The Mechanical Properties of Wood by Samuel J. Record

S >> Samuel J. Record >> The Mechanical Properties of Wood

Pages:
1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15

The wood to be tested is cut into blocks 3" X 3" X 1', and these
are weighed to the nearest grain just before placing in the
apparatus. Steam from the boiler at a pressure of about 43
pounds per square inch is ejected from a nozzle in such a way
that particles of fine quartz sand are caught up and thrown
violently against the block which is being rotated. Only
superheated steam strikes the block, thus leaving the wood dry.
The test is continued for two minutes, after which the specimen
is removed and immediately weighed.

By comparison with the original weight the loss from abrasion is
determined, and by comparison with a certain wood chosen as a
standard, a coefficient of wear-resistance can be obtained. The
amount of wear will vary more or less according to the surface
exposed, and in these tests quarter-sawed material was used with
the edge grain to the blast.

_Indentation_: The tool used for this test consists of a punch
with a hemispherical end or steel ball having a diameter of
0.444 inch, giving a surface area of one-fourth square inch. It
is fitted with a guard plate, which works loosely until the
penetration has progressed to a depth of 0.222 inch, whereupon
it tightens. (See Fig. 43.) The effect is that of sinking a ball
half its diameter into the specimen. This apparatus is fitted
into the movable head of the static testing machine.

[Illustration: FIG. 43.--Design of tool for testing the hardness
of woods by indentation.]

The wood to be tested is cut square with the grain into
rectangular blocks measuring 2" X 2" X 6". A block is placed on
the platform and the end of the punch forced into the wood at
the rate of 0.25 inch per minute. The operator keeps moving the
small handle of the guard plate back and forth until it
tightens. At this instant the load is read and recorded.

Two penetrations each are made on the tangential and radial
surfaces, and one on each end of every specimen tested.

In choosing the places on the block for the indentations, effort
should be made to get a fair average of heartwood and sapwood,
fine and coarse grain, early and late wood.

Another method of testing by indentation involves the use of a
right-angled cone instead of a ball. For details of this test as
used in New South Wales see _loc. cit._, pp. 86-87.

CLEAVAGE TEST

A static testing machine and a special cleavage testing device
are required. (See Fig. 44.) The latter consists essentially of
two hooks, one of which is suspended from the centre of the top
of the cage, the other extended above the movable head.

[Illustration: FIG. 44.--Design of tool for cleavage test.]

The specimens are 2" X 2" X 3.75". At one end a one-inch hole is
bored, with its centre equidistant from the two sides and 0.25
inch from the end. (See Fig. 45.) This makes the cross section
to be tested 2" X 3". Some of the blocks are cut radially and
some tangentially, as indicated in the figure.

[Illustration: FIG. 45.--Design of cleavage test specimen.]

The free ends of the hooks are fitted into the notch in the end
of the specimen. The movable head of the machine is then made to
descend at the rate of 0.25 inch per minute, pulling apart the
hooks and splitting the block. The maximum load only is taken
and the result expressed in pounds per square inch of width. A
piece one-half inch thick is split off parallel to the failure
and used for moisture determination.

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.

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" X 2" X 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"
X 2".

[Illustration: FIG. 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]

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

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

[Illustration: FIG. 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 deg. 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 formulae
commonly used for computations are as follows:

5.1 M
(1) T = -------
c^{3}

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.

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 repetitively 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 substituting 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 substituted 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
passing 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 analyses 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
principal 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 butt 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_

Field notes as outlined in Form--_a_ Shipment Description,
Manual of the Branch of Products, will be fully and carefully
made by the collector. The age of each tree selected will be
recorded and any other information likely to be of interest or
importance will also be made a part of these field notes. Each
log will have the bark left on. It will be plainly marked in
accordance with directions given under Detailed Instructions.
All material will be shipped to the laboratory immediately after
being cut. No trees will be cut until the collector is notified
that the laboratory is ready to receive the material.

DETAILED INSTRUCTIONS

_Part of Tree to be Tested_

(_a_) For determining the value of tree and locality and the
influence on the mechanical properties of distance from the
pith, a 4-foot bolt will be cut from the top end of each 16-foot
butt log.

(_b_) For investigating the variation of properties with the
height of timber in the tree, all the logs from one average tree
will be used.

