[Footnote 56: The methods of timber testing described here are
for the most part those employed by the U. S. Forest Service.
See Cir. 38 (rev. ed.), 1909.]
WORKING PLAN
Preliminary to making a series of timber tests it is very
important that a working plan be prepared as a guide to the
investigation. This should embrace: (1) the purpose of the
tests; (2) kind, size, condition, and amount of material needed;
(3) full description of the system of marking the pieces; (4)
details of any special apparatus and methods employed; (5)
proposed method of analyzing the data obtained and the nature of
the final report. Great care should be taken in the preparation
of this plan in order that all problems arising may be
anticipated so far as possible and delays and unnecessary work
avoided. A comprehensive study of previous investigations along
the same or related lines should prove very helpful in outlining
the work and preparing the report. (For sample working plan see
Appendix.)
FORMS OF MATERIAL TESTED
In general, four forms of material are tested, namely: (1) large
timbers, such as bridge stringers, car sills, large beams, and
other pieces five feet or more in length, of actual sizes and
grades in common use; (2) built-up structural forms and
fastenings, such as built-up beams, trusses, and various kind of
joints; (3) small clear pieces, such as are used in compression,
shear, cleavage, and small cross-breaking tests; (4)
manufactured articles, such as axles, spokes, shafts,
wagon-tongues, cross-arms, insulator pins, barrels, and packing
boxes.
As the moisture content is of fundamental importance (see WATER
CONTENT, above.), all standard tests are usually made in the
green condition. Another series is also usually run in an
air-dry condition of about 12 per cent moisture. In all cases
the moisture is very carefully determined and stated with the
results in the tables.
SIZE OF TEST SPECIMENS
The size of the test specimen must be governed largely by the
purpose for which the test is made. If the effect of a single
factor, such as moisture, is the object of experiment, it is
necessary to use small pieces of wood in order to eliminate so
far as possible all disturbing factors. If the specimens are too
large, it is impossible to secure enough perfect pieces from one
tree to form a series for various tests. Moreover, the drying
process with large timbers is very difficult and irregular, and
requires a long period of time, besides causing checks and
internal stresses which may obscure the results obtained.
On the other hand, the smaller the dimensions of the test
specimen the greater becomes the relative effect of the inherent
factors affecting the mechanical properties. For example, the
effect of a knot of given size is more serious in a small stick
than in a large one. Moreover, the smaller the specimen the
fewer growth rings it contains, hence there is greater
opportunity for variation due to irregularities of grain.
Tests on large timbers are considered necessary to furnish
designers data on the probable strength of the different sizes
and grades of timber on the market; their coefficients of
elasticity under bending (since the stiffness rather than the
strength often determines the size of a beam); and the manner of
failure, whether in bending fibre stress or horizontal shear. It
is believed that this information can only be obtained by direct
tests on the different grades of car sills, stringers, and other
material in common use.
When small pieces are selected for test they very often are
clear and straight-grained, and thus of so much better grade
than the large sticks that tests upon them may not yield unit
values applicable to the larger sizes. Extensive experiments
show, however, (1) that the modulus of elasticity is
approximately the same for large timbers as for small clear
specimens cut from them, and (2) that the fibre stress at
elastic limit for large beams is, except in the weakest timbers,
practically equal to the crushing strength of small clear pieces
of the same material.[57]
[Footnote 57: Bul. 108, U. S. Forest Service: Tests of
structural timbers, pp. 53-54.]
MOISTURE DETERMINATION
In order for tests to be comparable, it is necessary to know the
moisture content of the specimens at the zone of failure. This
is determined from disks an inch thick cut from the timber
immediately after testing.
In cases, as in large beams, where it is desirable to know not
only the average moisture content but also its distribution
through the timber, the disks are cut up so as to obtain an
outside, a middle, and an inner portion, of approximately equal
areas. Thus in a section 10" x 12" the outer strip would be one
inch wide, and the second one a little more than an inch and a
quarter. Moisture determinations are made for each of the three
portions separately.
The procedure is as follows:
(1) Immediately after sawing, loose splinters are removed and
each section is weighed.
(2) The material is put into a drying oven at 100° C. (212° F.)
and dried until the variation in weight for a period of
twenty-four hours is less than 0.5 per cent.
(3) The disk is again carefully weighed.
(4) The loss in weight expressed in per cent of the dry weight
indicates the moisture content of the specimen from which the
specimen was cut.
MACHINE FOR STATIC TESTS
The standard screw machines used for metal tests are also used
for wood, but in the case of wood tests the readings must be
taken "on the fly," and the machine operated at a uniform speed
without interruption from beginning to end of the test. This is
on account of the time factor in the strength of wood. (See
SPEED OF TESTING MACHINE, below.)
