Issue 41

A. Cernescu, Frattura ed Integrità Strutturale, 41 (2017) 307-313; DOI: 10.3221/IGF-ESIS.41.41 309 strain and respectively plane stress state. C Si Mn P S Cr Ni W Al Cu N 0.15 0.22 1.46 0.04 0.02 0.05 0.03 0.03 0.04 0.02 0.01 Table 1 : The chemical composition of steel, [%]. Material Young’s modulus [MPa] Yield strength [MPa] Ultimate strength [MPa] Elongation [%] Low-alloy steel 205000 255 368 30 Table 2 : The mechanical properties of tested material. Fatigue crack growth rate in plane strain state Two CT samples were tested for plane strain conditions with dimensions shown in Fig. 2 and a thickness of 10 mm. One sample was tested at constant amplitude loading with stress ratio 0.1 and maximum load of 8 kN. Figure 2 : The dimensions of CT samples used in fatigue crack growth rate tests, (units: mm). At the second sample, after fatigue pre-cracking it was introduced an overloading cycle with maximum load of about 20 kN and then continued testing at constant amplitude loading with maximum load of 8 kN and stress ratio 0.1. Fatigue crack growth rate in plane stress state For plane stress state were used two CT samples with the same dimensions as in Fig. 2 but thickness of 2.4 mm. In all cases the sample dimensions respect the conditions required by ASTM E 647. After fatigue pre-cracking both samples were tested at constant amplitude loading with maximum load of 2 kN and stress ratio 0.1. Also, to both samples were applied an overloading cycle with maximum force higher with 26 % of the maximum force of constant amplitude loading and respectively 82 %. During the entire fatigue crack propagation tests have been recorded the applied force and respectively the crack opening displacement (COD) corresponding to each loading cycle. Also, the compliance technique was used to determine the crack length. R ESULTS AND DISCUSSIONS ll samples were examined by scanning electron microscopy (SEM) aiming to crack evolution before and after the overloading cycle. For plane strain state it was determined the variation of fatigue crack growth rate function of stress intensity factor range for constant amplitude loading with stress ratio 0.1, fig. 3. A