Defining the Technique

Figure 5-65. Double Shield for Reducing External Noise Signals.

SECTION IV ULTRASONIC INSPECTION OF BONDED STRUCTURES

TM-1-1500-335-23 Nondestructive Inspection Methods Manual
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T.O. 33B-1-1 5-61 5.3.7.1.1 Information on many of the above items can be obtained by visual examination of the test part and study of applicable manuals and drawings. Examination of failed parts is helpful for obtaining information on the location of and type of discontinuity causing failure. 5.3.7.2 Defining the Technique. The information required by paragraph 5.3.7.1, along with the information in this chapter, is used to establish the technique variables. In addition, if welds are to be inspected, refer to T.O. 00-25-224. Items that need to be defined are listed below and described in more detail in the subsequent paragraphs. a. Inspection surfaces. b. Mode(s) of inspection: longitudinal, shear and/or surface wave, contact or immersion. c. Scanning plan d. Reference standard(s). e. Transfer method. f. Frequency. g. Search unit. h. Requirements for special wedges or shoes. i. Surface preparation required and method to be used. j. Couplant. 5.3.7.2.1 Inspection Surfaces, Scan Plan and Mode(s). The expected location and orientation of discontinuities, along with accessibility of the inspection area, are used to help define which surfaces will be used for sound entry, the mode(s) of sound energy that will be used and the scanning procedure. The sound should be directed normal to the expected plane of the largest surface of the discontinuity. Therefore, straight beam inspection would be used to locate laminar discontinuities, and angle beam inspection would be used to locate internal discontinuities that are not parallel to the inspection surface. The beam angle should be chosen to be perpendicular to the largest surface of the discontinuity. In the case of discontinuities at the inspection surface, surface wave inspection may be a better choice. 5.3.7.2.2 Reference Standard. The reference standard should be fabricated from material with the same acoustic properties as the test part. When possible, the reference standard should be of the same alloy, heat treat condition, same hot/cold working condition, and surface condition as the test part. The transfer technique (paragraph 5.3.5) should be used to compensate for differences between the reference standard and the test part. The geometry of the reference standard should match the geometry of the test part so that the sound path will be the same. The simulated discontinuities should be in accordance with the applicable specification for the test part. Refer to MIL-STD-2154 for general information. 5.3.7.2.3 Frequency. The frequency is selected based upon the acceptance criteria and the acoustic properties of the test part. A good rule to remember is, “Use the highest frequency that will provide the necessary depth of penetration”. When geometry permits, the test part shall be checked at the intended frequency to verify that a strong back reflection is obtained. The frequency also should be appropriate for detecting the minimum size discontinuity anywhere in the test part (see paragraph 5.4). Frequencies of 2.25 MHz, 5 MHz and 10 MHz are popular inspection frequencies. When using both angle beam (either refracted shear or longitudinal wave) and straight beam (longitudinal wave) methods, the frequency

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