1998 Fiscal Year Final Research Report Summary
Fatigue strength and fracture mechanisms in severely drawn eutectoid steel wires
| Project/Area Number |
09650086
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| Research Category |
Grant-in-Aid for Scientific Research (C)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
Materials/Mechanics of materials
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| Research Institution | Iwate University |
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Principal Investigator |
KATAGIRI Kazumune IWATE Univ., Faculty of Eng., Professor, 工学部, 教授 (90029893)
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| Co-Investigator(Kenkyū-buntansha) |
KASABA Koichi IWATE Univ., Faculty of Eng., Assistant Professor, 工学部, 助手 (00271841)
SATO Tadashi IWATE Univ., Faculty of Eng., Associate Professor, 工学部, 助教授 (30003859)
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| Project Period (FY) |
1997 – 1998
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| Keywords | Fatigue strength / Eutectoid steel wire / fatigue crack growth / Drawing strain / Residual stress / Hardness / Bluing / Shot-peening |
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| Research Abstract |
Following results clarifying the effects of various factors on the fatigue strength of eutectoid steel wires were obtained through the experiments.
1. Within the drawing strain range of epsilon 0 to 3.8, fatigue strength increased monotonically up to 800MPa with increase in epsilon , as in the case of ferritc steel in which the degree of increase is low. In some cases, however it had a peak value at epsilon=2.5 depending on the fabrication condition. It increased with decrease in the wire diameter and did not depend on the lamellar spacing (0.01-0.02 mu m).
2. Associated with high density dislocations and the narrow lamellar spacing observed in TEM, the hardness and tensile residual stress in the surface layer increased with increase in epsilon up to Hv=520 and 1000 MPa.
3. While the tensile residual stress monotonically decreased with increase in bluing temperature and diminished at 500ﾟC, the hardness increased to a peak value of Hv=650 by bluing at 300ﾟC The fatigue limit increased to a peak of 900MPa by bluing at 400ﾟC due to combined effects of strain aging, recovery and relaxation of residual stress. By shot peeing, the residual stress decreased from 1000 to -300 MPa and the highest fatigue limit of 1200 MPa was obtained
4. Fatigue crack initiated from inclusions or the ferrite layer in the surface region and changed its direction at the boundary of pearlite colony. The ratio of the duration for fatigue crack initiation in the fatigue life-time increased with increase in epsilon
5. With increase in epsilon , the *K_<th> decreased through decrease in the roughness induced crack closure level due to decrease in the pearlite colony size. The fatigue crack resistance in Paris' region also increased and the "m-value" decreased.
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