Issue 30

G. Meneghetti et alii, Frattura ed Integrità Strutturale, 30 (2014) 191-200; DOI: 10.3221/IGF-ESIS.30.25 192 recognise the temperature measurements obtained from specimens loaded by stress amplitude higher or lower than the fatigue limit [6]. Giancane et al. analysed the non-uniform temperature distribution in the case of aluminium alloys [7]. In ref. [8] an experimental procedure was proposed to evaluate the energy dissipated as heat in a unit volume of material per cycle, Q, starting from temperature measurements. The Q parameter was then adopted as a new experimental damage index useful for fatigue life estimations. Recently, the use of the Q parameter enabled us to rationalise several experimental results generated from constant amplitude, push-pull, stress- or strain-controlled fatigue tests on plain and notched hot rolled AISI 304 L stainless steel specimens [9, 10] as well as from cold drawn un-notched bars of the same steel under fully-reversed axial and torsional fatigue loadings [11]. Here we recall that notched specimens had either lateral U- or V- notches, with root radii equal to 3 or 5 mm, or a central hole with radius equal to 8 mm. Fig. 1 shows the axial and the torsional fatigue test results in terms of net-section stress amplitude  an or  a , respectively, the mean fatigue curves and the 10%-90% scatter bands. The figure reports also the inverse slope k of the curves, the stress-based scatter index T  =  a,10% /  a,90% (T  ) and the life-based scatter index T N,  (T N,  ). In the case of strain-controlled fatigue tests, the stress amplitude reported in Fig. 1 is the value measured at half the fatigue life. Fig. 2 shows the same fatigue data re-analysed in terms of the Q parameter. In particular, the 10%-90% scatter band shown in the figure was fitted only on the fatigue data published in [10]. However, Fig. 2 shows that fatigue data obtained under axial and torsional fatigue tests [11] can be interpreted by the same scatter band. More than 120 fatigue data are included in the figure. 100 700 300  an ,  a [MPa] 200 500 10 2 10 3 10 4 10 5 10 6 10 7 N f , number of cycles to failure Strain controlled Stair case: broken, unbroken Plain material k=17.2; T  =1.19; T N,  =20.0 Hole, R=8 mm: k=8.9; T  =1.18; T N,  =4.3 U-notch, R=5 mm V-notch, R=3 mm k=5.8 T  =1.30 T N,  =4.5 Load ratio: -1 Axial load: k=18.9, T  =1.13, T N,  =10.0 Torsional load: k=18.7, T  =1.13, T N,  =9.02 Scatter bands: 10% - 90% survival probabilities. Data from [9,10] Data from [11] N A Figure 1 : Fatigue data analysed in terms of net-section stress amplitude. Scatter bands are defined for 10% and 90% survival probabilities. It is worth noting that the Q parameter is independent of the mechanical and thermal boundary conditions such as the specimen’s geometry, load test frequency and room temperature [10]. By applying the energy balance equation, it was shown [8] that Q can be evaluated by stopping the fatigue test and then measuring the cooling gradient immediately after the test has been interrupted, according to Eq. (1): T Q f c t         (1) where f is the load test frequency, T is temperature, t is time,  is the material density c is the material specific heat. Concerning the stainless steel material analysed in the present paper, the material density  and the specific heat c were experimentally measured and resulted 7940 kg/m 3 and 507 J/(kg K), respectively [12]. According to Eq. (1), it is possible to evaluate the thermal power (Q·f) dissipated in steady state conditions by measuring the cooling gradient just after the