Two types of fatigue monitoring methods are experimentally examined for metal structures; fatigue crack detection and fatigue life prediction. As the detection method, five kinds of crack 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 compact specimen by adhesives or painting, and performances of each sensor are examined through the fatigue tests. As the prediction method, a fatigue coupon indicator of adhesive-type is developed. The main body of the coupon is a center cracked thin plate with 60mm-Iong, 10mm-width and 0.25mm-thickness, made from mild steel. The middle part in length of the plate is coated by Teflon film and the whole is sandwiched in between two thin epoxy resin plates. The coupon is bonded on a smooth specimen by adhesives, and the performance is investigated by pulsating tension and fully reversed fatigue tests.
There is a possibility of fatigue failure in any repeatedly loaded structures. It would be useful to know when failure may occur. There are means of predicting this at the design stage, but in general the prediction can only be very approximate. In order to compensate the uncertainty, in-service inspections are carried out for most of the structures; Goranson (1983), Fujimoto (1991). While, inspections are carried out intermittently. Also, crack detection by inspection is usually probabilistic. If the present and former inspections fail to detect the initiated cracks, they are helpless for the resultant failure which may occur until the next inspection. The idea of structural monitoring was presented to cover such incompleteness of in-service inspections. There are two ways of monitoring for fatigue safety. One is to detect the initiated crack at early stage and the other is to predict crack initiation time in the future; Smith (1970), Gerardi (1990), Fujimoto (1994) (1995).
