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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15 THE MECHANICAL PROPERTIES OF WOOD
[Illustration: Frontispiece. _Photo by the author_.
Photomicrograph of a small block of western hemlock. At the top
is the cross section showing to the right the late wood of one
season's growth, to the left the early wood of the next season.
The other two sections are longitudinal and show the fibrous
character of the wood. To the left is the radial section with
three rays crossing it. To the right is the tangential section
upon which the rays appear as vertical rows of beads. X 35.]
THE MECHANICAL PROPERTIES OF WOOD
_Including a Discussion of the Factors Affecting the Mechanical
Properties, and Methods of Timber Testing_
BY SAMUEL J. RECORD, M.A., M.F. ASSISTANT PROFESSOR OF FOREST
PRODUCTS, YALE UNIVERSITY
FIRST EDITION FIRST THOUSAND
1914
BY THE SAME AUTHOR
Identification of the Economic Woods of the United States.
8vo, vi + 117 pages, 15 figures. Cloth, $1.25 net.
TO THE STAFF OF THE FOREST PRODUCTS LABORATORY, AT MADISON,
WISCONSIN IN APPRECIATION OF THE MANY OPPORTUNITIES AFFORDED AND
COURTESIES EXTENDED THE AUTHOR
PREFACE
This book was written primarily for students of forestry to whom
a knowledge of the technical properties of wood is essential.
The mechanics involved is reduced to the simplest terms and
without reference to higher mathematics, with which the students
rarely are familiar. The intention throughout has been to avoid
all unnecessarily technical language and descriptions, thereby
making the subject-matter readily available to every one
interested in wood.
Part I is devoted to a discussion of the mechanical properties
of wood--the relation of wood material to stresses and strains.
Much of the subject-matter is merely elementary mechanics of
materials in general, though written with reference to wood in
particular. Numerous tables are included, showing the various
strength values of many of the more important American woods.
Part II deals with the factors affecting the mechanical
properties of wood. This is a subject of interest to all who are
concerned in the rational use of wood, and to the forester it
also, by retrospection, suggests ways and means of regulating
his forest product through control of the conditions of
production. Attempt has been made, in the light of all data at
hand, to answer many moot questions, such as the effect on the
quality of wood of rate of growth, season of cutting, heartwood
and sapwood, locality of growth, weight, water content,
steaming, and defects.
Part III describes methods of timber testing. They are for the
most part those followed by the U.S. Forest Service. In schools
equipped with the necessary machinery the instructions will
serve to direct the tests; in others a study of the text with
reference to the illustrations should give an adequate
conception of the methods employed in this most important line
of research.
The appendix contains a copy of the working plan followed by the
U.S. Forest Service in the extensive investigations covering the
mechanical properties of the woods grown in the United States.
It contains many valuable suggestions for the independent
investigator. In addition four tables of strength values for
structural timbers, both green and air-seasoned, are included.
The relation of the stresses developed in different structural
forms to those developed in the small clear specimens is given.
In the bibliography attempt was made to list all of the
important publications and articles on the mechanical properties
of wood, and timber testing. While admittedly incomplete, it
should prove of assistance to the student who desires a fuller
knowledge of the subject than is presented here.
The writer is indebted to the U.S. Forest Service for nearly all
of his tables and photographs as well as many of the data upon
which the book is based, since only the Government is able to
conduct the extensive investigations essential to a thorough
understanding of the subject. More than eighty thousand tests
have been made at the Madison laboratory alone, and the work is
far from completion.
The writer also acknowledges his indebtedness to Mr. Emanuel
Fritz, M.E., M.F., for many helpful suggestions in the
preparation of Part I; and especially to Mr. Harry Donald
Tiemann, M.E., M.F., engineer in charge of Timber Physics at the
Government Forest Products Laboratory, Madison, Wisconsin, for
careful revision of the entire manuscript.
SAMUEL J. RECORD.
YALE FOREST SCHOOL, _July 1, 1914_.
