By using photoelastic coating technique, the author undertook a series of experiments on wood beam (red lauan), under the variation of the angle (θ) between the span direction and the longitudinal direction of wood, i. e. grain angle (θ) from 0゜ to 90゜, to measure the strain and to observe the failure form of the simply supported wood beam that carried a concentrated load P at midspan. The following conclusions may be drawn from the experiments :
1 ) The load-deflection curves are shown in Fig. 1. The strength, elastic modulus and stress in proportional limit, in bending were calculated from Fig. 1. These mechanical properties of wood beam are shown in Fig. 2, and are discussed. The strength value in bending to vary the grain angle θ are applied to HANKINSON's equation, and the coefficient n of the HANKINSON's equation equals 1.867 (n = 1.867). This value is the same as reported by other researchers, so it is considered that the strain distributions also was shown in expected range.
2 ) The photoelastically observed values (pattern in isoclinics (a), principal strain (b) and isochromatics (c) are shown in Fig. 3. Especially, patterns in principal strain (b) in Fig. 3 are illustrated against change of the grain to span direction.
3 ) The relationships between the variation with grain angle and normal strain, and shearing strain are shown in Fig. 4. In distribution of normal strain, compressive strains are attained to the maximum values in compression side (upper part) of wood beam, and tensile strains are attained to minimum values in tension side (lower part) of wood beam. The neutral axis shifts to tensile side along with the increase of the grain angle to span direction. Change in the distribution of the shearing strain are shown under the variation of the grain angle. The change may be divided into three major groupes of θ = 0゜, 22.5゜～ 45゜, and 67.5 ～90゜. Especrally in the groupe θ= 67.5゜ ～90゜, the deformations in part of wood beam take a U-shape as shown in Fig.8 (c, and so compressive strains are remarkably increased on compression side (upper part). Therefore, tensile and shearing strains are increased slightly on tension side (lower part) in wood beam, The bending strength in grain angle θ = 90゜ is dependent on the tensile strength perpendicular to grain, as the compressive deformation perpendicular to grain has the considerable plastic range.