Issue 35

Y. Matsuda et alii, Frattura ed Integrità Strutturale, 35 (2016) 1-10; DOI: 10.3221/IGF-ESIS.35.01 3 each specimen. Hydrogen content was measured using gas chromatography thermal desorption analysis (TDA). The rate of temperature increase was 0.028 °C/s. During hydrogen charging, the areas of fatigue cracking and small artificial holes were locally protected to prevent corrosion. After removing the protection, rotating bending fatigue tests were conducted for both specimens (S10C and S25C steels) at test frequencies of f = 1.6–3.2 Hz, corresponding to stress amplitudes of   a = 250 MPa. The fatigue tests at fatigue limit were conducted at f = 30 Hz and  a = 150 MPa for S10C torsional prestrained (  pre = 45.0 deg/mm) steel and at f = 30 Hz and  a = 135 MPa for S25C torsional prestrained (  pre = 20.0 deg/mm) steel. The stress intensity factor was calculated according to the handbook [5]. (a) Rotating bending fatigue test specimen (b) Hydrogen measurement specimen Figure 1 : The shapes and dimensions of specimens (mm). Figure 2 : Relationship between torque and specific angle of twist. R ESULTS AND DISCUSSION Effect of torsional prestrain on hydrogen entry properties ig. 3 shows the TDA hydrogen release profiles during continuous heating of hydrogen-precharged S10C and S25C specimens. As shown in Fig. 3(a), the hydrogen desorption peaks were located at around 100–150 °C for virgin and prestrained specimens for both S10C and S25C steels. The height of the peak increases with the prestrain due to the increasing density of dislocations, which act as hydrogen trap sites. Hydrogen released from around 20–200 °C is classified as diffusible hydrogen and causes hydrogen embrittlement because hydrogen can diffuse to crack tips or regions of stress concentration [6]. Small peaks corresponding to the so-called non-diffusible hydrogen were observed at temperatures close to 300 °C. F