Issue 37

D. Angelova et alii, Frattura ed Integrità Strutturale, 30 (2017) 60-68; DOI: 10.3221/IGF-ESIS.37.08 61 Our paper presents the results of research on fatigue behavior of austenitic stainless steels at different fatigue and fretting fatigue conditions, which will be used for future characterization, analysis and predictions of fatigue and fretting fatigue in products made of these steels, and subjected to hydrogen-environment influence. M ATERIALS AND EXPERIMENTAL WORK Materials tudies of our team on hydrogen influence on metals are focused on analyzing fatigue and fretting fatigue characteristics of three austenitic stainless steels ASTM 304, 316, 316L (SUS304, SUS316, SUS316L by Japanese Standard) Tab. 1 [1-8]. Tests A. Fretting fatigue tests The tests are carried out in the HYDROGENIUS Institute, Kyushu University, Japan  Kondo et all [3] examine the hydrogen influence on Steels 304, 316, 316L by two types of specimens: hydrogen charged and uncharged ones The fretting fatigue tests are carried out under the following conditions: 1. Using of an assembly shown in Fig. 1a and specimens represented in Fig. 1b; 2. Pressing contact pads (Fig. 1c) against the specimen by tightening the bar springs through clamping bolts; 3. Inducing fretting by the difference of deformation between the specimen and contact pads, when a constant amplitude cyclic bending moment applies to the assembled fatigue specimen; 4. The specimens and pads used in the tests are made of the same steel. Some specific characteristics of this fretting fatigue testing are connected with the following details [3]. The pads used are bridge type ones with 3mm contact length of each foot in the relative slip direction. The contact edge of each pad is square without chamfer. he 0.5mm foot length is chosen in order to generate a large relative slip range as the effect of Steel Chemical compositions of steels (wt %) C Si Mn P S Ni Cr Mo 304 0.06 0.51 0.92 0.033 0.004 8.08 18.8 – 316 0.05 0.49 1.31 0.030 0.027 10.22 17.0 2.04 316L 0.012 0.19 1.64 0.031 0.012 12.19 16.6 2.22 Steel Mechanical properties of steels Condition Yield stress R e (MPa) Ultimate stress R m (MPa) Elongation A (%) 304 Solution heat-treated 285 637 60 316 Solution heat-treated 286 598 59 316L Solution heat-treated 212 530 59 Table 1 : Characterisation of used steels (wt %). hydrogen on fretting can be observed clearer at large amount of fretting wear. The contact surfaces of both, the specimen and the pad, are finished by 400 Emery paper. A part of an experiment is carried out when the contact surfaces are finished only by grinding for investigating the effect of machining process on fretting fatigue strength. Before test starting the contact pressure between the specimen and the contact pad is set up at 100 MPa. It is found that after 10 million cycles of fretting the initial contact pressure drops roughly by 5%. The amount of relative slip is measured by using a small displacement sensor attached at the end of contact pad. The tangential force is measured by a strain gage pasted between the pad feet. The strain of pad is transferred into tangential force by elastic finite element stress analysis. Nominal stress S