Issue 41

G. Meneghetti et alii, Frattura ed Integrità Strutturale, 41 (2017) 299-306; DOI: 10.3221/IGF-ESIS.41.40 300 Figure 1 : A propagating fatigue crack and the assumed shape of the structural volume V c where the heat energy is to be averaged (a) and time-variant temperature for i-th pixel (b) . The averaged heat energy generated inside the structural volume can be evaluated as follows:        cd c S cd cL V c c * dSh Vf dVQ V Q 1 1 (1) where h is the heat flux existing at a point along the boundary of V c , S cd , through which heat energy is extracted by conduction. It is worth noting that Eq. (1) is valid under the hypothesis that the heat is extracted from V c only by conduction, which is not true, strictly speaking. However, it has been demonstrated that conduction is by far the most active heat dissipation mechanism in standard laboratory testing conditions [8]. The heat flux can be evaluated from the thermal gradients calculated from infrared temperature maps; therefore, referring to a two-dimensional problem, Eq. (1) can be written as follows [8]:                   * ( , ) 1 m c L c r R c T r Q z R d f V r (2) T m (r,  ) being the mean temperature, measured when the thermal equilibrium with surroundings is reached. Fig. 1b reports a typical temperature vs time acquisition at a point of a specimen or component after a fatigue test has started. If the temperature field is monitored by means of an infrared camera, Fig. 1b is the pixel-by-pixel temperature vs time history and it shows that temperature increases until the mean temperature level stabilizes, while the alternating component due to the thermoelastic effect still exists. If we consider a sampling window taken after thermal equilibrium with the surroundings is achieved (between t s and t* in Fig. 1b), the mean temperature T i m for i-th pixel is defined as follows:    max 1 max n i j j i m T T n (3) where T j i are the temperature data acquired at a sampling rate f acq and n max = f acq ·(t*- t s ) is the number of picked-up samples between the time t s (j=1) and the time t* (j=n max ). Eq. (2) is based on the radial temperature gradient, therefore it has been referred as spatial gradient technique. An alternative method to evaluate Q* has also been proposed, which is based on the cooling gradient measured after the machine test has been stopped and therefore it has been referred as cooling gradient technique [8]. However, the latter method has not been applied here. The aim of the present paper is to use the parameter Q* to correlate crack growth data generated from tension- compression axial fatigue tests on specimens machined from 4-mm-thick, hot-rolled AISI 304L steel plates. The plan of T j i time T i m Thermoelastic oscillations t s t * T i a n max /f acq (b) (a)