The standard machines for static tests can be used for
transverse bending, compression, tension, shear, and cleavage. A
common form consists of three main parts, namely: (1) the
straining mechanism, (2) the weighing apparatus, and (3) the
machinery for communicating motion to the screws.
The straining mechanism consists of two parts, one of which is a
movable crosshead operated by four (sometimes two or three)
upright steel straining screws which pass through openings in
the platform and bear upward on the bed of the machine upon
which the weighing platform rests as a fulcrum. At the lower
ends of these screws are geared nuts all rotated simultaneously
by a system of gears which cause the movable crosshead to rise
and fall as desired.
The stationary part of the straining mechanism, which is used
only for tension and cleavage tests, consists of a steel cage
above the movable crosshead and rests directly upon the weighing
platform. The top of the cage contains a square hole into which
one end of the test specimen may be clamped, the crosshead
containing a similar clamp for the other end, in making tension
tests.
For testing long beams a special form of machine with an
extended platform is used. (See Fig. 29.)
The weighing platform rests upon knife edges carried by primary
levers of the weighing apparatus, the fulcrum being on the bed
of the machine, and any pressure upon it is directly transmitted
through a series of levers to the weighing beam. This beam is
adjusted by means of a poise running on a screw. In operation
the beam is kept floating by means of another poise moved back
and forth by a screw which is operated by a hand wheel or
automatically. The larger units of stress are read from the
graduations along the side of the beam, while the intermediate
smaller weights are observed on the dial on the rear end of the
beam.
The machine is driven by power from a shaft or a motor and is so
geared that various speeds are obtainable. One man can operate
it.
In making tests the operation of the straining screws is always
downward so as to bring pressure to bear upon the weighing
platform. For tests in tension and cleavage the specimen is
placed between the top of the stationary cage and the movable
head and subjected to a pull. For tests in transverse bending,
compression, and cleavage the specimen is placed between the
movable head and the platform, and a direct compression force
applied.
Testing machines are usually calibrated to a portion of their
capacity before leaving the factory. The delicacy of the
weighing levers is verified by determining the number of pounds
necessary to move the beam between the stops while a load of
1,000 pounds rests on the platform. The usual requirement is
that ten pounds should accomplish this movement.
The size of machine suitable for compression tests on 2" X 2"
sticks or for 2" X 2" beams with 26 to 36-inch span has a
capacity of 30,000 pounds.
SPEED OF TESTING MACHINE
In instructions for making static tests the rate of application
of the stress, _i.e._, the speed of the machine, is given
because the strength of wood varies with the speed at which the
fibres are strained. The speed of the crosshead of the testing
machine is practically never constant, due to mechanical defects
of the apparatus and variations in the speed of the motor, but
so long as it does not exceed 25 per cent the results will not
be appreciably affected. In fact, a change in speed of 50 per
cent will not cause the strength of the wood to vary more than 2
per cent.[58]
[Footnote 58: See Tiemann, Harry Donald: The effect of the speed
of testing upon the strength and the standardization of tests
for speed. Proc. Am. Soc. for Testing Materials, Vol. VIII,
Philadelphia, 1908.]
Following are the formulæ used in determining the speed of the
movable head of the machine in inches per minute (n):
(1) For endwise compression n = Z l
Z l^{2}
(2) For beams (centre loading) n = ---------
6h
Z l^{2}
(3) For beams (third-pointloading) n = ---------
5.4h
Z = rate of fibre strain per inch of fibre length.
l = span of beam or length of compression specimen.
h = height of beam.
The values commonly used for Z are as follows:
Bending large beams Z = 0.0007
Bending small beams Z = 0.0015
Endwise compression-large specimens Z = 0.0015
Endwise compression-small " Z = 0.003
Right-angled compression-large " Z = 0.007
Right-angled compression-small " Z = 0.015
Shearing parallel to the grain Z = 0.015
Example: At what speed should the crosshead move to give the
required rate of fibre strain in testing a small beam 2" X 2" X
30". (Span = 28".) Substituting these values in equation (2)
above:
(0.0015 X 28^2)
n = ----------------- = 0.1 inch per minute.
(6 X 2)
In order that tests may be intelligently compared, it is
important that account be taken of the speed at which the stress
was applied. In determining the basis for a ratio between time
and strength the rate of strain, which is controllable, and not
the ratio of stress, which is circumstantial, should be used. In
other words, the rate at which the movable head of the testing
machine descends and not the rate of increase in the load is to
be regulated. This ratio, to which the name _speed-strength
modulus_ has been given, may be expressed as a coefficient
which, if multiplied into any proportional change in speed, will
give the proportional change in strength. This ratio is derived
from empirical curves. (See Table XVII.)
