Issue 37

M. Kurek et alii, Frattura ed Integrità Strutturale, 37 (2016) 221-227; DOI: 10.3221/IGF-ESIS.37.29 224 a f mA N    log log   (15) being     mAmA , , , the coefficients of the regression equations for bending and torsion, respectively. The values of coefficients of the above regression equations for each analysed material are listed in Tab. 1. Eq. (3) proposed by Carpinteri et al. involves only fatigue limits, and can be applied for 2 B ranging from 1 to 3 . By analysing the values of coefficients  m and  m listed in Tab. 1, all materials can be noted to be characterized by values of  m different from those of  m , which means that such materials have non-parallel mutual fatigue characteristics. Therefore, a dependence of  angle on the ratio between bending strength and torsion strength in correspondence to a given number of loading cycles, fi N , is proposed in next Section. Material Bending Torsion ) (T min  fi N ) ( ) ( 2 fi a fi a N N B     Number of tests  A  m  A  m [°] [cycles] D30 [8] 30.50 10.75 25.40 9.20 8 2000000 1.496 6 GGG40 [9] 32.39 10.95 35.48 12.41 1 1000000 1.110 15 10HNAP [10] 30.88 * 9.50 * 25.28 8.20 45 2000000 1.874 108 PA4 (6082) [11] 23.80 8.00 21.40 7.70 45 2000000 1.680 45 30CrNiMo8 [12] 27.54 8.05 69.56 24.62 0 100000 1.500 9 CuZn40Pb2 [13] 19.99 5.86 45.30 17.17 16 1000000 0.920 55 GTS45 [9] 53.00 19.40 35.50 12.80 20 250000 1.265 11 Cast Iron IC2 [8] 23.7 8.80 44.00 19.50 0 1000000 1.155 4 Hard Steel 982FA [8] 36.60 12.10 49.50 18.60 14 1000000 1.550 11 SM45C [14] 31.10 10.30 49.40 18.60 0 100000 1.402 5 SUS304 [15] 19.8 * 7.04 * 22.5 8.7 5 2500 1.379 18 *push-pull Table 1: Coefficients of regression Eqs. (14) and (15) and fatigue properties of the examined materials. The number of tests is also reported. F ATIGUE STRENGTH SCATTER CALCULATION n order to analyse how the fatigue life is influenced by the value of  angle, simulation studies are carried out by assuming  ranging between 0° and 45°, with an increment equal to 1 °. For each of the 46 angle values, the parameter B is computed according to Eq. (10), whereas the parameter K is a constant according to Eq. (11) and depends only on the fatigue material properties. Fig. 1 shows the value of the parameter B against the  angle (Fig. 1(a)) and that of the parameter K (Fig. 1(b)) for 10HNAP steel [10]. In order to perform a suitable analysis of the fatigue strength scatter, the logarithmic dependence of the ratio between the experimental and calculated fatigue strength should be examined. A new method to determine such a scatter has been proposed by Walat et al. [16], who have defined the root mean square error: n N N E n i cal    1 exp 2 log (16) I