Issue 37

B. Jo et alii, Frattura ed Integrità Strutturale, 37 (2016) 28-37; DOI: 10.3221/IGF-ESIS.37.05 36 same geometry specimen. The strain-life curves of both the case and the core material were estimated by using Roessle and Fatemi’s equation [9] as: ߝ ௔ ൌ ସ.ଶହ ሺு஻ሻାଶଶହ ா ൫2ܰ ௙ ൯ ି଴.଴ଽ ൅ ଴.ଷଶሺு஻ሻ మ ିସ଼଻ሺு஻ሻାଵଽଵ଴଴଴ ா ሺ2ܰ ௙ ሻ ି଴.ହ଺ (9) Fig. 9 presents the axial strain-life data of the carburized specimen and estimated the axial strain-life curve of the case and the core material. It can be seen from this figure that in the long-life region (sub-surface cracked failure region), the experimental data of the carburized specimens are in between the case and the core curves, but more close to the case curve, similar to previous results on the case-hardening steel [5]. In the short-life region (surface cracked failure region) the carburized data are below both the case and the core curves. Figure 9: Superimposed strain-life curves for the case and the core and prediction results. CONCLUSIONS his study was carried out for better understanding the cyclic deformation and fatigue behavior of carburized steel. Monotonic and cyclic deformation tests under both axial and torsional loading conditions were performed. Axial and rotating bending fatigue tests were also conducted. Based on the experimental results and analysis presented here, the following conclusions can be made: 1) The carburized steel investigated in this study exhibited cyclic softening under axial loading. Predicted curve based on the two-layer model for the cyclic deformation behavior was a little under-estimated, but comparatively similar to the experimental curve. 2) Under torsional loading, the carburized steel exhibited cyclic softening behavior. Predicted results for cyclic shear deformation showed that the maximum principal strain criterion provides good agreement with the experimental data, whereas von Mises criterion significantly under-estimates the cyclic stress-strain curve. 3) Failure mode of both the axial and the rotational bending was mostly sub-surface cracking, except for high load amplitude region with surface cracking. For this carburized steel, the fatigue strength of axial loading was about 70% of that of rotating bending specimen on the basis of fatigue strength at 10 6 cycles, slightly lower than that of commonly expected. This seems to be due to stress gradient effects being more significant for carburized steels. 4) For both axial fatigue and rotating bending fatigue, the predicted results based on the simple two-layer model were comparatively similar to the experimental data, except for surface failure region at short life. T