PSI - Issue 19

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

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4. Investigation of fatigue strength reduction of grinding finish materials 4.1. Investigation items

In order to figure out essential cause of fatigue strength reduction of grinding finish materials by Type 2, following factors, which are considered to possibly affect fatigue strength of the material, were measured and investigated. – hardness of surface – residual stress – local strain – grinding finish unique surface characteristic 4.2. Hardness of surface Fig. 8 shows measurement results of hardness on the axial cross section. As shown in Fig. 8, hardness near the surfaces of all specimens is increased by surface finishing compared to the hardness inside the specimen. Especially, the amount of increase of hardness by grinding is much larger than emery polishing. In addition, although the increase of hardness by emery polishing is limited within a range of only 50  m below the surface, grinding finish still increases the hardness of the material at the depth of 100  m below the surface. However, among all the 3 types of grinding finish conditions, the increase tendency of hardness is almost the same. Hence, increase of hardness should not be the main factor of fatigue reduction of Type 2 grinding finish. 4.3. Residual stress Measurement results of residual stress in axial direction for each surface finish are described in Fig. 9. The major difference between emery polishing and grinding is that residual stress of emery polishing is compression but that of grinding is tension until 500  m depth to the surface. Also, the residual stress profile from 50  m to 500  m depth after Type 1 and 2 grinding is almost the same. In contrast, residual stress on surface of Type 1 and Type 2 grinding finish is 400 MPa tension and 0 MPa respectively. In general, residual tensile stress acts as tensile mean stress and reduces fatigue strength of the material. However, in Fig. 5, fatigue strength of emery polishing and Type 1 ground specimen is almost same. What’s more, fatigue strength of Type 2 specimen which has no tensile residual stress on surface is much lower than that of Type 1 specimen with 400MPa of tensile residual stress. Based on these comparisons, residual stress is not an explainable reason for fatigue reduction of Type 2 grinding finish specimens.

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○ Emery polishing ■ Grinding (Type 1) ◆ Grinding (Type 2) ● Grinding (Type 3)

○ Emery polishing ■ Grinding (Type 1) ◆ Grinding (Type 2)

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Fig. 8 Hardness of axial cross section.

Fig. 9 Residual stress of axial direction.