There is an increasing demand for evaluation of fracture toughness of long time used pressure vessels. This paper proposes a small-size specimen, which uses parallel laser welds of small-sample extracted from running vessels. The constraint effect by hard region of laser welds is strong enough to cause brittle fracture in the small-size specimen. Sometimes the constraint of laser welds in the small-size specimen is higher than that of the full-thick base metal specimen. To estimate the fracture toughness of the thick base metal specimen, this paper applies the local approach, which employs the Weibull stress as a fracture driving force of the material. It has been shown that the distribution of the critical Weibull stress at brittle fracture initiation for the small-size specimen with hard zones and homogeneous specimens are almost the same. This means that the critical Weibull stress is a material property independent of the specimen type. Based on the Weibull stress criterion, the fracture toughness of every thick base metal specimen is expected to be predicted from the fracture toughness results of the small-size specimen. The procedure proposed in this paper is useful to evaluate the fracture toughness of pressure vessels in service from the test results by small sample extracted from the vessels.
