to measure the damping energy with precision. The results obtained by an experimental investigation on a common austenitic steel (AISI 304) highlight that the hysteresis areas defined by the DIC displacements, and those found by the testing machine outputs have substantial differences in value. The thermal variations and the areas of the hysteresis loops, both linked to the plastic energy, were also compared, showing a reliable correlation.

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Abstract. In this work is indicated how it could be possible to evaluate the limit stress of the thermo-elastic phase of deformation by thermo-analysing the surface of the specimen during a static traction test. Adding the temperature curve measured on a small area of the surface (the hottest) to the classic stress-strain curve, it is possible to evaluate a limit temperature T 0 coincident with the beginning of the non linear trend of the curve. The corresponding stress value is coincident with the fatigue limit of the analyzed component. As an example, the results of traction tests performed on two notched specimens, where the change of linearity in the temperature curve during static traction test was evident, are reported. The corresponding value of stress was a good approximation of the fatigue limit for R = -1, determined by the conventional method. The aim of the reported examples in this paper must be interpreted as support to the basic principle of the method and not as the results of a complete experimental planning of which we will comment in an another occasion.1 Nomenclature E l energy required for fatigue fracture ĭ l integral of a surface point temperature, proportional to E l , at fatigue fracture ı stress ı 0 fatigue limit for R = -1 (limit stress above which some crystal is plasticized)) ı i minimum stress ı s maximum stress ı m mean stress ı p plastic stress ı 0,2 yield stress ı r fracture strength/original cross sectional area and t r is the test time with thermo plastic behaviour f loading frequency R stress ratio İ strain İ 0 strain corresponding to ı 0 a

In this work is indicated how it could be possible to evaluate the limit stress of the thermo-elastic phase of deformation by thermo-analysing the surface of the specimen during a static traction test. Adding the temperature curve measured on a small area of the surface (the hottest) to the classic stress-strain curve, it is possible to evaluate a limit temperature T0 coincident with the beginning of the non linear trend of the curve. The corresponding stress value is coincident with the fatigue limit of the analyzed component. As an example, the results of traction tests performed on two notched specimens, where the change of linearity in the temperature curve during static traction test was evident, are reported.