Development of a Field Inspection System for Detection of Cracks Under Installed Fasteners

The work that I'm going to describe was funded by the Air Force Materials Lab from February 1972 to February 1974. It is described in an AFML report, TR74-80, available from the Air Force. This report describes a more practical type of problem than has been discussed so far. Our work was directed towards building an ultrasonic system to be used on aircraft in the field to detect cracks around fastener holes with the fastener in place. With this system we are able to detect cracks and locate them relative to the depth of the hole. Estimates of crack size by this method are not very accurate. Disciplines Materials Science and Engineering | Structures and Materials This 6. transducers is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/cnde_yellowjackets_1975/45 DEVELOPMENT OF A FIELD INSPECTION SYSTEM FOR DETECTION OF CRACKS UNDER INSTALLED FASTENERS W. Woodmansee The Boeing Company Seattle, Washington The work that I'm going to describe was funded by the Air Force Materials Lab from February 1972 to February 1974 . It is described in an AFML report, TR74-80, available from the Air Force. This report describes a more practical type of problem than has been discussed so far. Our work was directed towards building an ultrasonic system to be used on aircraft in the fie ld to detect cracks around fastener hol es with the fastener in place. With this system we are able to detect cracks and locate them relative to the depth of the hole. Estimates of crack size by this method are not very accurate. At present, hole inspection is largely performed by visual or radiographic methods which are only sensitive to rather large cracks around fasteners. Alternatively , the fastener may be removed and an eddy current probe passed into the hole to sense the presence of cracks. There are good reasons for not taking the fastener out if you can avoid it. In the process of removal and replacement, holes are frequently damaged and become in worse condition than before fastener removal. Some of the advantages of ultrasonic methods are listed in Fig. 1. It is not necessary to remove the fastener, you can look at holes more rapidly, and the possible damage to holes by fastener removal and reinstallation is avoided. The ultrasonic method should be a more rapict and less costly inspection technique and, by early crack detection , one can frequently dri ll out a hole to enlarge it and replace the fastener with a larger fastener and continue to use the structure. What we are attempting to do is to detect cracks at the locations seen in Fig. 2 at the base of the countersink in flush head fasteners and at the upper and lower surfaces in straight holes. In al l cases we are restricted to detection of cracks in the skin layer in which sound is injected. Reference will be made to radial depth and bore length of cracks as seen in the sketch in Fig. 2. Typically, these are about the same dimension for cracks found in aircraft structures. These cracks are detected with conventional shear wave ul trasonic pulse echo techniques. The specific requirements of our contract, given in Fig. 3, included the use of the shear wave techn ique. A pilot study by a separate organization, prior to our work on this problem, suggested this was the most likely way to improve the inspection of these holes. We were to automate the techni que and to produce a prototype unit which could be taken to the field and demonstrated on operational Air Force aircraft.