SOME CONNECTIONS BETWEEN AE TESTING OF LARGE STRUCTURES AND SMALL SAMPLES

Abstract It is possible to transfer results from laboratory specimens to field tests when the AE waves are measured and understood according to sound theoretical principles. There are several other advantages to measuring the waves. Various types of noise, ever present in testing situations, can be distinguished from the wave modes of interest by examining the characteristics of measured signals. This paper provides some basic wave propagation examples which equip the reader with the knowledge necessary to understand how to approach making a transition from all the database material already collected in certain field testing applications and put such testing on a firmer physical basis. For example, typical measurements of signal strength may be grossly in error. A way to prevent this is described. Reflections severely influence parameter data from laboratory specimens and must be eliminated in order to compare with field data where reflections are not as prevalent. In both laboratory and field tests it is the direct part of the wave which should be measured. To do this, it is necessary to set the instrument so that the signal strengths or energies of the direct waves are measured. Even when looking at the direct waves, care must be taken in analyzing signal strengths. Measuring the signal strength with an envelope beginning with the extensional mode and ending after the flexural mode would be grossly in error since the space in between the modes doesn't count. The envelope must take the two modes into account separately. Thus, simply checking an instrument with a function generator using a single burst is inadequate. A dual burst closely spaced in time, say 20 to 100 us apart, would be a better test.