Size Effect of Wood in Bending

In this study, theory of size effect of wood in bending is discussed. Prior to theoretical study, measurement of size effect on Western hemlock (Tuga heterophylla Sarg.) in bending was carried out. Size effect factor of MOR（Modulus of rupture）is 58.0 and that of proportional limit is 34.5. But size effect of MOE（Modulus of elastisity） was not recognized. Change of strain distribution with increase of load was measured, and the following behavior was observed : Plastic deformation begins at compression surface near load of proportional lmit of load-deflection curve, and after that, neutral axis moves to the tension side with increase of load. Assuming that (i)stress-strain relation in compression is elastoplastic and (ii) size effect in compression dose not exist, the following equation was derived for the relation between size effect in bending and that in tension ; mb = mi log (k2/k1) / log (3k2-1/1+k2)・1+k1/3k1-1) where k2 = (d1/d2)3/mi k1, k1 : ratio tensile strength and compressive strength of beam with size d1, b : in bending, i : in tension, m : factor of size effect i. e. ó2cr/ó1cr = (d1/d2)3/m, ó1cr, ó2cr : MOR of beam with height of d1, and d2, respectively