PSI - Issue 19

Shota Hasunuma et al. / Procedia Structural Integrity 19 (2019) 194–203 Shota Hasunuma, Ogawa Takeshi/ Structural Integrity Procedia 00 (2019) 000 – 000

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vessel is determined from the effects of the material variability, specimen size, surface finish, environment and other effects: in Japan Society of Mechanical Engineering (2012). Among these effects, the effect of the surface finish (i.e., the machined surface) on the fatigue life remains unknown and requires more research. Especially, in real structures and components, cracks initiate from regions of concentrated stress due to low cycle fatigue: in Suresh(2005). It is thus important to reveal the effect of the machined surface layer on the low cycle fatigue life. In addition, we can determine the safety factor if the low cycle fatigue life of a component having a machined surface layer can be predicted quantitatively. It is therefore also important to propose a method for predicting the fatigue life of a component with a machined surface layer. Generally, the effect of the machined surface layer on low cycle fatigue is weak. However, in our previous research on low-alloy steel, austenitic stainless steel, and Ni-based super alloy, the low cycle fatigue life was strongly affected by the machined surface layer; Hasunuma et al., (2011a); Hasunuma et al., (2011b); Hasunuma et al., (2015). The machined surface is distinguished by three factors, namely the variation in surface shape, variation in material properties and generation of residual stress. The variation in surface shape includes the roughness, stress concentration and scratches. The variation in material properties includes plastic deformation, hardening and microstructural transformation. In order to reveal the effect of machined surface layer on low cycle fatigue life, we have to separate the effect of surface morphology, material property variations and residual stress. The present study investigates a method for predicting the fatigue life of a component with a machined surface layer under the condition of low cycle fatigue. Specimens were first machined under different conditions to have different machined surface layers. In order to separate the effect of surface morphology and material property variations, fatigue tests were carried out for specimens, which had four different surface finishes, and the effect of the machined surface layer on the fatigue life was investigated. Finally, a method for predicting the fatigue life that considers the surface machined layer was investigated. 2. Preparing specimen with different machined surface layers The present study used austenitic stainless steel, SUS316L. Tables 1 and 2 give the chemical compositions and mechanical properties, respectively. Fig. 1 presents the round-bar shape of the specimens, with diameter d = 8 mm and gauge length G.L. = 11.5 mm. The machining conditions of the specimens are given in Table 3. The specimens were machined at different cut depth, a p , and spindle speeds, n , to produce different machined surface layers. Feed speed of f = 0.1 mm/rev was used in all cases. A cutting tip for stainless steel (VBMT 16 04 04-MM 2025, Sandvik) was used to machine the specimens.

Table 1 Chemical composition (mass%). C Si Mn

P

S

Ni

Cr

Mo 2.02

0.022

0.48

1.37

0.032

0.021

12.02

17.23

Table 2 Mechanical properties. Yield stress[MPa]

Tensile strength[MPa]

Elongation[%]

Young ’s modulus[GPa]

232

556

60

193

Fig.1 Specimen geometry of round bar specimen.