TENSION TEST PARALLEL TO THE GRAIN

Since the tensile strength of wood parallel to the grain is greater than the compressive strength, and exceedingly greater than the shearing strength, it is very difficult to make satisfactory tension tests, as the head and shoulders of the test specimen (which is subjected to both compression and shear) must be stronger than the portion subjected to a pure tensile stress.

Various designs of test specimens have been made.  The one first employed by the Division of Forestry[61] was prepared as follows:  Sticks were cut measuring 1.5” X 2.5” X 16”.  The thickness at the centre was then reduced to three-eighths of an inch by cutting out circular segments with a band saw.  This left a breaking section of 2.5” X 0.375”.  Care was taken to cut the specimen as nearly parallel to the grain as possible, so that its failure would occur in a condition of pure tension.  The specimen was then placed between the plane wedge-shaped steel grips of the cage and the movable head of the static machine and pulled in two.  Only the maximum load was recorded. (See Fig. 46, No. 1.)

[Illustration:  FIG. 46.—­Designs of tension test specimens used in United States.]

[Footnote 61:  Bul.  No. 8:  Timber physics, Part II., 1893, p. 7.]

The difficulty of making such tests compared with the minor importance of the results is so great that they are at present omitted by the U.S.  Forest Service.  A form of specimen is suggested, however, and is as follows:  “A rod of wood about one inch in diameter is bored by a hollow drill from the stick to be tested.  The ends of this rod are inserted and glued in corresponding holes in permanent hardwood wedges.  The specimen is then submitted to the ordinary tension test.  The broken ends are punched from the wedges."[62] (See Fig. 46, No. 2.)

[Footnote 62:  Cir. 38:  Instructions to engineers of timber tests, 1906, p. 24.]

The form used by the Department of Forestry of New South Wales[63] is as shown in Fig. 47.  The specimen has a total length of 41 inches and is circular in cross section.  On each end is a head 4 inches in diameter and 7 inches long.  Below each head is a shoulder 8.5 inches long, which tapers from a diameter of 2.75 inches to 1.25 inches.  In the middle is a cylindrical portion 1.25 inches in diameter and 10 inches long.

[Illustration:  FIG. 47.—­Design of tension test specimen used in New South Wales.]

[Footnote 63:  Warren, W.H.:  The strength, elasticity, and other properties of New South Wales hardwood timbers, 1911, pp. 58-62.]

In making the test the specimen is fitted in the machine, and an extensometer attached to the middle portion and arranged to record the extension between the gauge points 8 inches apart.  The area of the cross section then is 1.226 square inches, and the tensile strength is equal to the total breaking load applied divided by this area.