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| TABLE XVII TABLE XVII |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| SPEED-STRENGTH MODULI AND RELATIVE INCREASE IN STRENGTH AT RATES OF FIBRE STRAIN INCREASING IN GEOMETRICAL RATIO. (Tiemann, _loc. cit._) |
| (Values in parentheses are approximate) |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| Rate of fibre strain. | | | | | | | |
| Ten-thousandths inch | 2/3 | 2 | 6 | 18 | 54 | 162 | 486 |
| per minute per inch | | | | | | | |
|-------------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------|
| C | Speed of crosshead. | | | | | | | |
| O | Inches per minute | 0.000383 | 0.00115 | 0.00345 | 0.0103 | 0.0310 | 0.0931 | .279 |
| M | | | | | | | | |
| P |---------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------|
| R | Specimens | Wet | Dry | All | Wet | Dry | All | Wet | Dry | All | Wet | Dry | All | Wet | Dry | All | Wet | Dry | All | Wet | Dry | All |
| E |---------------------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------|
| S | Relative | | | | | | | | | | | | | | | | | | | |
| S | crushing | | 100.0 | 100.0 | 100.0 | 103.4 | 100.8 | 101.5 | 107.5 | 102.7 | 103.8 | 113.9 | 105.5 | 107.9 | 121.3 | 108.3 | 116.4 | 128.8 | 110.0 |118.9 |
| I | strength | | | | | | | | | | | | | | | | | | | |
| O | | | | | | | | | | | | | | | | | | | | |
| N | Speed-strength | | 0.017 |(0.006)|(0.009)| 0.033 | 0.012 | 0.016 | 0.047 | 0.021 | 0.029 | 0.053 | 0.027 | 0.039 | 0.060 | 0.023 | 0.049 |(0.052)|(0.015)|(0.040)|
| | modulus, _T_ | | | | | | | | | | | | | | | | | | | |
|---+---------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------|
| | Speed of crosshead. | | | | | | | |
| | Inches per minute | 0.0072 | 0.0216 | 0.0648 | 0.194 | 0.583 | 1.75 | 5.25 |
| B | | | | | | | | |
| E |---------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------|
| N | Specimens | Wet | Dry | All | Wet | Dry | All | Wet | Dry | All | Wet | Dry | All | Wet | Dry | All | Wet | Dry | All | Wet | Dry | All |
| D |---------------------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------|
| I | Relative | | | | | | | | | | | | | | | | | | | | | |
| N | crushing | 97.4 | 99.0 | 98.2 | 100.0 | 100.0 | 100.0 | 105.1 | 102.1 | 103.7 | 111.3 | 105.8 | 108.1 | 117.9 | 108.6 | 112.7 | 123.7 | 109.6 | 116.3 | 126.3 | 110.3 | 118.9 |
| G | strength | | | | | | | | | | | | | | | | | | | | | |
| | | | | | | | | | | | | | | | | | | | | | | |
| | Speed-strength |(0.014)|(0.005)| 0.012 | 0.033 | 0.014 | 0.026 | 0.049 | 0.026 | 0.037 | 0.053 | 0.033 | 0.038 | 0.049 | 0.014 | 0.035 | 0.038 | 0.006 | 0.025 |(0.023)|(0.004)|(0.014)|
| | modulus, _T_ | | | | | | | | | | | | | | | | | | | | | |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| NOTE.--The usual speeds of testing at the U.S. Forest Service laboratory are at rates of fibre strain |
| of 15 and 10 ten-thousandths in. per min. per in. for compression and bending respectively. |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
BENDING LARGE BEAMS
_Apparatus_: A static bending machine (described above), with a
special crosshead for third-point loading and a long platform
bearing knife-edge supports, is required. (See Fig. 29.)
[Illustration: FIG. 29.--Static bending test on large beam. Note
arrangement of wire and scale for measuring deflection; also
method of applying load at "third-points."]
_Preparing the material_: Standard sizes and grades of beams and
timbers in common use are employed. The ends are roughly squared
and the specimen weighed and measured, taking the
cross-sectional dimensions midway of the length. Weights should
be to the nearest pound, lengths to the nearest 0.1 inch, and
cross-sectional dimensions to the nearest 0.01 inch.
_Marking and sketching_: The butt end of the beam is marked _A_
and the top end _B_. While facing _A_, the top side is marked
_a_, the right hand _b_, the bottom _c_, the left hand _d_.
Sketches are made of each side and end, showing (1) size,
location, and condition of knots, checks, splits, and other
defects; (2) irregularities of grain; (3) distribution of
heartwood and sapwood; and on the ends: (4) the location of the
pith and the arrangement of the growth rings, (5) number of
rings per inch, and (6) the proportion of late wood.
