Measurement of the shearing properties of wood by in-plane shear test using a thin specimen

In this research, in-plane shear tests were conducted for obtaining the shearing properties of wood. Thin strips of western hemlock (Tsuga heterophylla Sarg.) were used for the specimens. Two circular holes were cut along the axial centerline of the wider surface (longitudinal-tangential plane), and then two slots were cut from the holes asymmetrically to each other by varying the inclined angle of the slot with respect to the axial centerline. Shear stress was induced in the area between the holes, which is called the shear zone, by applying a tension load along the long axis of the specimen; the shear modulus, shear strength, and principal strain angle were measured from the shear stress/shear strain relation obtained. Independently of the in-plane shear tests, Iosipescu shear tests were conducted, and the validity of the in-plane shear tests was examined by comparing the test results with those obtained by the Iosipescu shear tests. In addition to the tests, stress distribution in the in-plane shear test was calculated by the finite element method, and the results were compared with those obtained by the actual in-plane shear tests. The following three results were obtained. First, the shear moduli obtained by the in-plane shear tests were close enough to those obtained by the Iosipescu shear tests throughout the range of inclined angle examined here. Additionally, the calculated result of the finite element method also verified the validity of the in-plane shear test for measuring the shear modulus. Thus, the in-plane shear test method examined was effective for measuring the shear modulus. Second, a failure was initiated at the hole edge because of the stress concentration, which was also confirmed by the finite element calculation, and it immediately propagated along the grain in the early stage of the test. Thus, it was difficult to obtain the proportional limit stress and realistic shear strength by the in-plane shear test because of the catastrophic failure in the early stage of the test. Third, since it is desirable that the stress field in the shear zone is close to the pure shear stress condition, we recommend that the slot runs outward from the hole when conducting the in-plane shear test in spite of the independence of the shear strength on the inclined angle.