Piezoelectric Effect in Wood

The work reported here indicates piezoelectricity has promise as a tool for nondestructive measurement of local strain in a dynamic system, and its voltage patterns may furnish clues to nondestructive methods of determining the effect of growth rate, density, knots, and other conditions on a dynamic system. This particular method was developed in the continuing search for methods of nondestructive testing. In the initial experimentation the research was designed to study adhesive bonds in scarf joints. The system did provide encouraging correlation between signal response and certain glue bond defects; however, the limited correlation between signals and defects proved not to be the principal benefit derived from the research. The proposed research was designed to examine in more detail the dynamic response characteristics, including the elastic wave length of sizeable specimens subjected to strain. Three important phases were to be investigated: 1) The wood characteristics which might be classed as environmental, 2) the wood characteristics which might be termed modifying conditions or defects and 3) crystallinity. The most practical method for signal display was the use of a differential preamplifier input to an oscilloscope. The differential input required two electrodes. The voltage observed after impact of the wood specimen is caused by the piezoelectric property of the wood. After further refinement it may be possible to obtain useful measurements of local strain in a dynamic system. Wood properties such as density and growth rate, as well as local defects, affect the magnitude and physical orientation of the piezoelectric patterns. In a dynamic system, the mechanical properties of wood related to wave propagation are of equal importance.

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