Development of Crack Monitoring Sensor for Ship Structure

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The applicability of structural monitoring is discussed for crack-type damages of metal ships and offshore structures. Two types of monitoring methods are examined ; perception of existing crack and forecast of crack damage.As the perception of existing crack, the concept of health monitoring is employed. Five types of line sensors, i.e. conductive film-sensor, conductive-paint sensor, plastic optical-fiber sensor, glass optical-fiber sensor and carbon-fiber sensor are made on an experimental basis. The sensors are bonded on the surface of a welded joint model and two types of specimens by adhesives or painting, and fatigue tests are carried out. In these tests, the appearance that the fatigue cracks break the sensors is closely observed. Further, electric resistance of sensors or light strength output of laser beam are measured to perceive the instant of failure during the fatigue test. Based on the results of the experiments, performances of each type sensor are discussed.In order to forecast fatigue damage, method to attach sacrificial specimen on member is examined. Sacrificial specien is designed such that magnified member stress is transmitted to the specimen and thus results in earlier crack initiation than the membe. For this purpose, a rectangle thin plate with 60mm-length, 10mm-width and 0.25mm-thick, which has an artificial crack in the center, is developed. The middle part in length of the specimen is coated by teflon film and the whole of the specimen is sandwiched in between two thin epoxy resin plates. The sacrificial specimen is bonded on a smooth fatigue specimen by epoxy resin, and fatigue tests were carried out under pulsating tension and fully reversed stress conditions. From the test result, it is concluded that the sacrificial specimen developed in this study can forecast the fatigue initiation of member for both stress conditions.