numero25

G. Qian et alii, Frattura ed Integrità Strutturale, 25 (2013) 7-14; DOI: 10.3221/IGF-ESIS.25.02 8 M ATERIAL AND EXPERIMENTAL METHOD our-glass shape specimens (Fig. 1) of a structural steel 40Cr (0.4% C, 1% Cr) were tested with a rotary bending machine operating at a frequency of 52.5 Hz and the testing environments were of three types, namely laboratory air, fresh water and 3.5% NaCl aqueous solution. The average size of original austenite grains is 11.2  m and the average yield stress is 1501 MPa. Vickers microhardness indentation on the heat-treated specimen gives the average value of 545 kgf/mm 2 with the uniform distribution over the specimen cross section. The pH values of fresh water and 3.5% NaCl aqueous solution are 7.70 and 7.47, respectively. Based on the fatigue test data and scanning electron microscopy (SEM) observations of fracture surfaces, the effect of environment on the fatigue behavior at high cycle and VHCF regimes was examined. Figure 1 : Schematic drawing of hour-glass shape specimen for rotary bending fatigue test (dimensions in mm). S-N CURVES or specimens tested in laboratory air (triangles in Fig. 2), single crack originated from the surface of the specimens with fatigue life or the number of cycles to failure N f less than 10 7 loading cycles and the corresponding stress levels are above 700 MPa, whereas the crack started from subsurface for the specimens with N f beyond 10 7 loading cycles and the stress levels are below 700 MPa. For fatigue testing in fresh water, a similar stepwise S-N curve is presented (squares in Fig. 2), but the stress corresponds to the transition part of the S-N curve is dramatically decreased. For the fatigue tests in NaCl aqueous solution, the S-N curve (circles symbols in Fig. 2) displays a continuously descending shape. The fatigue strength is even lower than that tested in water from high cycle to VHCF regime, implying that the effect of NaCl aqueous solution on the degradation of fatigue strength for the structural steel is more remarkable than that of water media. 10 5 10 6 10 7 10 8 0 200 400 600 800 1000 Maximum Stress  max (MPa) Number of Cycles to Failure In air In water In 3.5% NaCl Figure 2: S-N curves for specimens tested at laboratory air, fresh water and 3.5% NaCl aqueous solution, hollow symbols: crack origination at surface, solid symbols: crack origination at subsurface, semi-solid symbols: mixed crack origination. H F