Issue 17

B. Atzori et alii, Frattura ed Integrità Strutturale, 17 (2011) 15-22; DOI: 10.3221/IGF-ESIS.17.02 18 The grain size was assessed according to ASTM E112 standard [5]. Two specimens were analysed and three measurements were done on each of them: in particular, two measurements were performed in the cross-section (one in the mid thickness and one below specimen’s surface) and one on the surface of the sheet. The mean values are listed in Tab. 2. One can note that the grains are bigger in the external surface, as it can be expected in the case of rolling sheets. Finally, measurements of Vickers micro-hardness (applied load 0.2 kg) were carried out. Tab. 3 shows the results obtained in the middle of the thickness and at 0.2 mm below the surface. Specimen 1 [  m] Specimen 2 [  m] Below surface 37 27 Mid-thickness 30 27 Surface 41 35 Table 2 : Experimentally measured grain size. Specimen 1 Specimen 2 Below surface 175 163 151 155 157 151 Mid-thickness 169 162 162 170 163 167 Table 3 : Measured values of Vickers micro-hardness (applied load 0.2 kg). Synthesis of experimental data in terms of stress amplitude The material fatigue limit was evaluated according to a shortened stair-case procedure according to Dixon’s rule [6], which involved 7 specimens. Tests were stopped after 10 million cycles. As a results, the fatigue limit in terms of stress amplitude  A  ,-1 resulted equal to 217 MPa. The Wöhler curve, shown in Fig. 4, was evaluated via statistical analysis of the available fatigue data, by assuming log- normal distribution of the number of cycles to failure. In the same figure the fatigue limit  A  ,-1 , the 10-90% scatter band, the value of the inverse slope k of Wöhler curve, the scatter index T  (T  =  A  ,-1,10% /  A  ,-1,90% ), the scatter index T N,  (T N,  = T  k ) and the number of cycles N A , which corresponds to the knee point of the curve, are listed. Figure 4 : Wöhler curve and 10%-90% scatter band of the AISI 304 L stainless steel. Synthesis of experimental data in terms Q parameter As already discussed, the evaluation of Q parameter is based on the measurement of the cooling rate just after a sudden interruption of the fatigue test, according to Eqs. (3) and (4). The material density, experimentally measured by using Archimedes method and a Sartorius 1801 balance, with a resolution of 10 -7 kg, was 7940 kg/m 3 . By using a calorimeter, the specific heat c was determined equal to 507 J·kg -1 ·K -1 . R=-1 T  =1.20 k=12.9 T N,  =10.5  A  = 217 MPa N A =165800 300 200 10 4 10 5 10 6 10 7 10 8 N. cycles  a [MPa] 10% 90% broken run-out broken Stair case procedure 140