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9/8/98 AC 43.13-1B (3) Underlying structures contain dif- ferences in; core material, cell size, thickness, and height, back skin material and thickness, doublers (material and thickness), closure member attachments, foam adhesive, steps in skins, internal ribs, and laminates (number of layers, layer thickness, and layer material). (4) The top only or top and bottom skin of a bonded structure may be accessible. b. Application of Ultrasonic Inspection. Application to bonded structures must be ex- amined in detail because of the many inspec- tion methods and structural configurations. The advantages and limitations of each in- spection method should be considered, and reference standards (representative of the structure to be inspected) should be ultrasoni- cally inspected to verify proposed techniques. c. Internal Configuration. Complete in- formation on the internal configuration of the bonded test part must be obtained by the in- spector. Drawings should be reviewed, and when necessary, radiographs of the test part should be taken. Knowledge of details such as the location and boundaries of doublers, ribs, etc., is required for valid interpretation of ul- trasonic inspection results. The boundaries of internal details should be marked on the test part using a grease pen or other easily remov- able marking. d. Reference Standards. Standards can be a duplicate of the test part except for the controlled areas of unbond. As an option, simple test specimens, which represent the varied areas of the test part and contain con- trolled areas of unbond, can be used. Refer- ence standards must meet the following re- quirements. (1) The reference standard must be similar to the test part regarding material, ge- containing: closure members, core splices, stepped skins, and internal ribs similar to the test part if bonded areas over or surrounding these details are to be inspected. (2) The reference standard must contain bonds of good quality except for controlled ar- eas of unbond fabricated as explained below. (3) The reference standard must be bonded using the adhesive and cure cycle pre- scribed for the test part. e. Types of Defects. Defects can be sepa- rated into five general types to represent the various areas of bonded sandwich and laminate structures as follows: (1) Type I. Unbonds or voids in an outer skin-to-adhesive interface. (2) Type II. Unbonds or voids at the adhesive-to-core interface. (3) Type III. Voids between layers of a laminate. (4) Type IV. Voids in foam adhesive or unbonds between the adhesive and a closure member at core-to-closure member joints. (5) Type V. Water in the core. f. Fabrication of NDI Reference Stan- dards. Every ultrasonic test unit should have sample materials that contain unbonds equal to the sizes of the minimum rejectable unbonds for the test parts. Information on minimum rejectable unbond sizes for test parts should be obtained from the OEM’s manuals, FAA re- quirements, or the cognizant FAA Aircraft Certification Office (ACO) engineer. One or more of the following techniques can be used in fabricating reference defects; however, since bonding materials vary, some of the methods may not work with certain materials. ometry, and thickness. This includes Par 5-94 Page 5-45PDF Image | AFS-640
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