The number of rings per inch and the proportion of late wood
should always be determined along a radius or a line normal to
the rings. The average number of rings per inch is the total
number of rings divided by the length of the line crossing them.
The proportion of late wood is equal to the sum of the widths of
the late wood crossed by the line, divided by the length of the
line. Rings per inch should be to the nearest 0.1; late wood to
the nearest 0.1 per cent.
Since in large beams a great variation in rate of growth and
relative amount of late wood is likely in different parts of the
section, it is advisable to consider the cross section in three
volumes, namely, the upper and lower quarters and the middle
half. The determination should be made upon each volume
separately, and the average for the entire cross section
obtained from these results.
At the conclusion of the test the failure, as it appears on each
surface, is traced on the sketches, with the failures numbered
in the order of their occurrence. If the beam is subsequently
cut up and used for other tests an additional sketch may be
desirable to show the location of each piece.
_Adjusting specimen in machine_: The beam is placed in the
machine with the side marked _a_ on top, and with the ends
projecting equally beyond the supports. In order to prevent
crushing of the fibre at the points where the stress is applied
it is necessary to use bearing blocks of maple or other hard
wood with a convex surface in contact with the beam. Roller
bearings should be placed between the bearing blocks and the
knife edges of the crosshead to allow for the shortening due to
flexure. (See Fig. 29.) Third-point loading is used, that is,
the load is applied at two points one-third the span of the beam
apart. (See Fig. 30.) This affords a uniform bending moment
throughout the central third of the beam.
[Illustration: FIG. 30.--Two methods of loading a beam, namely,
third-point loading (upper), and centre loading (lower).]
_Measuring the deflection_: The method of measuring the
deflection should be such that any compression at the points of
support or at the application of the load will not affect the
reading. This may be accomplished by driving a small nail near
each end of the beam, the exact location being on the neutral
plane and vertically above each knife-edge support. Between
these nails a fine wire is stretched free of the beam and kept
taut by means of a rubber band or coiled spring on one end.
Behind the wire at a point on the beam midway between the
supports a steel scale graduated to hundredths of an inch is
fastened vertically by means of thumb-tacks or small screws
passing through holes in it. Attachment should be made on the
neutral plane.
The first reading is made when the scale beam is balanced at
zero load, and afterward at regular increments of the load which
is applied continuously and at a uniform speed. (See SPEED OF
TESTING MACHINE, above.) If desired, however, the load may be
read at regular increments of deflection. The deflection
readings should be to the nearest 0.01 inch. To avoid error due
to parallax, the readings may be taken by means of a reading
telescope about ten feet distant and approximately on a level
with the wire. A mirror fastened to the scale will increase the
accuracy of the readings if the telescope is not used. As in all
tests on timber, the strain must be continuous to rupture, not
intermittent, and readings must be taken "on the fly." The
weighing beam is kept balanced after the yield point is reached
and the maximum load, and at least one point beyond it, noted.
_Log of the test_: The proper log sheet for this test consists
of a piece of cross-section paper with space at the margin for
notes. (See Fig. 32.) The load in some convenient unit (1,000 to
10,000 pounds, depending upon the dimensions of the specimen) is
entered on the ordinates, the deflection in tenths of an inch on
the abscissæ. The increments of load should be chosen so as to
furnish about ten points on the stress-strain diagram below the
elastic limit.
As the readings of the wire on the scale are made they are
entered directly in their proper place on the cross-section
paper. In many cases a test should be continued until complete
failure results. The points where the various failures occur are
indicated on the stress-strain diagram. A brief description of
the failure is made on the margin of the log sheet, and the form
traced on the sketches.
_Disposal of the specimen_: Two one-inch sections are cut from
the region of failure to be used in determining the moisture
content. (See MOISTURE DETERMINATION, above.) A two-inch section
may be cut for subsequent reference and identification, and
possible microscopic study. The remainder of the beam may be cut
into small beams and compression pieces.
_Calculating the results_: The formulæ used in calculating the
results of tests on large rectangular simple beams loaded at
third points of the span are as follows:
0.75 P
(1) J = --------
b h
l (P_{1} + 0.75 W)
(2) r = --------------------
b h^{2}
l (P + 0.75 W)
(3) R = ----------------
b h^{2}
P_{1} l^{3}
(4) E = ---------------
4.7 D b h^{3}
0.87 P_{1} D
(5) S = --------------
2 V
b, h, l = breadth, height, and span of specimen, inches.
D = total deflection at elastic limit, inches.
P = maximum load, pounds.
P_{1} = load at elastic limit, pounds.
