Feasibility of Using the Piezoelectric Properties of Wood for Structural Health Monitoring

Abstract

Wood has many talents, including, among others, the possession of growth characteristics that provide it with stiffness and strength in key directions, a naturally superior resistance to fatigue loading compared with many other structural materials, and piezoelectric properties that, as we are wont to say, must be there for some reason. A piezoelectric material is one that becomes electrically polarized when mechanical stress is applied, a so-called direct effect. A converse effect is the onset of deformation when the material is placed in an electric field. These are characteristics exhibited by many materials, among them many polymers, and wood is a piezo-active polymer. The most widely accepted reason given for the piezoelectric property in wood comes from the microfibrils that make up part of the cell wall structure. These cylindrically-shaped structures, which have diameters in the 10-30 nm range, consist of regions with both parallel and more amorphous arrangements of chains of cellulose molecules. It is in the more orderly parallel portions of this structure, called crystallites because in the aggregate they have a crystalline structure, that the piezoelectric effect has its origin. Piezoelectric properties of wood are correlated with the degree of crystallinity. Given that piezoelectricity exists in wood, can this electrical property be used in such applications as structural health monitoring since, as reported below, its voltage potential appears to be proportional to the magnitude of applied stress? The results reported herein are from a computational study into the feasibility of using surficial voltage or charge measurements to predict mechanical stress unobtrusively for the purposes of structural health monitoring. The present study is focused on modeling dry wood subjected to static loads; future research will include wood with moisture and cyclic loads.