Examples of Dynamic Testing in the Field of Structures and Materials
Authors: McHenry Douglas , Hosticka Harold E. 
Published: July 2009

In the minds of many engineers, the subject of dynamic testing is shrouded in an aura of deep mystery. This comes about, no doubt, because of the great advances which have been made recently in the fields which require intricate and expensive testing equipment and elaborate techniques. The complete vibration record of an airplane in flight is apt to involve rather spectacular instrumentation. However, these elaborate applications need not blind the engineer to the more everyday cases in which dynamic observations may be useful. Probably the simplest of all dynamic tests is the well-known “hammer test,” in which the quality of a material or a structural part is judged by a rather crude determination of its natural frequency and damping capacity. Even this crude test may acquire quantitative significance if the tone is matched with a musical note of known pitch. Dynamic testing equipment, too, may often be simple and inexpensive. A great deal may often be learned about a vibrating machine part or structural member by gluing a bit of fine sandpaper to it, illuminating the sandpaper obliquely, and observing the motion of the particles through a microscope suspended in such a way as to be free from vibration. If a record of the vibration is desired, something as simple as the de Forest scratch gage may suffice (Fig. 1). This gage, which costs only a few dollars, simply scratches a record of the movements onto a polished metal plate for later study under a microscope. However, these simple cases are perhaps as rare as the very elaborate ones mentioned previously. In this paper, examples will be given of some rather ordinary engineering applications which fall in an in-between category. The examples described in the following pages are from the records of a large and diversified engineering organization engaged in the construction of dams, powerplants, irrigation canals, and related structures. As in most engineering organizations, the equipment available to the Bureau of Reclamation for making dynamic measurements is not unlimited. Certain basic units such as recording oscillographs, Carlson strainmetefs, geophones, and SR-4 bonded resistancewire strain gages have been purchased specifically for this use; other items, such as oscilloscopes and motion picture cameras, are borrowed from their regular duty and adapted for this work. For specific problems where commercially available instruments cannot be applied, special units are designed and constructed in the laboratories. When appraising a new problem for its most expedient solution, first consideration is given to apparatus already on hand, and in consequence the instrumentation is more often adequate than ideal. It should be clear that the examples given in this paper do not necessarily describe the best instrumentation of the problem, but rather that instrumentation which gave the most economical and yet satisfactory solution to the problem at the time. From the viewpoint of the testing engineer, a structure is simply an assemblage of materials set upon a foundation. However, if the desired structural results are to be obtained, it must be designed so that the proper materials are assembled in the proper manner on an adequate foundation, and it happens not infrequently that this result can be achieved or verified only by laboratory or field tests upon the materials, the foundation, or the structure. Even if the structure is intended only for static loading, certain types of dynamic tests may be of value in indicating the quality and physical properties of the materials and the foundation, or in estimating the over-all continuity and adequacy of the structure.