E = modulus of elasticity, pounds per square inch.
r = fibre stress at elastic limit, pounds per sq. inch.
R = modulus of rupture, pounds per square inch.
S = elastic resilience or work to elastic limit, inch-pounds
per cu. in.
J = greatest calculated longitudinal shear, pounds per square
inch.
V = volume of beam, cubic inches.
W = weight of the beam.
In large beams the weight should be taken into account in
calculating the fibre stress. In (2) and (3) three-fourths of
the weight of the beam is added to the load for this reason.
BENDING SMALL BEAMS
_Apparatus_: An ordinary static bending machine, a steel I-beam
bearing two adjustable knife-edge supports to rest on the
platform, and a special deflectometer, are required. (See Fig.
31.)
[Illustration: FIG. 31.--Static bending test on small beam. Note
the use of the deflectometer with indicator and dial for
measuring the deflection; also roller bearings between beam and
supports.]
_Preparing the material_: The specimens may be of any convenient
size, though beams 2" X 2" X 30" tested over a 28-inch span, are
considered best. The beams are surfaced on all four sides, care
being taken that they are not damaged by the rollers of the
surfacing machine. Material for these tests is sometimes cut
from large beams after failure. The specimens are carefully
weighed in grams, and all dimensions measured to the nearest
0.01 inch. If to be tested in a green or fresh condition the
specimens should be kept in a damp box or covered with moist
sawdust until needed. No defects should be allowed in these
specimens.
_Marking and sketching_: Sketches are made of each end of the
specimen to show the character of the growth, and after testing,
the manner of failure is shown for all four sides. In obtaining
data regarding the rate of growth and the proportion of late
wood the same procedure is followed as with large beams.
_Adjusting specimen in machine_: The beam should be correctly
centred in the machine and each end should have a plate with
roller bearings between it and the support. Centre loading is
used. Between the movable head of the machine and the specimen
is placed a bearing block of maple or other hard wood, the lower
surface of which is curved in a direction along the beam, the
curvature of which should be slightly less than that of the beam
at rupture, in order to prevent the edges from crushing into the
fibres of the test piece.
_Measuring the deflection_: The method of measuring deflection
of large beams can be used for small sizes, but because of the
shortness of the span and consequent slight deformation in the
latter, it is hardly accurate enough for good work. The special
deflectometer shown in Fig. 31 allows closer reading, as it
magnifies the deflection ten times. It rests on two small nails
driven in the beam on the neutral plane and vertically above the
supports. The fine wire on the wheel at the base of the
indicator is attached to another small nail driven in the beam
on the neutral plane midway between the end nails. All three
nails should be in place before the beam is put into the
machine. The indicator is adjustable by means of a thumb-screw
at the base and is set at zero before the load is applied.
Deflections are read to the nearest 0.001 inch. For rate of
application of load see SPEED OF TESTING MACHINE, above. The
speed should be uniform from start to finish without stopping.
Readings must be made "on the fly."
_Log of the test_: The log sheets used for small beams (see Fig.
32) are the same as for large sizes and the procedure is
practically identical. The stress-strain diagram is continued to
or beyond the maximum load, and in a portion of the tests should
be continued to six-inch deflection or until the specimen fails
to support a load of 200 pounds. Deflection readings for equal
increments of load are taken until well beyond the elastic
limit, after which the scale beam is kept balanced and the load
read for each 0.1 inch deflection. The load and deflection at
first failure, the maximum load, and any points of sudden change
should be shown on the diagram, even though they do not occur at
one of the regular points. A brief description of the failure
and the nature of any defects is entered on the log sheet.
[Illustration: FIG. 32.--Sample log sheet, giving full details
of a transverse bending test on a small pine beam.]
_Calculating the results_: The formulæ used in calculating the
results of tests on small rectangular simple beams are as
follows:
0.75 P
(1) J = --------
b h
1.5 P_{1} l
(2) r = -------------
b h^{2}
1.5 P l
(3) R = ---------
b h^{2}
P_{1} l^{3}
(4) E = -------------
4 D b h^{3}
P_{1} D
(5) S = ---------
2 V
The same legend is used as in BENDING LARGE BEAMS. The weight of
the beam itself is disregarded.
ENDWISE COMPRESSION
_Apparatus_: An ordinary static testing machine and a
compressometer are required. (See Fig. 33.)
[Illustration: FIG. 33.--Endwise compression test, showing
method of measuring the deformation by means of a
compressometer.]
_Preparing the material_: Two classes of specimens are commonly
used, namely, (1) posts 24 inches in length, and (2) small clear
blocks approximately 2" X 2" X 8". The specimens are surfaced on
all four sides and both ends squared smoothly and evenly. They
are carefully weighed, measured, rate of growth and proportion
of late wood determined, as in bending tests. After the test a
moisture section is cut and weighed. Ordinarily these specimens
should be free from defects.
