The Mechanical Properties of Wood / Including a Discussion of the Factors Affecting the Mechanical Properties, and Methods of Timber Testing

by Samuel J. Record

TENSION TEST AT RIGHT ANGLES TO THE GRAIN

A static testing machine and a special testing device ([see Fig. 48]) are required. The latter consists essentially of two double hooks or clamps, one of which is suspended from the centre of the top of the cage, the other extended above the movable head. The specimens are 2" × 2" × 2.5". At each end a one-inch hole is bored with its centre equidistant from the two sides and 0.25 inch from the ends. This makes the cross section to be tested 1" × 2".

Figure 48

Design of tool and specimen for testing tension at right angles to the grain.

The free ends of the clamps are fitted into the notches in the ends of the specimen. The movable head of the machine is then made to descend at the rate of 0.25 inch per minute, pulling the specimen in two at right angles to the grain. The maximum load only is taken and the result expressed in pounds per inch of width. A piece one-half inch thick is split off parallel to the failure and used for moisture determination.

TORSION TEST[⁶⁴]

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