円孔

The purpose of this study is to obtain basic data on the influence of the stress concentration on the strength of wooden members with an artificial circular hole in them. Three kinds of test materials were used, namely, Hemlock (Tsugahetellohylla Sarg.), ninelayer plywood and threelayer particleboard. A circular hole was bored at the center of the specimen, and was varied by the diameter of it. Tensile and bending tests of three materials were performed. The bending tests were carried out by center loading method and the bending moment was parallel to the plane on which the hole was located. Repetitive bending tests were also carried out for Hemlock. Notch sensitivity (η) was calculated based on the relationship between stress concentrated factor (α) and the ratio of fracture load of control specimen to that of holed specimen (β), and from the calculated value of η, the effect of the hole on the strength reduction of the specimen was estimated. The results obtained are as follows. In the cases of multilayer materials such as wood (earlywood layer-latewood layer), plywood and particleboard, the tensile and bending strength of these specimens with a hole are affected more pronouncedly by the layer construction than by the stress concentration due to a circular hole. For example, when the material has stronger layers in both surfaces as particleboard, it has little strength reduction by drilling a hole in inner layer. When a bending load is applied, the specimen tends to fracture on compression side at lower load level. The strength reduction due to a circular hole in bending test is greater than that in tensile test. In tensile and bending test, the value of notch sensitivity in the case of Hemlock is largest of the three materials tested. The value for particleboard, which is porous material, is smallest. And plywood shows a mendium value of them. In the repetitive bending test with the constant deflection of specimen (Hemlock), the fatigue strength is scarcely effected by stress concentration caused by a circular hole because it is affected chifly by stress relaxation.