CONTENTS
PREFACE
PART I THE MECHANICAL PROPERTIES OF WOOD
Introduction
Fundamental considerations and definitions
Tensile strength
Compressive or crushing strength
Shearing strength
Transverse or bending strength: Beams
Toughness: Torsion
Hardness
Cleavability
PART II FACTORS AFFECTING THE MECHANICAL PROPERTIES OF
WOOD
Introduction
Rate of growth
Heartwood and sapwood
Weight, density, and specific gravity
Color
Cross grain
Knots
Frost splits
Shakes, galls, pitch pockets
Insect injuries
Marine wood-borer injuries
Fungous injuries
Parasitic plant injuries
Locality of growth
Season of cutting
Water content
Temperature
Preservatives
PART III TIMBER TESTING
Working plan
Forms of material tested
Size of test specimens
Moisture determination
Machine for static tests
Speed of testing machine
Bending large beams
Bending small beams
Endwise compression
Compression across the grain
Shear along the grain
Impact test
Hardness test: Abrasion and indentation
Cleavage test
Tension test parallel to the grain
Tension test at right angles to the grain
Torsion test
Special tests
Spike pulling test
Packing boxes
Vehicle and implement woods
Cross-arms
Other tests
APPENDIX
Sample working plan of United States Forest
Service
Strength values for structural timbers
BIBLIOGRAPHY
Part I: Some general works on mechanics, materials of
construction, and testing of materials
Part II: Publications and articles on the mechanical
properties of wood, and timber testing
Part III: Publications of the United States Government on
the mechanical properties of wood, and timber
testing
ILLUSTRATIONS
Frontispiece Photomicrograph of a small block of western
hemlock
1. Stress-strain diagrams of two longleaf pine beams
2. Compression across the grain
3. Side view of failures in compression across the
grain
4. End view of failures in compression across the
grain
5. Testing a buggy-spoke in endwise compression
6. Unequal distribution of stress in a long column due
to lateral bending
7. Endwise compression of a short column
8. Failures of a short column of green spruce
9. Failures of short columns of dry chestnut
10. Example of shear along the grain
11. Failures of test specimens in shear along the
grain
12. Horizontal shear in a beam
13. Oblique shear in a short column
14. Failure of a short column by oblique shear
15. Diagram of a simple beam
16. Three common forms of beams--(1) simple,
(2) cantilever, (3) continuous
17. Characteristic failures of simple beams
18. Failure of a large beam by horizontal shear
19. Torsion of a shaft
20. Effect of torsion on different grades of hickory
21. Cleavage of highly elastic wood
22. Cross-sections of white ash, red gum, and eastern
hemlock
23. Cross-section of longleaf pine
24. Relation of the moisture content to the various
strength values of spruce
25. Cross-section of the wood of western larch showing
fissures in the thick-walled cells of the late
wood
26. Progress of drying throughout the length of a
chestnut beam
27. Excessive season checking
28. Control of season checking by the use of S-irons
29. Static bending test on a large beam
30. Two methods of loading a beam
31. Static bending test on a small beam
32. Sample log sheet, giving full details of a
transverse bending test on a small pine beam
33. Endwise compression test
34. Sample log sheet of an endwise compression test on
a short pine column
35. Compression across the grain
36. Vertical section of shearing tool
37. Front view of shearing tool
38. Two forms of shear test specimens
39. Making a shearing test
40. Impact testing machine
41. Drum record of impact bending test
42. Abrasion machine for testing the wearing qualities
of woods
43. Design of tool for testing the hardness of woods
by indentation
44. Design of tool for cleavage test
45. Design of cleavage test specimen
46. Designs of tension test specimens used in United
States
47. Design of tension test specimen used in New South
Wales
48. Design of tool and specimen for testing tension at
right angles to the grain
49. Making a torsion test on hickory
50. Method of cutting and marking test specimens
51. Diagram of specific gravity apparatus
TABLES
I. Comparative strength of iron, steel, and wood
II. Ratio of strength of wood in tension and in
compression
III. Right-angled tensile strength of small clear
pieces of 25 woods in green condition
IV. Results of compression tests across the grain on
51 woods in green condition, and comparison with
