Sec. 1-1] 



MECHA.XICAL I.XI'CT THAXS DTCERS 



17 



about 10 per cent. The breakdown voltage varies with the com- 

 position of the alloy. A comparative graph for different test materials 

 is shown in Fig. (1-1)14. 



For references see Anon., Bell Lab. Record, 20, 27S (1942); Am.Soc. Testing 



Materials, Standard B, p. 110; J. Herenguel and li. Negond, M eta u.r -corrosion, 

 20, 1 (1945). 



1-15. Ultrasonic Method 



The method uses an ultrasonic output transducer T, as shown in 

 Fig. (1-1)15. The oscillating end of the transducer is brought in 

 acoustical contact with the mate- 

 rial, and the frequency of the 

 oscillation is varied. At a certain 

 frequency / and at harmonics of 

 this frequency, standing waves are 

 set up in the wall; the internal 

 damping of the material causes a 

 sharp increase of the loading of 

 the oscillator, and the power ab- 

 sorption from the transducer rises 



in the form of a resonance curve. The thickness t of the test object 

 can be found from 



n = 



Variable 

 frequency 

 oscillator 



Fig. (1-1)15. Schematic diagram of an 

 ultrasonic thickness gauge. 



/ 



C 



2/ 



where n is an integer ( 1 , 2, 3, . . .), c the sound velocity in the material 

 of the test object, and/ the resonance frequency. 



Steel walls ranging in thickness from 0.125 to 12 in. have been 

 measured with this method. The accuracy is better than 5 per cent. 

 The system permits measurement of a wall thickness from one side 

 without requiring access to the other side, but the method requires 

 the knowledge of c, the propagation velocity of the longitudinal 

 sound waves in the material. 



L. Bergmann, "Der Ultraschall," 5th ed., pp. 536ff, S. Hirzel Verlag, Zurich, 

 1949; G. N. Branson, Electronics, 21, 88 (January, 1948); also, B. Carlin, 

 Electronics, 21, 76 (November, 1948). A summarizing paper describing different 

 modifications of the ultrasonic method, such as pulse-echo methods, has been 

 published by A. Lutsch and W. Bohme, Arch. tech. Messen, V 1 124-6, May, 1957. 



1-16. Radiation Thickness Gauges 



(Alpha-, Beta-, Gamma-, X-ray Gauges) 



The gauge consists of a shielded source for penetrating radiation 

 and a radiation detector such as an ionization chamber, a Geiger 

 counter, or a scintillation counter. Variation of the thickness of the 



