Remote Tapping Inspection Method Employing a String Shooter

Abstract Tapping inspections of structures are commonly performed through manual efforts by inspectors. However, in the case of nuclear structures, there may be situations where it is preferable to avoid close proximity to the objects under inspection. Moreover, there is a desire for the automation of these inspections. This study proposes a completely new tapping inspection method that enables inspections without the need to approach the object. Our specific approach involves the use of a string shooter for tapping inspections. Known commonly as a toy that launches a looped string into the air using two rollers, we hypothesized that attaching a steel ball for impact to this string could enable remote tapping of the inspection object. Consequently, we attached a 4mm diameter stainless steel ball to the string of a string shooter and attempted to detect a circular void defect with a diameter of 300mm within a concrete test specimen measuring 600mm × 600mm × 175mm from a distance. When the specimen was randomly tapped from a 2-meter distance, a unique acoustic signal was generated above the void defect. By assigning colors to the impact points based on the intensity of the acoustic frequency spectrum, we were able to differentiate the area corresponding to the void defect from healthy areas by color, thus facilitating the identification of void defect areas. This method allows for tapping inspections to be conducted remotely from several centimeters to several meters away, and it is capable of tapping inspection surfaces in any direction, from horizontal to vertical. By mounting the inspection device on a drone, it also becomes possible to inspect high places.