1991 年 40 巻 453 号 p. 656-662
Failure causes for welded joints and brazing material in three cases were analyzed by means of fractography.
During the work of attaching a 180 tonf sternpost to the stern frame in a dockyard, two small plates welded on the rudder horn to support the sternpost came off near the weld metal, and the sternpost dropped to ground. No weld defects were observed in the fillet weld. However, both the throat thickness and the leg length in the fillet weld were smaller than those required in joint design. Therefore, it was concluded that the weld metal could not withstand the weight of sternpost.
When butt-welded steel sheet piles were driven into ground using a hydraulic pile-driver under construction of sewage work, a sheet pile failed at the welded joint. Many weld defects, such as lack of fusion, lack of penetration and some discontinuities were recognized on the fracture surface, and a half of the cross-sectional area of the sheet pile was occupied by these weld defects. The stress applied on the true cross section excluding the weld defects was found to exceed the yield strength of the sheet pile. Hence, the cause of failure for the sheet pile was concluded to be the welding defects resulting from poor workmanship.
When a new circular saw consisting of 16 blades was used to cut a board, some of the blades suddenly failed and a fragment of blade chip stuck in an eye of the operator. The material of blade chip broken was WC-Co alloy and was brazed on a shank made of high carbon steel. The failure pass was limited in the WC-Co alloy. As a result of fractographic examination, a crack of about 150μm length was observed in a chip of the blade failed. The stress intensity factor of the blade chip approached the lower limit of the scatter of KIC for WC-Co alloy. Hence, the cause of failure was related to the pre-existing crack in the blade chip.