Downloaded from https://www.everyspec.com AMCP 706-196 effectively removed In the return. Also, fluws Lamb waves are elastic vibrations analogous that are not accessible by a simple geometry to ripples in the whole material. These waves can be detected by letting the pulse zig-zag,, have proven useful in detecting nonbonded from one boundary to anothar until a flaw is areas in laminated structures where vibration reached, Thus, rather complex geometries In localized areas induced by the Lamb waves can be probed by this method. can be sensed. The Lamb wave technique is capable of detecting cracks that extend as little as 0.001 In. below the surface of a 8-7.2 TRANSMISSION METHOD material.
The tiansmission method is very similar to the pulse echo technique. A pulse Is In. 8.7.4 ADVANTAGES AND DICADVAN- troduced at one boundary of a material and TAGES sensed at anothe;,. The energy transmitted past the flaw is attenuated due to reflection. One of the major advantages of ultrasonic The energy received Is, therefore, less when test techniques is their ability to penetrate a flaw Is present than when there is no flaw. into a material to locate flaws. This depends This energy decrease indicates the surface on available power and sensitivity of the position of a flaw, but gives no measure of detection equipment; however, the technique depth.
has been used to locate flaws as deep as 30 ft down in a metallic bar, It alao permit rapid measurements and is economical, 8.7.3 RESONANCE METHOD relatively sensitive, and reasonably accurate for measuring flaw extent and porition, Ac- In the resonance method, a material is cessibility to a single surface ii adequate for excited at its thickness resonant frequency, detecting many flaws and aikoma;ies, The material is driven by a transducer which is, in turn, controlled by a variable frequency The resolution of ultrasonic test rnethods oscillator. When the resonant frequency is depends on the frequency of the ultrasonic reached, a standing wave will be established propagntion.
The higher the frequency, the between the material faces. As the surface smaller the defect that can be resolved. A of the material is scanned, any change in reso- Ilifltini, factor is that absorption oa' ultrasonic nant frequency not associated with material energy Increases with increasing frequency. , i thickness indicates a flaw. This method is Therefore, a trade-off between frequency and used for thickness measurement as well as for available energy must be made. Generally flaw detection. Both longitudinal and trans- available equipment permits detection of flaws verse ultrasonic waves are used. Special prop- with dimensions in the 0.001 to 0,005 in.
agation modes called Rayleigh waves and Lamb range, waves are less frequently used. The inconvenience of getting ultrasonic 1 Rayleigh waves are surface waves analogous energy Into and detecting the enerey from a to ripples on water ani ,;an be generated by specimen is one of the major disadvantagem of controlling the angle of incidence of the Input u.ltrasonic methods. Alit does not provide the ultrasonic energy. The Rayleigh wave tech- proper impedance match between the trams- ndque is useful for scanning across the surface ducer and specimen, ao that liquid couplants of an item for flaws near the surface. A dis- such as oil, water, or glycerine are iequired.
tinct advantage of this method is its useful- Another major disadvantage Is in readout of .F ness in investigating curved surfaces. the information. Display of pulse positions on -` ( L/ L n .•x * `+m + ` r r e ` b ...... .... .... . . w . t + ,t .. ....-.. •.. . . .. . . ..... ...... I, .. . .... .. .. .......... . ... .. ... . I I . .. . -'• . . . . ..... ' .......