Volume 23, Issue 2 (March 1995)
Comparison of Optically and Mechanically Measured Deformations in a Finger-Jointed Wood Tension Specimen
An advanced finite element program was developed to predict the elastic stresses and strains in a specimen of finger-jointed lumber subjected to uniaxial tension. In this study, two techniques were examined to experimentally determine the deformations in the finger joint. The first method involved photographing at incrementally increasing load levels, predefined targets mounted on the finger joint and measuring the changing distances between targets using stereoscopic instruments. The second method consisted of placing microstrain gages at key locations in the finger-jointed region of the lumber.
The results showed that even when the most sensitive film was used with a camera and planar lens, the photographic technique lacked the sensitivity to accurately measure the small deformations in the finger joint. The actual errors in deformation measurement were approximately twice the magnitude expected from an analysis of the properties of the film being used. Much of the error was due to the thermal expansion of the film in the stereoscopic instrument used to measure deformation. The microstrain gage technique was shown to be sufficiently sensitive to measure the deformations in the finger joint. However, the time, effort, and cost required to mount the tiny sensors with appropriate attention to their alignment made this technique impractical for any experiment other than those that involved placing only a modest number of sensors on a small number of specimens.