Selecting a gauge length to measure parallel-to-grain strain in southern pine

For foil-bonded or mechanically attached gauges to accurately measure strains in wood, the gauge must be sufficiently large to cover a representative area of annual growth-ring increment. Too small a gauge (0.25 in.) or too large a gauge (4.0 in.) will sometimes record strain that is not representative of the average macroscopic deformation of the material under the gauge. In this study, foil-bonded gauges ranging in size from 0.25 inch to 4.0 inches and a 2.18-inch clip-on electrical transducer were randomly attached at selected locations (consisting of earlywood, latewood, and combined earlywood-latewood zones) on a southern yellow pine specimen. The specimen was subjected to a constant axial tension load. The results indicate that optimal gauge length, representative of the material response around a point, ranges from 2.0 to 2.5 inches for measuring strains parallel to the grain in southern yellow pine tension specimens.

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