Detection of Cracks Under Installed Fasteners

Résumé du livre

THE OVERALL OBJECTIVE OF THIS 2-YEAR PROGRAM IS TO DEVELOP RELIABLE NONDESTRUCTIVE INSPECTION METHODS FOR DETECTING CRACKS UNDER INSTALLED FASTENERS, EMPHASIZING IMPROVEMENT OF THE ULTRASONIC SHEAR WAVE METHOD. A SIGNIFICANT ASPECT OF THIS PROGRAM IS THE DEVELOPMENT OF AN ULTRASONIC FASTENER HOLE SCANNING AND SIGNAL DISPLAY SYSTEM SUITABLE FOR ON-LINE INSPECTION OF AIRPLANES. THE PAST 6-MONTH EFFORT INCLUDED EVALUATION OF POTENTIAL ULTRASONIC TECHNIQUES AND DEVELOPMENT OF THE MOST PROMISING TECHNIQUE FOR THE DETECTION OF CRACKS INITIATING AT THE BASES OF COUNTERSINKS AND AT THE UPPER EDGES OF STRAIGHT HOLES. AS WITH THE CASE OF LOWER SURFACE HOLE CRACKS, A SIGNIFICANT DIFFERENCE WAS EXPERIENCED BETWEEN THE ULTRASONIC RESPONSE FROM NOTCHES AND CRACKS OF THE SAME SIZE. A WIDE VARIATION IN CRACK RESPONSE WAS EXPERIENCED. THE MINIMUM RADIAL DEPTH CRACK DETECTED AT THE BASE OF A COUNTERSINK AND AT THE UPPER EDGE OF A STRAIGHT HOLE WAS 0.040 IN. HOWEVER, A CRACK WITH AN ESTIMATED RADIAL DEPTH OF 0.080 IN. WAS NOT DETECTED AT THE UPPER SURFACE OF A STRAIGHT HOLE. THE INFLUENCE OF AN ALUMINUM/SEALANT INTERFACE ON A REFLECTED SHEAR WAVE IS STUDIED. CURVES ARE DEVELOPED RELATING REFLECTED ENERGY TO ANGLE OF INCIDENCE. TRANSDUCER REQUIREMENTS THAT SUPPORT THE FASTENER HOLE INSPECTION APPROACH HAVE BEEN FURTHER IDENTIFIED. PORTABLE SYSTEMS FOR SCANNING AROUND FASTENER HOLES AND FOR SIGNAL DISPLAY AND SCAN CONTORL HAVE BEEN DESIGNED AND PROTOTYPE HARDWARE FABRICATED. THE PORTABLE FIELD SYSTEM INCLUDES MANUAL OR AUTOMATIC SCANNING AND PERMITS THE SELECTION OF DATA PRESENTATION AS A RADIAL DEFLECTING CIRCULAR SCAN OAS A CIRCULAR OR LINEAR B-SCAN. THE SYSTEM IS DESCRIBED AND ITS FUNCTIONS EXPLAINED.