Revisiting the neutral axis in wood beams

For wood beams, it is often assumed that the neutral axis (NA) is located at the centroid of the beam. This would be the case for isotropic and homogeneous materials but these prerequisites are not valid for wood. The varying grain patterns and knots located throughout wood make wood anisotropic and non-homogeneous. Knowledge of the true location of the neutral axis would facilitate a better understanding of the mechanical behavior of wood beams. To analyze this question, a digital image correlation technique (speckle photography) was applied while wood beams with the dimensions 25×25 mm2 and 38×89 mm2 were loaded in bending. The NA was determined by axial strain plots. From the data obtained it is was observed that the neutral axis of 25×25 mm2 beams is located below the centroid while for 38×89 mm2 beams NA was above the centroid. As would be expected, knots change the location of the NA depending on their location in the beam.