| Budget Amount *help |
¥2,000,000 (Direct Cost: ¥2,000,000)
Fiscal Year 1988: ¥700,000 (Direct Cost: ¥700,000)
Fiscal Year 1987: ¥1,300,000 (Direct Cost: ¥1,300,000)
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| Research Abstract |
It is recognized that CTOD results of the heat affected zone(HAZ) in multipass welded joints have large data scatter, and low critical CTOD results are caused by small areas of limited cleavage resistance within the coarse-grain HAZ; these areas are called local brittle zones(LBZ's). It is pointed out that the greatest metallurgical contributor to low toughness of LBZ's for low carbon steels is martensite islands which generate at intercritically reheated coarse-grain regions(IRCG).
The soft program which iso-thermal lines corresponding to various peak temperatures can be automatically drawn on an etched cross section of welded joint by thermal analysis was developed, and HAZ's are classified into various parts with different microstructures in accordance with matallurgical contributors to toughness levels. From the fatigue crack sampling information about each CTOD test specimen of a multipass welded joint, the relation between LBZ size intersected by the fatigue crack and critical CTO
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D value was investigated.
As a result, it becomes clear that critical CTOD value decreases with increasing the maximum size of LBZ with the most deteriorated microstructure interseced by a fatigue crack. However in the case of the size greater than 2mm, the critical CTOD value stayes almost constant. This value is considered to be minimum value of critical CTOD's. Simular trend was obserbed in Charpy impact tests.
From the view point of structural integrity, it is the most important to prevent the initiation of brittle fracture from a small sized surface crack. It was planed to study the relation between critical CTOD's obtained by above mentioned normal CTOD tests with a through-thickness(T.T.) type fatigue pre-crack and those for a semi-elliptical surface crack.
However, the method to estimate the CTOD for a semi-elliptical surface flaw has not been established. Then the conversion method to CTOD from mouth COD and crack angle at surface for a semi-elliptical surface crack was developed. By using the method, it becomes clear that if a tip of the surface crack locates at LBZ, the CTOD of the surface crack at the brittle fracture initiation is well agreement with the minimum CTOD value obtained by T.T. type CTOD tests. In other words, it can be said that only the T.T. type specimens of which the crack intersects LBZ with the size greater than 2mm give valid critical ctod's. Less
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