Issue 37

B. Jo et alii, Frattura ed Integrità Strutturale, 37 (2016) 28-37; DOI: 10.3221/IGF-ESIS.37.05 32 σ ൌ σ ୡୟୱୣ ୅ ౙ౗౩౛ ୅ ൅ σ ୡ୭୰ୣ ୅ ౙ౥౨౛ ୅ (2) where σ is the average stress for the carburized specimen and A is the total cross section area. From approximated material properties of both the case and the core layer, the deformation behavior of the carburized specimen was predicted and compared with experimental results. Predicted and experimental cyclic stress-strain curves are shown in Fig. 4(b). As can be seen from this figure, the predicted curve is a little bit underestimated, but comparatively similar to the experimental curve. This indicates that a simple two- layer model can be used to predict the cyclic deformation behavior for the carburized steel in the absence of real data. The small gap between the predicted and experimental data is attributed to the difference of microstructural and mechanical properties between the two core materials investigated. As an example, the beneficial influence of grain refinement on the mechanical properties such as strength and fatigue limit in steel could be well explained by Hall-Petch equation which indicates that the strength of a metal is proportional to the inverse of the square root of the grain size. And also, grain refinement (grain size reduction) is a method to increase the toughness of a metal. Therefore, it is estimated that the predicted curve will be even closer to the experimental data, considering that the material used in this study has a finer grain, as shown in Fig. 2, resulting in superior mechanical properties to those of the core material used for prediction. Table 2: Axial monotonic and cyclic deformation properties of the material investigated. Figure 4: Monotonic and cyclic axial deformation behaviors (a) and prediction curves (b) . Monotonic and Cyclic Torsional Deformation Behaviors The monotonic and cyclic torsional deformation behavior of the materials can be also expressed by a Ramberg-Osgood type equation as: ߛ ൌ ߛ ௘ ൅ ߛ ௣ ൌ ఛ ீ ൅ ቀ ఛ ௄ ೚ ቁ ଵ/௡ ೚ (3) where  ,  e ,  p ,  , G, K o and n o are total shear strain, elastic shear strain, plastic shear strain, shear stress, shear modulus, shear strength coefficient, and shear strain hardening exponent, respectively. For cyclic loading, stress amplitude (  a or Δ  /2) and strain amplitudes (  a or Δ  /2) are used in Eq. (3) and K o and n o are replaced by cyclic strength coefficient, K o ', and cyclic strain hardening coefficient, n o ', respectively. Monotonic Cyclic E (GPa) S y (MPa) EL. (%) K (MPa) n K′ (MPa) n′ S y ′ (MPa) Carburized 201 1025 2.82 3883.6 0.1637 2633.5 0.1294 - Case 208 1125 1.7 11109 0.372 16053 0.289 2665 Core 214 990 27 2276 0.138 2137 0.134 927