1992 Fiscal Year Final Research Report Summary
Study on theoretical assessment method of K_<th> and assessment method of fatigue life for a short crack
| Project/Area Number |
02452183
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| Research Category |
Grant-in-Aid for General Scientific Research (B)
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| Allocation Type | Single-year Grants |
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| Research Field |
船舶構造・建造
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| Research Institution | KYUSHU UNIVERSITY |
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Principal Investigator |
TOYOSADA Masahiro KYUSHU UNIV.,FACULTY OF ENGN.,PROFESSOR, 工学部, 教授 (30188817)
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| Co-Investigator(Kenkyū-buntansha) |
NIWA Toshio KYUSHU UNIV.,FACULTY OF ENGN.,RESEARCH ASSOCIATE, 工学部, 助手 (10208267)
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| Project Period (FY) |
1990 – 1992
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| Keywords | Fatigue Crack Propagation / Crack Opening and Closing / RPG Load / DELTAK_<RP> / Short Crack / Fatigue Life Assessment by Unified Theorem |
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| Research Abstract |
From the consideration about the small change of compliance for a cracked body under cyclic loadings, it can be postulated that fatigue crack dose not grow if no damage occurs in the vicinity of a crack tip.Then we propose an effective stress intensity factor range (DELTAK_<RP>) corresponding to the period in which re-tensile plastic zone's generates at a crack tip for a fatigue crack propagation parameter. Moreover an automatic controlled system was developed to measure Re-tensile Plastic zone's Generated load (RPG load).
Then the various types of fatigue crack propagation test were conducted by using above developed system. For a long crack, it becomes clear that DELTAK_<RP> gives the quantitative effect of stress ratio on fatigue crack propagation rate (da/dN) and the logarithmic curve of DELTAK_<RP> - da/dN appears to be linear in the wide range from very slow crack growth rate region to stable crack growth region, which means that DELTAK_<th> is not a material constant. It is also confirmed that the logarithmic curve of DELTAK_<RP> - da/dN for a physically small crack is in good agreement with that for a long crack, considering that fatigue cracks initiate and propagate as suface cracks.
For a microstructual short crack, it is postulated that dislocations do not excess barrieres such as a grain boundary and a pearlite zone. Then equevalent DELTAK is defined from a plastic zone size and equivalent DELTAK_<RP> is obtained from measured RPG load. Therefore crack propagation curves are calculated for a crack emanated from a notch root without distingurishing crack initiation and propagation life and are well in agreement with exprimental ones. This result seems to mean the establishment of fatigue life assessment method by unified theorem.
Moreover the simulated program by a fatigue propagation model in an arbitrary stress distribution field based upon the Dugdale model is also developed.
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