PSI - Issue 19

Motoki Nakane et al. / Procedia Structural Integrity 19 (2019) 284–293 Author name / Structural Integrity Procedia 00 (2019) 000–000

291

8

4.4. Local strain It can be recognized in Fig. 3 that, unlike the lathe machining finish, the surface profile of grinding finish is not constant and some scratch is very sharp. So, elastic-plastic finite element analysis (FEA) was performed to examine local strain at bottom of sharp scratch generated by grinding finish. Based on the measured surface profile, two kinds of notch models, model 1 and model 2 described in Table 5, were subjected to FEA. For comparison, analysis of local strain for lathe finishing surface with Rz =140  m was also performed. Results are shown in Table 5. Local strain obtained from FEM is average of 100  m area from surface because averaged strain within a few grain size is dominant for initiation of fatigue crack. The calculated strain concentration for model 1 and 2 is around 1.5 and this number is close to K sf for Type 2 grinding conditions presented in Fig. 7. On the other hand, strain concentration of lathe machining finishing is 4.1 and this number is not align with the correction of fatigue strength for lathe machining finishing specimen shown in Fig. 6. Therefore, it can be concluded that local strain is not dominant factor for reduction of fatigue strength of Type 2 grinding condition.

Table 5. Local strain calculation conditions and results of strain concentrations. Surface finish Width of scratch (  m) Depth of scratch (  m) Radius of scratch (  m) Nominal strain (%)

Local strain (%)

Strain concentration

Grinding (model 1) Grinding (model 2) Lathe machining

39

3.2 8.5 140

2.6 37 325

0.28 0.34 0.82

1.4 1.7 4.1

100 600

0.2

4.5. Grinding finish unique surface characteristic Macroscopic and microscopic observation for ground surface and fracture surface of the fatigue test specimen is conducted in order to investigate trigger of fatigue strength reduction by Type 2 grinding finishing. Fig. 10 shows macroscopic photo of Type 2 grinding finished fatigue test specimen. It can be said that grinding finished surface has mainly two surface textures. One is streaky texture which can be regarded as usual grinding scratch and the other is groove texture. In this paper, this groove texture is called as ‘groove-like scratch’. Cross sectional and birds eye view of fatigue crack is presented in Fig. 11. The fatigue crack is initiated from the bottom of groove-like scratch and is communicated along the groove-like scratch which is perpendicular to the cyclic loading direction. Fig. 12 shows fracture surfaces of two kinds of Type 2 grinding finished fatigue test specimens. For both specimen, groove-like scratches can be found at the fatigue crack initiation. Length of groove-like scratches are 1mm for  ta =0.115% and 0.5mm for  ta =0.2% test condition. Typical example of cross sectional view of groove-like scratch is shown in Fig. 13. Groove-like scratches are composed of groove and covering, and the groove shape and covering would be generated by local plastic deformation of the material due to the multiple grinding work at same place, same direction. To clarify the effect of grinding scratch including groove-like scratch on the fatigue strength reduction, depth a and width w of scratches were measured for untested Type 2 and Type 3 specimens. The measurement range of the specimen is 15mm along the axial direction and results are presented in Fig. 14. From Fig. 14(a), the scratch depth of Type 2 appears deeper than Type 3 grinding. This would be attributed to the fact that Type 2 grinding is worked on the same place, same direction many times. The results of groove-like scratch depth of w =8  m and a / w =1.38 at fatigue crack initiation of Type 2 grinding test specimen used for  ta =0.115% fatigue test are also indicated in Fig. 14. This Type 2 ground specimen shows most fatigue strength reduction. Fig. 14(a) shows that groove-like scratch depth of the specimen for most fatigue reduction specimen is not so deep compared to other groove-like scratches. On the other hand, a / w for most fatigue reduction specimen is significantly large, and this result expresses that groove-like scratch of the specimen is much sharper than the others. In addition, a / w of Type 2 grinding test specimen used for  ta =0.2% fatigue test is a / w =0.86 and this value is also relatively larger than the others. Consequently, it can be concluded that Type 2 grinding is a distinct surface finish process that has potential to generate very sharp and long groove-like scratch which will lead to fatigue reduction of the materials. Therefore, the