white oak
V. Relation of fibre stress at elastic limit in
bending to the crushing strength of blocks cut
therefrom in pounds per square inch
VI. Results of endwise compression tests on small
clear pieces of 40 woods in green condition
VII. Shearing strength along the grain of small clear
pieces of 41 woods in green condition
VIII. Shearing strength across the grain of various
American woods
IX. Results of static bending tests on small clear
beams of 49 woods in green condition
X. Results of impact bending tests on small clear
beams of 34 woods in green condition
XI. Manner of first failure of large beams
XII. Hardness of 32 woods in green condition, as
indicated by the load required to imbed a
0.444-inch steel ball to one-half its diameter
XIII. Cleavage strength of small clear pieces of 32
woods in green condition
XIV. Specific gravity, and shrinkage of 51 American
woods
XV. Effect of drying on the mechanical properties of
wood, shown in ratio of increase due to reducing
moisture content from the green condition to
kiln-dry
XVI. Effect of steaming on the strength of green
loblolly pine
XVII. Speed-strength moduli, and relative increase in
strength at rates of fibre strain increasing in
geometric ratio
XVIII. Results of bending tests on green structural
timbers
XIX. Results of compression and shear tests on green
structural timbers
XX. Results of bending tests on air-seasoned
structural timbers
XXI. Results of compression and shear tests on
air-seasoned structural timbers
XXII. Working unit stresses for structural timber
expressed in pounds per square inch
PART I THE MECHANICAL PROPERTIES OF WOOD
INTRODUCTION
The mechanical properties of wood are its fitness and ability to
resist applied or external forces. By external force is meant
any force outside of a given piece of material which tends to
deform it in any manner. It is largely such properties that
determine the use of wood for structural and building purposes
and innumerable other uses of which furniture, vehicles,
implements, and tool handles are a few common examples.
Knowledge of these properties is obtained through
experimentation either in the employment of the wood in practice
or by means of special testing apparatus in the laboratory.
Owing to the wide range of variation in wood it is necessary
that a great number of tests be made and that so far as possible
all disturbing factors be eliminated. For comparison of
different kinds or sizes a standard method of testing is
necessary and the values must be expressed in some defined
units. For these reasons laboratory experiments if properly
conducted have many advantages over any other method.
One object of such investigation is to find unit values for
strength and stiffness, etc. These, because of the complex
structure of wood, cannot have a constant value which will be
exactly repeated in each test, even though no error be made. The
most that can be accomplished is to find average values, the
amount of variation above and below, and the laws which govern
the variation. On account of the great variability in strength
of different specimens of wood even from the same stick and
appearing to be alike, it is important to eliminate as far as
possible all extraneous factors liable to influence the results
of the tests.
The mechanical properties of wood considered in this book are:
(1) stiffness and elasticity, (2) tensile strength, (3)
compressive or crushing strength, (4) shearing strength, (5)
transverse or bending strength, (6) toughness, (7) hardness, (8)
cleavability, (9) resilience. In connection with these,
associated properties of importance are briefly treated.
In making use of figures indicating the strength or other
mechanical properties of wood for the purpose of comparing the
relative merits of different species, the fact should be borne
in mind that there is a considerable range in variability of
each individual material and that small differences, such as a
few hundred pounds in values of 10,000 pounds, cannot be
considered as a criterion of the quality of the timber. In
testing material of the same kind and grade, differences of 25
per cent between individual specimens may be expected in
conifers and 50 per cent or even more in hardwoods. The figures
given in the tables should be taken as indications rather than
fixed values, and as applicable to a large number collectively
and not to individual pieces.