_Sketching_: Sketches are made of each end of the specimens to
show the character of the growth. After testing, the manner of
failure is shown for all four sides, and the various parts of
the failure are numbered in the order of their occurrence.
_Adjusting specimen in machine_: The compressometer collars are
adjusted, the distance between them being 20 inches for the
posts and 6 inches for the blocks. If the two ends of the blocks
are not exactly parallel a ball-and-socket block can be placed
between the upper end of the specimen and the movable head of
the machine to overcome the irregularity. If the blocks are true
they can simply be stood on end upon the platform and the
movable head allowed to press directly upon the upper end.
_Measuring the deformation_: The deformation is measured by a
compressometer. (See Fig. 33.) The latter registers to 0.001
inch. In the case of posts the compression between the collars
is communicated to the four points on the arms by means of brass
rods; with short blocks, as in Fig. 33, the points of the arms
are in direct contact with the collars. The operator lowers the
fulcrum of the apparatus by moving the micrometer screws at such
a rate that the set-screw in the rear end of the upper lever is
kept barely touching the fixed arm below it, being guided by a
bell operated by electric contact.
_Log of the test_: The load is applied continuously at a uniform
rate of speed. (See SPEED OF TESTING MACHINE, above.) Readings
are taken from the scale of the compressometer at regular
increments of either load or compression. The stress-strain
diagram is continued to at least one deformation point beyond
the maximum load, and in event of sudden failure, the direction
of the curve beyond the maximum point is indicated. A brief
description of the failure is entered on the log sheet. (See
Fig. 34.)
[Illustration: FIG. 34.--Sample log sheet of an endwise
compression test on a short pine column.]
In short specimens the failure usually occurs in one or several
planes diagonal to the axis of the specimen. If the ends are
more moist than the middle a crushing may occur on the extreme
ends in a horizontal plane. Such a test is not valid and should
always be culled. If the grain is diagonal or the stress is
unevenly applied a diagonal shear may occur from top to bottom
of the test specimen. Such tests are also invalid and should be
culled. When the plane (or several planes) of failure occurs
through the body of the specimen the test is valid. It may
sometimes be advantageous to allow the extreme ends to dry
slightly before testing in order to bring the planes of failure
within the body. This is a perfectly legitimate procedure
provided no drying is allowed from the sides of the specimen,
and the moisture disk is cut from the region of failure.
_Calculating the results:_ The formulæ used in calculating the
results of tests on endwise compression are as follows:
P
(1) C = -----
A
P_{1}
(2) c = -------
A
P_{1} l
(3) E = ---------
A D
P D
(4) S = -----
2 V
C = crushing strength, pounds per square inch.
c = fibre strength at elastic limit, pounds per square inch.
A = area of cross section, square inches.
l = distance between centres of collars, inches.
D = total shortening at elastic limit, inches.
V = volume of specimen, cubic inches.
Remainder of legend as in BENDING LARGE BEAMS, above.
COMPRESSION ACROSS THE GRAIN
_Apparatus_: An ordinary static testing machine, a bearing
plate, and a deflectometer are required. (See Fig. 35.)
[Illustration: FIG. 35.--Compression across the grain. Note
method of measuring the deformation by means of a
deflectomoter.]
_Preparing the material_: Two classes of specimens are used,
namely, (1) sections of commercial sizes of ties, beams, and
other timbers, and (2) small, clear specimens with the length
several times the width. Sometimes small cubes are tested, but
the results are hardly applicable to conditions in practice. In
(2) the sides are surfaced and the ends squared. The specimens
are then carefully measured and weighed, defects noted, rate of
growth and proportion of late wood determined, as in bending
tests. (See BENDING LARGE BEAMS, above.) After the test a
moisture section is cut and weighed.
_Sketching_: Sketches are made as in endwise compression tests.
(See ENDWISE COMPRESSION, above.)
_Adjusting specimen in machine_: The specimen is laid
horizontally upon the platform of the machine and a steel
bearing plate placed on its upper surface immediately beneath
the centre of the movable head. For the larger specimens this
plate is six inches wide; for the smaller sizes, two inches
wide. The plate in all cases projects over the edges of the test
piece, and in no case should the length of the latter be less
than four times the width of the plate.
_Measuring the deformation_: The compression is measured by
means of a deflectometer (see Fig. 35), which, after the first
increment of load is applied, is adjusted (by means of a small
set screw) to read zero. The actual downward motion of the
movable head (corresponding to the compression of the specimen)
is multiplied ten times on the scale from which the readings are
made.
