Issue 37

B. Jo et alii, Frattura ed Integrità Strutturale, 37 (2016) 28-37; DOI: 10.3221/IGF-ESIS.37.05 33 Monotonic shear deformation behavior for the carburized (case hardened) specimens is well represented by the Ramberg-Osgood relation. The measured shear properties are shown in Tab. 3. Fig. 5(a) illustrates monotonic and cyclic torque-strain behavior, showing that this carburized steel exhibits also cyclic softening behavior under torsional loading, similar to cyclic softening obtained from a thick case obtained from induction hardened steel in [6]. And also, the cyclic fatigue crack was found to be nucleated on the surface and to be propagated along about 45 o helix (spiral crack pattern), which is the plane of maximum principal strain. Table 3: Monotonic shear deformation properties of the carburized steel and axial cyclic properties of the materials investigated Figure 5: (a) Monotonic and cyclic torque-strain behavior and, (b) Predicted vs. experimental cyclic stress-strain curves For predictions, since cyclic shear strength coefficient, K o ', and cyclic shear strain hardening exponent, n o ', data for neither the case nor the core were widely available, predictions were performed by using the axial cyclic data of another case- hardening material [5, 7] on the basis of similar hardness level and similar microstructure. The axial cyclic data used in this prediction are presented Tab. 3. In this case, the following criteria were used to relate shear properties to axial properties: von Mises: ܭ ௢ ᇱ ൌ ௄ᇱ √ଷ ቀ ଵ √ଷ ቁ ௡ᇱ ܽ݊݀ ݊ ௢ᇱ ൌ ݊ ᇱ (4) Maximum Principal Strain: ܭ ௢ ᇱ ൌ ௄ᇱ ଵାఔ ቀ ଵ ଶ ቁ ௡ᇱ ܽ݊݀ ݊ ௢ᇱ ൌ ݊′ (5) The value of Poisson ratio, ν, used in this study was 0.3. The shear stress-strain relationships for both the case and the core sections were calculated by using above criteria, and then the total torque carried by the carburized specimens was obtained. At high strain region, yielding of the core material was also considered. The total torque of the carburized specimen can then be calculated from: ܶ ௧௢௧௔௟ ൌ ܶ ௖௢௥௘ ൅ ܶ ௖௔௦௘ ൌ 2 ߨ ׬ ߬ ݎ ଶ ݀ ݎ ൅ 2 ߨ ׬ ߬ ݎ ଶ ݀ ݎ ஽/ଶ ௔ ೎೚ೝ೐ ௔ ೎೚ೝ೐ ଴ (6) where a core is the core radius, D is the diameter of the specimen, and  is the shear stress at radius r. Fig. 5(b) shows the predicted results. As shown in this figure, for this carburized steel, maximum principal strain criterion is in good agreement with the experimental data, whereas von Mises criterion significantly under-estimates the cyclic Monotonic shear Axial cyclic G (GPa)  y (MPa) K o (MPa) n o K′ (MPa) n′ S y ′(MPa) Carburized 77.47 498.5 1703 0.1977 - - - Case 80.00 - - - 16053 0.289 2665 Core 74.62 - - - 2041 0.193 613