Apparatus: The torsion test is made in a Riehle-Miller torsional testing machine or its equivalent. (See Fig. 49.)

[Footnote 64: Wood is so seldom subjected to a pure stress of this kind that the torsion test is usually omitted.]

Figure 49

Figure 49

Making a torsion test on hickory.

Preparation of material: The test pieces are cylindrical, 1.5 inches in diameter and 18 inches gauge length, with squared ends 4 inches long joined to the cylindrical portion with a fillet. The dimensions are carefully measured, and the usual data obtained in regard to the rate of growth, proportion of late wood, location and kind of defects. The weight of the cylindrical portion of the specimen is obtained after the test.

Making the test: After the specimen is fitted in the machine the load is applied continuously at the rate of 22° per minute. A troptometer is used in measuring the deformation. Readings are made until failure occurs, the points being entered on the cross-section paper. The character of the failure is described. Moisture determinations are made by the disk method.

Results: The conditions of ultimate rupture due to torsion appear not to be governed by definite mathematical laws; but where the material is not overstrained, laws may be assumed which are sufficiently exact for practical cases. The formulæ commonly used for computations are as follows:




 5.1 M
(1) T = -------



c3







 114.6 T f
(2) G = -----------



a c





a = angle measured by troptometer at elastic limit, in degrees.

c = diameter of specimen, inches.

f = gauge length of specimen, inches.

G = modulus of elasticity in shear across the grain, pounds per square inch.

M = moment of torsion at elastic limit, inch-pounds.

T = outer fibre torsional stress at elastic limit, pounds per square inch.