Issue 17

B. Atzori et alii, Frattura ed Integrità Strutturale, 17 (2011) 15-22 ; DOI: 10.3221/IGF-ESIS.17.02 17 equipped with a 100 kN load cell. The static behaviour was investigated by means of tensile tests under displacement control with a crosshead speed equal to 2 mm/min. During the static test the axial strain was measured by means of a MTS extensometer having a gauge length of 25 mm. The fatigue tests were carried out under load control, with a sinusoidal wave, nominal load ratio R (R =  min /  max ) equal to -1 and a test frequency variable in the range of 2-36 Hz depending on the applied stress level. To investigate the material fatigue behaviour, constant amplitude fatigue tests were carried out up to the specimen failure. Concerning the two load level fatigue tests, some specimens were fatigued at a stress level higher than the fatigue limit for a significant fraction of fatigue life. Then the stress level was decreased lower than the fatigue limit and kept constant up to 10 millions of cycles or specimen failure. Temperature increments were monitored by means of an AGEMA THV 900 LW/ST infrared camera able to detect infrared radiation in the range of wave lengths between 8 and 12  m with a resolution of 0.1 °C. The thermal images were post processed by using the dedicated software AGEMA research 2.1. R30 48 50 50 12 205 (a) 113 30 30 30 R30 10 7 (b) Figure 2 : Specimen geometry for static (a) and fatigue (b) tests. E XPERIMENTAL RESULTS n order to characterise the material static behaviour, five tests were carried out. The mean value of elastic modulus E, engineering tensile strength  R , proof strength  p0,2 and true fracture strain A% are summarised in Tab. 1. By means of an electrolytic etching (stainless steel anode and cathode, voltage 1.2 V, current 0.2 A) on a 60% nitric acid solution the microstructure was analysed. A typical example is shown in Fig. 3: the white matrix represents the austenitic phase while the dark zones inside the grains are ferrite, which represents the 1% of the volume. E [MPa]  p0,2 [MPa]  R [MPa] A% 194750 315 699 59% Table 1 : Material properties of AISI 304L stainless steel (a) (b) Figure 3 : Example of microstructure observed in the cross section: mid-thickness (a) and below surface (b) . I 60  m 60  m