Proposal of Evaluation of Fracture Toughness Deterioration of Pressure Vessels in Service Using Small-Size Specimens

This paper describes a procedure proposed for fracture toughness evaluation of pressure vessels in use. A small-size specimen of thickness B = 2.5 to 5.0 mm with a couple of hard zones enclosing a notch is presented. Main attention is focused on how to create the hard zone in the small-size specimen. Preliminary FE-analysis indicates that the distance e from the hard zone to notch plane and the strength ratio Sr = sigma sub Y exp H / sigma sub Y exp B are major controlling factors of the near notch-tip constraint, where sigma sub Y exp H and sigma sub Y exp B are the yield stresses of the hard zone and base material, respectively. In order to obtain a sufficient plastic constraint, the e and Sr should be e < 0.16B and Sr > 2. The CO sub 2 laser welding is effective to realize such constrained state of specimens. The laser welds are hard enough and a rectangular hard zone can be created under a selected welding condition. The peak temperature in the region sandwiched by the laser welds is less than 200 deg C, and the level of welding residual stress is at most plus /minus100 MPa. Accordingly, the influence of laser welding on the mechanical and metallurgical properties of the test region is very marginal. The fracture toughness test verifies the validity of the small-size specimen with the laser welds. The fracture toughness measured by the small-size specimen is almost identical with the toughness of the full-thick specimen of base metal.