Non Destructive Testing Basics

Nondestructive material testing with ultrasonics is more than 40 years old. From the very first examinations, using ultrasonic oscillations for detection of flaws in different materials, it has become a classical test method based on measurements with due regard to all the important influencing factors. Today it is expected that ultrasonic testing, supported by great advances in instrument technology, give reproducible test results within narrow tolerances.
This assumes exact knowledge of the influencing factors and the ability to apply these in testing technology. Not all influences have to be seriously regarded by the operator. In many cases some of the influences can be neglected without exceeding the permitted measurement tolerances. Due to this, the test sequence is simplified and the testing time reduced. Despite this, the future belongs to the qualified operator who carries out his task responsibly and who continuously endeavors to keep his knowledge at the latest state of the art.

At the beginning of the fifties the technician only knew radiography (x-ray or radioactive isotopes) as a method for detection of internal flaws in addition to the methods for nondestructive testing of material surfaces, e.g. the dye penetrant and magnetic particle method. After the Second World War the ultrasonic method, as described by Sokolovin 1935 and applied by Firestone 1940, was further developed so that very soon instruments were available for ultrasonic testing of materials. The ultrasonic principle is based on the fact that solid materials are good conductors of sound waves. Whereby the waves are not only reflected at the interfaces but also by internal flaws (material separations, inclusions etc.). The interaction effect of sound waves with the material is stronger the smaller the wave length, this means the higher the frequency of the wave.

This means that ultrasonic waves must be used in a frequency range between about 0.5 MHz and 25 MHz and that the resulting wave length is in mm. With lower frequencies, the interaction effect of the waves with internal flaws would be so small that detection becomes questionable. Both test methods, radiography and ultrasonic testing, are the most frequently used methods of testing different test pieces for internal flaws, partly covering the application range and partly extending it. This means that today many volume tests are possible with the more economical and non-risk ultrasonic test method, on the other hand special test problems are solved, the same as before, using radiography. In cases where the highest safety requirements are demanded (e.g. nuclear power plants, aerospace industry) both methods are used.