The Mechanical Properties of Wood Part 15

SPECIAL TESTS

_Spike-pulling Test_

Spike-pulling tests apply to problems of railroad maintenance, and the results are used to compare the spike-holding powers of various woods, both untreated and treated with different preservatives, and the efficiency of various forms of spikes.

Special tests are also made in which the spike is subjected to a transverse load applied repet.i.tively by a blow.

For details of tests and results see:

Cir. 38, U.S.F.S.: Instructions to engineers of timber tests, p. 26. Cir. 46, U.S.F.S.: Holding force of railroad spikes in wooden ties. Bul. 118, U.S.F.S,: Prolonging the life of cross-ties, pp. 37-40.

_Packing Boxes_

Special tests on the strength of packing boxes of various woods have been made by the U.S. Forest Service to determine the merits of different kinds of woods as box material with the view of subst.i.tuting new kinds for the more expensive ones now in use. The methods of tests consisted in applying a load along the diagonal of a box, an action similar to that which occurs when a box is dropped on one of its corners. The load was measured at each one-fourth inch in deflection, and notes were made of the primary and subsequent failures.

For details of tests and results, see:

Cir. 47, U.S.F.S.: Strength of packing boxes of various woods.

Cir. 214, U.S.F.S.: Tests of packing boxes of various forms.

_Vehicle and Implement Woods_

Tests were made by the U.S. Forest Service to obtain a better knowledge of the mechanical properties of the woods at present used in the manufacture of vehicles and implements and of those which might be subst.i.tuted for them. Tests were made upon the following materials: hickory buggy spokes (see Fig. 5); hickory and red oak buggy shafts; wagon tongues; Douglas fir and southern pine cultivator poles.

Details of the tests and results may be found in:

Cir. 142, U.S.F.S.: Tests on vehicle and implement woods.

_Cross-arms_

In tests by the U.S. Forest Service on cross-arms a special apparatus was devised in which the load was distributed along the arm as in actual practice. The load was applied by rods pa.s.sing through the pinholes in the arms. Nuts on these rods pulled down on the wooden bearing-blocks shaped to fit the upper side of the arm. The lower ends of these rods were attached to a system of equalizing levers, so arranged that the load at each pinhole would be the same. In all the tests the load was applied vertically by means of the static machine.

See Cir. 204, U.S.F.S.: Strength tests of cross-arms.

_Other Tests_

Many other kinds of tests are made as occasion demands. One kind consists of barrels and liquid containers, match-boxes, and explosive containers. These articles are subjected to shocks such as they would receive in transit and in handling, and also to hydraulic pressure.

One of the most important tests from a practical standpoint is that of built-up structures such as compounded beams composed of small pieces bolted together, mortised joints, wooden trusses, etc. Tests of this kind can best be worked out according to the specific requirements in each case.

APPENDIX

SAMPLE WORKING PLAN OF THE U.S. FOREST SERVICE

MECHANICAL PROPERTIES OF WOODS GROWN IN THE UNITED STATES

Working Plan No. 124

PURPOSE OF WORK

It is the general purpose of the work here outlined to provide:

(_a_) Reliable data for comparing the mechanical properties of various species;

(_b_) Data for the establishment of correct strength functions or working stresses;

(_c_) Data upon which may be based a.n.a.lyses of the influence on the mechanical properties of such factors as:

Locality;

Distance of timber from the pith of the tree;

Height of timber in the tree;

Change from the green to the air-dried condition, etc.

The mechanical properties which will be considered and the princ.i.p.al tests used to determine them are as follows:

Strength and stiffness-- Static bending; Compression parallel to grain; Compression perpendicular to grain; Shear.

Toughness-- Impact bending; Static bending; Work to maximum load and total work.

Cleavability-- Cleavage test.

Hardness-- Modification of Janka ball test for surface hardness.

MATERIAL

_Selection and Number of Trees_

The material will be from trees selected in the forest by one qualified to determine the species. From each locality, three to five dominant trees of merchantable size and approximately average age will be so chosen as to be representative of the dominant trees of the species. Each species will eventually be represented by trees from five to ten localities. These localities will be so chosen as to be representative of the commercial range of the species. Trees from one to three localities will be used to represent each species until most of the important species have been tested.

The 16-foot b.u.t.t log will be taken from each tree selected and the entire merchantable hole of one average tree for each species.

_Field Notes and Shipping Instructions_