_Log of the test_: The load is applied continuously and at
uniform speed (see SPEED OF TESTING MACHINE, above), until well
beyond the elastic limit. The compression readings are taken at
regular load increments and entered on the cross-section paper
in the usual way. Usually there is no real maximum load in this
case, as the strength continually increases as the fibres are
crushed more compactly together.
_Calculating the results_: Ordinarily only the fibre stress at
the elastic limit (c) is computed. It is equal to the load at
elastic limit (P_{1}) divided by the area under the plate (B).
{ P_{1} }
{ c = ------- }
{ B }
SHEAR ALONG THE GRAIN
_Apparatus_: An ordinary static testing machine and a special
tool designed for producing single shear are required. (See
Figs. 36 and 37.) This shearing apparatus consists of a solid
steel frame with set screws for clamping the block within it
firmly in a vertical position. In the centre of the frame is a
vertical slot in which a square-edged steel plate slides freely.
When the testing block is in position, this plate impinges
squarely along the upper surface of the tenon or lip, which, as
vertical pressure is applied, shears off.
[Illustration: Fig. 36.--Vertical section of shearing tool.]
[Illustration: FIG. 37.--Front view of shearing tool with test
specimen and steel plate in position for testing.]
_Preparing the material_: The specimens are usually in the form
of small, clear, straight-grained blocks with a projecting tenon
or lip to be sheared off. Two common forms and sizes are shown
in Figure 38. Part of the blocks are cut so that the shearing
surface is parallel to the growth rings, or tangential; others
at right angles to the growth rings, or radial. It is important
that the upper surface of the tenon or lip be sawed exactly
parallel to the base of the block. When the form with a tenon is
used the under cut is extended a short distance horizontally
into the block to prevent any compression from below.
[Illustration: FIG. 38.--Two forms of shear test specimens.]
In designing a shearing specimen it is necessary to take into
consideration the proportions of the area of shear, since, if
the length of the portion to be sheared off is too great in the
direction of the shearing face, failure would occur by
compression before the piece would shear. Inasmuch as the
endwise compressive strength is sometimes not more than five
times the shearing strength, the shearing surface should be less
than five times the surface to which the load is applied. This
condition is fulfilled in the specimens illustrated.
Shearing specimens are frequently cut from beams after testing.
In this case the specific gravity (dry), proportion of late
wood, and rate of growth are assumed to be the same as already
recorded for the beams. In specimens not so taken, these
quantities are determined in the usual way. The sheared-off
portion is used for a moisture section.
_Adjusting specimen in machine_: The test specimen is placed in
the shearing apparatus with the tenon or lip under the sliding
plate, which is centred under the movable head of the machine.
(See Fig. 39.) In order to reduce to a minimum the friction due
to the lateral pressure of the plate against the bearings of the
slot, the apparatus is sometimes placed upon several parallel
steel rods to form a roller base. A slight initial load is
applied to take up the lost motion of the machinery, and the
beam balanced.
[Illustration: FIG. 39.--Making a shearing test.]
_Log of the test_: The load is applied continuously and at a
uniform rate until failure, but no deformations are measured.
The points noted are the maximum load and the length of time
required to reach it. Sketches are made of the failure. If the
failure is not pure shear the test is culled.
The shearing strength per square inch is found by dividing the
{ P }
maximum load by the cross-sectional area. { Q = --- }
{ A }
IMPACT TEST
_Apparatus_: There are several types of impact testing
machines.[59] One of the simplest and most efficient for use
with wood is illustrated in Figure 40. The base of the machine
is 7 feet long, 2.5 feet wide at the centre, and weighs 3,500
pounds. Two upright columns, each 8 feet long, act as guides for
the striking head. At the top of the column is the hoisting
mechanism for raising or lowering the striking weights. The
power for operating the machine is furnished by a motor set on
the top. The hoisting-mechanism is all controlled by a single
operating lever, shown on the side of the column, whereby the
striking weight may be raised, lowered, or stopped at the will
of the operator. There is an automatic safety device for
stopping the machine when the weight reaches the top.
[Footnote 59: For description of U.S. Forest Service automatic
and autographic impact testing machine, see Proc. Am. Soc. for
Testing Materials, Vol. VIII, 1908, pp. 538-540.]
[Illustration: FIG. 40.--Impact testing machine.]
The weight is lifted by a chain, one end of which passes over a
sprocket wheel in the hoisting mechanism. On the lower end of
the chain is hung an electro-magnet of sufficient magnetic
strength to support the heaviest striking weights. When it is
desired to drop the striking weight the electric current is
broken and reversed by means of an automatic switch and current
breaker. The height of drop may be regulated by setting at the
desired height on one of the columns a tripping pin which throws
the switch on the magnet and so breaks and reverses the current.