(_c_) For investigating the effect of drying the wood, the bolt
next below that provided for in (_a_) will be used in the case
of one tree from each locality.

_Marking and Grouping of Material_

The marking will be standard except as noted. Each log will be
considered a "piece." The piece numbers will be plainly marked
upon the butt end of each log by the collector. The north side
of each log will also be marked.

When only one bolt from a tree is used it will be designated by
the number of the log from which it is cut. Whenever more than
one bolt is taken from a tree, each 4-foot bolt or length of
trunk will be given a letter (mark), _a, b, c,_ etc., beginning
at the stump.

All bolts will be sawed into 2-1/2" X 2-1/2" sticks and the
sticks marked according to the sketch, Fig. 50. The letters _N,
E, S,_ and _W_ indicate the cardinal points when known; when
these are unknown, _H, K, L,_ and _M_ will be used. Thus, _N5,
K8, S7, M4_ are stick numbers, the letter being a part of the
stick number.

[Illustration: FIG. 50.--Method of cutting and marking test
specimens.]

Only straight-grained specimens, free from defects which will
affect their strength, will be tested.

_Care of Material_

No material will be kept in the bolt or log long enough to be
damaged or disfigured by checks, rot, or stains.

_Green material_: The material to be tested green will be kept
in a green state by being submerged in water until near the time
of test. It will then be surfaced, sawed to length, and stored
in damp sawdust at a temperature of 70 deg.F. (as nearly as
practicable) until time of test. Care should be taken to avoid
as much as possible the storage of green material in any form.

_Air-dry material_: The material to be air-dried will be cut
into sticks 2-1/2" X 2-1/2" X 4'. The ends of these sticks will
be paraffined to prevent checking. This material will be so
piled as to leave an air space of at least one-half inch on each
side of each stick, and in such a place that it will be
protected from sunshine, rain, snow, and moisture from the
ground. The sticks will be surfaced and cut to length just
previous to test.

_Order of Tests_

The order of tests in all cases will be such as to eliminate so
far as possible from the comparisons the effect of changes of
condition of the specimens due to such factors as storage and
weather conditions.

The material used for determining the effect of height in tree
will be tested in such order that the average time elapsing from
time of cutting to time of test will be approximately the same
for all bolts from any one tree.

_Tests on Green Material_

The tests on all bolts, except those from which a comparison of
green and dry timber is to be gotten, will be as follows:

_Static bending_: One stick from each pair. A pair consists of
two adjacent sticks equidistant from the pith, as _N_7 and _N_8,
or _H_5 and _H_6.

_Impact bending_: Four sticks; one to be taken from near the
pith; one from near the periphery; and two representative of the
cross section.

_Compression parallel to grain_: One specimen from each stick.
These will be marked "1" in addition to the number of the stick
from which they are taken.

_Compression perpendicular to grain_: One specimen from each of
50 per cent of the static bending sticks. These will be marked
"2" in addition to the number of the stick from which they are
cut.

_Hardness_: One specimen from each of the other 50 per cent of
the static bending sticks. These specimens will be marked "4."

_Shear_: Six specimens from sticks not tested in bending or from
the ends cut off in preparing the bending specimens. Two
specimens will be taken from near the pith; two from near the
periphery; and two that are representative of the average
growth. One of each two will be tested in radial shear and the
other in tangential shear. These specimens will have the mark
"3."

_Cleavage_: Six specimens chosen and divided just as those for
shearing. These specimens will have the mark "5." (For sketches
showing radial and tangential cleavage, see Fig. 45.)

When it is impossible to secure clear specimens for all of the
above tests, tests will have precedence in the order in which
they are named.

_Tests to Determine the Effect of Air-drying_

These tests will be made on material from the adjacent bolts
mentioned in "_c_" under Part of Tree to be Tested. Both bolts
will be cut as outlined above. One-half the sticks from each
bolt will be tested green, the other half will be air-dried and
tested. The division of green and air-dry will be according to
the following scheme:

STICK NUMBERS

Lower bolt, 1, 4, 5, 8, 9, } Tested
etc. } green
Upper bolt, 2, 3, 6, 7, 10, }

Lower bolt, 2, 3, 6, 7, 10, } Air-dried
etc. } and
Upper bolt, 1, 4, 5, 8, 9, } tested

All green sticks from these two bolts will be tested as if they
were from the same bolt and according to the plan previously
outlined for green material from single bolts. The tests on the
air-dried material will be the same as on the green except for
the difference of seasoning.

