Issue 39

A. Risitano et alii, Frattura ed Integrità Strutturale, 39 (2017) 202-215; DOI: 10.3221/IGF-ESIS.39.20 204 evaluation of transition zone. In the present paper, a new approach is proposed to investigate the fatigue limit during tensile static tests. Particularly, a numerical approach to estimate specific energy is developed, based on the method proposed by Chrysochoos and Louche [14, 15, 19]. Therefore, the authors estimated the dissipation and the area where the first increase of energy occurs. The macro average stress value applied (load/area) on the specimen, when the first increase of temperature occurs, corresponds to conventional fatigue limit of material. In this paper a qualitative method, to improving an already proposed approach of the authors by using a different energetic parameter instead of the temperature is here presented. AISI 304 specimens with rectangular cross-sections were tested using static test and fatigue tests (R= -1). During static tensile and fatigue tensile tests, IR camera has been used to evaluate energy dissipations. In addition, the fatigue limit was determined using thermographic method and results have been compared to static results. M ATERIAL AND METHODS he tests were performed in two phases. In the first phase, the energy dissipation of the specimens was evaluated during tensile static test by a IR camera. In the second phase, the conventional fatigue limit was found during fatigue tests by thermographic method [1,3, 4]. The results of the two phases were compared. The material was AISI 304 steel; eight specimens were produced. Fig. 1 illustrates the shape and dimensions of the specimen. Four specimens were used for the static tensile tests and four were used for the fatigue tests employing thermographic method. All the specimens are black painted and their thermal emission coefficient was 0,92±5. A Zwick Z100 universal tester was used for the static tensile tests. During the static tests, the surface of the specimen was monitored using a FLIR_SC 3000 infrared camera. The tests were performed at a crosshead speed of 3 mm/min; the average nominal strain was ~9.09  10 -4 s -1 (calculated by speed test/gage length): physical transformation can be considered quasi-static. In Tab. 1, the material characteristics are reported. In Tab. 2, time constant and spatial resolution for the first phase of the tests are reported. In the second phase, the fatigue tests were performed using an INSTRON 8872 test machine and surface temperature was recorded during the test using an infrared machine FLIR-SC 3000 (the acquisition frequency of the IR image was 0.2 Hz). For the second phase, the load ratio was R= -1 and the test fatigue frequency was 5 Hz. density thermal conductivity specific heat coefficient of thermal expansion yield stress [kg/m 3 ] [W/mK] [J/kgK] [1/K·10 -6 ] [MPa] 7900 15 502.4 17 325 Table 1: Thermomechanical properties of AISI 304. Number of test  c,i Dimension of pixel in x direction Dimension of pixel in y direction Acquisition frequency IR [s] [mm] [mm] [Hz] 1 503 0.4000 0.4000 1 2 503 0.4000 0.4000 2 3 503 0.4000 0.4000 3 4 503 0.4167 0.4167 4 Table 2: Time constant and spatial resolution for the four tensile static tests. T HEORY AND C ALCULATION o simplify the theoretical approach, according Chrysochoos and Louche [14, 15, 19], the following hypotheses were used in analysing the heat released during the static tensile tests: 1. A quasi-static physical transformation during the static test occurs; thus permitting the use of classic rules T