There are four striking weights, weighing respectively 50, 100,
250, and 500 pounds, any one of which may be used, depending
upon the desired energy of blow. When used for compression tests
a flat steel head six inches in diameter is screwed into the
lower end of the weight. For transverse tests, a well-rounded
knife edge is screwed into the weight in place of the flat head.
Knife edges for supporting the ends of the specimen to be
tested, are securely bolted to the base of the machine.
The record of the behavior of the specimen at time of impact is
traced upon a revolving drum by a pencil fixed in the striking
head. (See Fig. 41.) When a drop is made the pencil comes in
contact with the drum and is held in place by a spring. The drum
is revolved very slowly, either automatically or by hand. The
speed of the drum can be recorded by a pencil in the end of a
tuning fork which gives a known number of vibrations per second.
[Illustration: FIG. 41.--Drum record of impact bending test.]
One size of this machine will handle specimens for transverse
tests 9 inches wide and 6-foot span; the other, 12 inches wide
and 8-foot span. For compression tests a free fall of about 6.5
feet may be obtained. For transverse tests the fall is a little
less, depending upon the size of the specimen.
The machine is calibrated by dropping the hammer upon a copper
cylinder. The axial compression of the plug is noted. The energy
used in static tests to produce this axial compression under
stress in a like piece of metal is determined. The external
energy of the blow (_i.e._, the weight of the hammer X the
height of drop) is compared with the energy used in static tests
at equal amounts of compression. For instance:
Energy delivered, impact test 35,000 inch-pounds
Energy computed from static test .26,400 " "
Efficiency of blow of hammer .75.3 per cent.
_Preparing the material_: The material used in making impact
tests is of the same size and prepared in the same way as for
static bending and compression tests. Bending in impact tests is
more commonly used than compression, and small beams with
28-inch span are usually employed.
_Method_: In making an impact bending test the hammer is allowed
to rest upon the specimen and a zero or datum line is drawn. The
hammer is then dropped from increasing heights and drum records
taken until first failure. The first drop is one inch and the
increase is by increments of one inch until a height of ten
inches is reached, after which increments of two inches are used
until complete failure occurs or 6-inch deflection is secured.
The 50-pound hammer is used when with drops up to 68 inches it
is reasonably certain 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 is used.
_Results_: The tracing on the drum (see Fig. 41) represents the
actual deflection of the stick and the subsequent rebounds for
each drop. The distance from the lowest point in each case to
the datum line is measured and its square in tenths of a square
inch entered as an abscissa on cross-section paper, with the
height of drop in inches as the ordinate. The elastic limit is
that point on the diagram where the square of the deflection
begins to increase more rapidly than the height of drop. The
difference between the datum line and the final resting point
after each drop represents the set the material has received.
The formulæ used in calculating the results of impact tests in
bending when the load is applied at the centre up to the elastic
limit are as follows:
3 W H l
(1) r = -----------
D b h^{2}
F S l^{2}
(2) E = -----------
6 D h
W H
(3) S = -------
l b h
H = height of drop of hammer, including deflection, inches.
S = modulus of elastic resilience, inch-pounds per cubic inch.
W = weight of hammer, pounds.
Remainder of legend as in BENDING LARGE BEAMS, above.
HARDNESS TEST: ABRASION AND INDENTATION
_Abrasion_: The machine used by the U.S. Forest Service is a
modified form of the Dorry abrasion machine. (See Fig. 42.) Upon
the revolving horizontal disk is glued a commercial sandpaper,
known as garnet paper, which is commonly employed in factories
in finishing wood.
[Illustration: FIG. 42.--Abrasion machine for testing the
wearing qualities of woods.]
A small block of the wood to be tested is fixed in one clamp and
a similar block of some wood chosen as a standard, as sugar
maple, at 10 per cent moisture, in the opposite, and held
against the same zone of sandpaper by a weight of 26 pounds
each. The size of the section under abrasion for each specimen
is 2" X 2". The conditions for wear are the same for both
specimens. The speed of rotation is 68 revolutions a minute.
The test is continued until the standard specimen is worn a
specified amount, which varies with the kind of wood under test.
A comparison of the wear of the two blocks affords a fair idea
of their relative resistance to abrasion.
Another method makes use of a sand blast to abrade the woods and
is the one employed in New South Wales.[60] The apparatus
consists essentially of a nozzle through which sand can be
propelled at a high velocity against the test specimen by means
of a steam jet.
[Footnote 60: See Warren, W.H.: The strength, elasticity, and
other properties of New South Wales hardwood timbers. Dept.
For., N.S.W., Sydney, 1911, pp. 88-95.]
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° 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 = -------
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°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, }