Issue 17

B. Atzori et alii, Frattura ed Integrità Strutturale, 17 (2011) 15-22; DOI: 10.3221/IGF-ESIS.17.02 19 The cooling rate after a sudden interruption of the fatigue test was measured by using the infrared camera and by considering the maximum value of the temperature inside on area encompassing the reduced section of the specimen. The maximum sampling frequency of the thermal images allowed by the available measuring system was 7 Hz. Fig. 5a shows an example of an infrared image: the rectangle identifies the area where the maximum surface temperature was detected. A typical example of the recorded temperature trend is plotted in Fig. 5b: t 0 indicates the time when the fatigue test was stopped while in the y-axis the temperature variation with respect to that stabilized before the test stop is shown. After evaluating the cooling gradient, Q was derived according to Eqs. (3) and (4). (a) (b) Figure 5 : Example of control area surrounding the specimen where the maximum temperature was analysed (a) and typical maximum temperature signal measured after a sudden interruption of fatigue test (b) (  a =210 MPa, run out). In order to evaluate the evolution of Q parameter, each fatigue test was interrupted several times. Fig. 6 shows the Q values plotted versus the number of cycles normalised with respect to the number of cycles to failure or, in the case of run-out specimens, with respect to 10 millions. It can be noted that the value of Q reached a constant value after about 50% of the total fatigue life. Moreover, the plotted curves show as soon as the stress amplitude is increased above the fatigue limit (  a > 220 MPa) then the stabilised values of Q increase of a factor 7 (from  100 kJ/(m 3 ·cycle) to  700 kJ/(m 3 ·cycle)). The fatigue data were analysed in terms of the stabilised energy parameter found during each fatigue test, by assuming a log-normal distribution of the number of cycles to failure, according to the following equation: cos k Q N t   (5) where N represents the number of cycles to failure and k is the inverse slope of the new fatigue curve. Figure 6 : Evolution of the specific energy loss versus the normalised fatigue life. 0 200 400 600 800 1000 1200 1400 0 0.2 0.4 0.6 0.8 1 260 MPa 240 MPa 230 MPa 220 MPa 210 MPa 190 MPa Q [kJ/(m 3 cycle)] N/N f