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AC 43.13-1B 9/8/98 (1) Standards for Types I, II, III, and IV unbonds can be prepared by placing discs of 0.006 inch thick (maximum) Teflon sheets over the adhesive in the areas selected for un- bonds. For Type II unbonds, the Teflon is placed between the core and adhesive. The components of the standard are assembled and the assembly is then cured. (2) Types I, II, and III standards can also be produced by cutting flat-bottomed holes of a diameter equal to the diameter of the unbonds to be produced. The holes are cut from the back sides of bonded specimens, and the depths are controlled to produce air gaps at the applicable interfaces. When using this method, patch plates can be bonded to the rear of the reference standard to cover and seal each hole. (3) Type II standards can be produced by locally undercutting (before assembly) the surface of the core to the desired size unbond. The depth of the undercut should be sufficient to prevent adhesive flow causing bonds be- tween the undercut core and the skin. (4) Type IV standards can be produced by removing the adhesive in selected areas prior to assembly. (5) Type V standards can be produced by drilling small holes in the back of the stan- dard and injecting varying amounts of water into the cells with a hypodermic needle. The small holes can then be sealed with a small amount of water-resistant adhesive. g. Inspection Coverage. Examples of several different configurations of bonded structure along with suggested inspection cov- erages with standard ultrasonic test instru- ments are shown in figure 5-21. In many cases, access limitations will not permit appli- cation of the suggested inspections in all of the areas shown. The inspection coverages and suggested methods contained in figure 5-21 and table 5-7 are for reference only. Details of the inspection coverage and inspections for a particular assembly should be obtained from the OEM’s manuals, or other FAA-approved requirements. h. Inspection Methods. Table 5-8 lists the various inspection methods for bonded structures along with advantages and disad- vantages of each inspection method. 5-95. BOND TESTING INSTRUMENTS. Standard ultrasonic inspection instruments can be used for bond testing as previously noted; however, a wide variety of bond testing in- struments are available for adaptation to spe- cific bonded structure inspection problems. a. General Principle. Two basic operat- ing principles are used by a variety of bond testers for single-sided bond inspection. (1) Ultrasonic resonance. Sound waves from a resonant transducer are transmitted into and received from a structure. A disbond in the structure will alter the sound wave charac- teristics, which in turn affect the transducer impedance. (2) Mechanical impedance. Low- frequency, pulsed ultrasonic energy is gener- ated into a structure. Through ultrasonic me- chanical vibration of the structure, the imped- ance or stiffness of the structure is measured, analyzed, and displayed by the instrumenta- tion. b. Operation. In general, operation of the adhesive bond test instruments noted is simi- lar. The test probe is moved over the surface in smooth overlapping strokes. The direction of the stroke with regard to the surface is gen- erally immaterial; however, when using the Sondicator models, the direction of the stroke becomes critical when the test probe is Page 5-46 Par 5-94PDF Image | AFS-640
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