Synopsis…It is well known that, in the testing of materials, the test results are influenced by the shape and dimensions of the test specimen. In the Charpy impact test on metals and sometimes on woods, there is used the "Impact value" to express the absorbed energy required to rupture divided by the nominal area of the section. But, there would be something doubtful in this expression which assumed that the absorbed energy would be proportional to the sectional area of the test specimen. Then, the author investigated to find the law of similitude to be applied to the impact test of wood.
The materials used are Lawson cypress and Mahogany and the tests were carried out by the Amsler-type universal wood testing machine. The capacity of the machine for impact is 10kg-m and the standard test specimen is of the sizes 2cm×2cm square section and 30cm length. Span is 24cm. The length and the span of the test specimen were always constant throughout the experiments and the depth and breadth were varied for several values between 0·5cm and 3·0cm.
The following conclusions were introduced…
(1) The relation between the energy required to rupture U and the dimensions of the test specimen may be expressed by the empirical formula
U=bd(A+Bd+Cdd²)
where b, the breadth, d, the depth of the specimen and A, B and C are constants independent of b and d. From the above formula it will be seen that, when the depth is constant, the energy is proportional to the breadth of the specimen.
(2) Energy/sectional-area which is the so-called "Impact value" have no meaning owing to the fact that the impact energy is not proportional to the sectional area of the specimen.
(3) When the sectional area is constant, the deeper the depth, the larger the energy.
(4) The absorbed energy rapidly decreases according to the increases of the ratio L/b or L/d (where L is the span).
(5) The relation between the equivalent modulus of rupture and the dimentions of the test specimen is expressed by the formula
σ=kd^-m
approximately, where k and m are constants and m is always smaller than unity. It is clear from the above formula that the equivalent modulus of rupture is independent of the breadth of the specimen.

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