The Mechanical Properties of Wood by Samuel J. Record
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Samuel J. Record >> The Mechanical Properties of Wood
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"(3) A high degree of steaming is injurious to wood in strength
and spike-holding power. The degree of steaming at which
pronounced harm results will depend upon the quality of the wood
and its degree of seasoning, and upon the pressure (temperature)
of steam and the duration of its application. For loblolly pine
the limit of safety is certainly 30 pounds for 4 hours, or 20
pounds for 6 hours."[52]
[Footnote 52: _Ibid._, p. 21. See also Cir. 108, p. 19, table
5.]
Experiments made at the Yale Forest School showed that steaming
above 30 pounds' gauge pressure reduces the strength of wood
permanently while wet from 25 to 75 per cent.
PRESERVATIVES
The exact effects of chemical impregnation upon the mechanical
properties of wood have not been fully determined, though they
have been the subject of considerable investigation.[53] More
depends upon the method of treatment than upon the preservatives
used. Thus preliminary steaming at too high pressure or for too
long a period will materially weaken the wood, (See TEMPERATURE,
above.)
[Footnote 53: Hatt, W. K.: Experiments on the strength of
treated timber. Cir. 39, U.S. Forest Service, 1906, p. 31.]
The presence of zinc chloride does not weaken wood under static
loading, although the indications are that the wood becomes
brittle under impact. If the solution is too strong it will
decompose the wood.
Soaking in creosote oil causes wood to swell, and accordingly
decreases the strength to some extent, but not nearly so much so
as soaking in water.[54]
[Footnote 54: Teesdale, Clyde II.: The absorption of creosote by
the cell walls of wood. Cir. 200, U. S. Forest Service, 1912, p.
7.]
Soaking in kerosene seems to have no significant weakening
effect.[55]
[Footnote 55: Tiemann, H.D.: Effect of moisture upon the
strength and stiffness of wood. Bul. 70, U. S. Forest Service,
1907, pp. 122-123, tables 43-44.]
PART III TIMBER TESTING[56]
[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 deg. C. (212 deg. 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 formulae 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.
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