The material will be tested at as near 12 per cent moisture as
is practicable. The approximate weight of the air-dried
specimens at 12 per cent moisture will be determined by
measuring while green 20 per cent of the sticks to be air-dried
and assuming their dry gravity to be the same as that of the
specimens tested green. This 20 per cent will be weighed as
often as is necessary to determine the proper time of test.

_Methods of Test_

All tests will be made according to Circular 38 except in case
of conflict with the instructions given below:

_Static bending_: The tests will be on specimens 2" X 2" X 30"
on 28-inch span. Load will be applied at the centre.

In all tests the load-deflection curve will be carried to or
beyond the maximum load. In one-third of the tests the
load-deflection curve will be continued to 6-inch deflection, or
till the specimen fails to support a 200-pound load. Deflection
readings for equal increments of load will be taken until well
past the elastic limit, after which the scale beam will be kept
balanced and the load read for each 0.1-inch deflection. The
load and deflection at first failure, maximum load and points of
sudden change, will be shown on the curve sheet even if they do
not occur at one of the regular load or deflection increments.

_Impact bending_: The impact bending tests will be on specimens
of the same size as those used in static bending. The span will
be 28 inches.

The tests will be by increment drop. The first drop will be 1
inch and the increase will be by increments of 1 inch till a
height of 10 inches is reached, after which increments of 2
inches will be used until complete failure occurs or 6-inch
deflection is secured.

A 50-pound hammer will be used when with drops up to 68 inches
it is practically certain that it will produce complete failure
or 6-inch deflection in the case of all specimens of a species.
For all other species, a 100-pound hammer will be used.

In all cases drum records will be made until first failure. Also
the height of drop causing complete failure or 6-inch deflection
will be noted.

_Compression parallel to grain_: This test will be on specimens
2" X 2" X 8" in size. On 20 per cent of these tests
load-compression curves for a 6-inch centrally located gauge
length will be taken. Readings will be continued until the
elastic limit is well passed. The other 80 per cent of the tests
will be made for the purpose of obtaining the maximum load only.

_Compression perpendicular to grain_: This test will be on
specimens 2" X 2" X 6" in size. The bearing plates will be 2
inches wide. The rate of descent of the moving head will be
0.024 inch per minute. The load-compression curve will be
plotted to 0.1 inch compression and the test will then be
discontinued.

_Hardness_: The tool shown in Fig. 43 (an adaptation of the
apparatus used by the German investigator, Janka) will be used.
The rate of descent of the moving head will be 0.25 inch per
minute. When the penetration has progressed to the point at
which the plate "_a_" becomes tight, due to being pressed
against the wood, the load will be read and recorded.

Two penetrations will be made on a tangential surface, two on a
radial, and one on each end of each specimen tested. The choice
between the two radial and between the two tangential surfaces
and the distribution of the penetrations over the surfaces will
be so made as to get a fair average of heart and sap, slow and
fast growth, and spring and summer wood. Specimens will be 2" X
2" X 6".

_Shear_: The tests will be made with a tool slightly modified
from that shown in Circular 38. The speed of descent of head
will be 0.015 inch per minute. The only measurements to be made
are those of the shearing area. The offset will be 1/8 inch.
Specimens will be 2" X 2" X 2-1/2" in size. (For definition of
offset and form of test specimen, see Fig. 38.)

_Cleavage_: The cleavage tests will be made on specimens of the
form and size shown in Fig. 45. The apparatus will be as shown
in Fig. 44. The maximum load only will be taken and the result
expressed in pounds per inch of width. The speed of the moving
head will be 0.25 inch per minute.

_Moisture Determinations_

Moisture determinations will be made on all specimens tested
except those to be photographed or kept for exhibit. A 1-inch
disk will be cut from near the point of failure of bending and
compression parallel specimens, from the portion under the plate
in the case of the compression perpendicular specimens, and from
the centre of the hardness test specimens. The beads from the
shear specimens will be used as moisture disks. In the case of
the cleavage specimens a piece 1/2 inch thick will be split off
parallel to the failure and used as a moisture disk.

Pages:
1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15