FUNDAMENTAL CONSIDERATIONS AND DEFINITIONS
Study of the mechanical properties of a material is concerned
mostly with its behavior in relation to stresses and strains,
and the factors affecting this behavior. A ~stress~ is a
distributed force and may be defined as the mutual action (1) of
one body upon another, or (2) of one part of a body upon another
part. In the first case the stress is _external_; in the other
_internal_. The same stress may be internal from one point of
view and external from another. An external force is always
balanced by the internal stresses when the body is in
equilibrium.
If no external forces act upon a body its particles assume
certain relative positions, and it has what is called its
_natural shape and size_. If sufficient external force is
applied the natural shape and size will be changed. This
distortion or deformation of the material is known as the
~strain~. Every stress produces a corresponding strain, and
within a certain limit (see _elastic limit_, in FUNDAMENTAL
CONSIDERATIONS AND DEFINITIONS, above) the strain is directly
proportional to the stress producing it.[1] The same intensity
of stress, however, does not produce the same strain in
different materials or in different qualities of the same
material. No strain would be produced in a perfectly rigid body,
but such is not known to exist.
[Footnote 1: This is in accordance with the discovery made in
1678 by Robert Hooke, and is known as _Hooke's law_.]
Stress is measured in pounds (or other unit of weight or force).
A ~unit stress~ is the stress on a unit of the sectional
{ P }
area. { Unit stress = --- } For instance, if a load (P) of one
{ A }
hundred pounds is uniformly supported by a vertical post with a
cross-sectional area (A) of ten square inches, the unit
compressive stress is ten pounds per square inch.
Strain is measured in inches (or other linear unit). A ~unit
strain~ is the strain per unit of length. Thus if a post 10
inches long before compression is 9.9 inches long under the
compressive stress, the total strain is 0.1 inch, and the unit
l 0.1
strain is --- = ----- = 0.01 inch per inch of length.
L 10
As the stress increases there is a corresponding increase in the
strain. This ratio may be graphically shown by means of a
diagram or curve plotted with the increments of load or stress
as ordinates and the increments of strain as abscissae. This is
known as the ~stress-strain diagram~. Within the limit mentioned
above the diagram is a straight line. (See Fig. 1.) If the
results of similar experiments on different specimens are
plotted to the same scales, the diagrams furnish a ready means
for comparison. The greater the resistance a material offers to
deformation the steeper or nearer the vertical axis will be the
line.
[Illustration: FIG. 1.--Stress-strain diagrams of two longleaf
pine beams. E.L. = elastic limit. The areas of the triangles
0(EL)A and 0(EL)B represent the elastic resilience of the dry
and green beams, respectively.]
There are three kinds of internal stresses, namely, (1)
~tensile~, (2) ~compressive~, and (3) ~shearing~. When external
forces act upon a bar in a direction away from its ends or a
direct pull, the stress is a tensile stress; when toward the
ends or a direct push, compressive stress. In the first instance
the strain is an _elongation_; in the second a _shortening_.
Whenever the forces tend to cause one portion of the material to
slide upon another adjacent to it the action is called a
_shear_. The action is that of an ordinary pair of shears. When
riveted plates slide on each other the rivets are sheared off.
These three simple stresses may act together, producing compound
stresses, as in flexure. When a bow is bent there is a
compression of the fibres on the inner or concave side and an
elongation of the fibres on the outer or convex side. There is
also a tendency of the various fibres to slide past one another
in a longitudinal direction. If the bow were made of two or more
separate pieces of equal length it would be noted on bending
that slipping occurred along the surfaces of contact, and that
the ends would no longer be even. If these pieces were securely
glued together they would no longer slip, but the tendency to do
so would exist just the same. Moreover, it would be found in the
latter case that the bow would be much harder to bend than where
the pieces were not glued together--in other words, the
_stiffness_ of the bow would be materially increased.
~Stiffness~ is the property by means of which a body acted upon
by external forces tends to retain its natural size and shape,
or resists deformation. Thus a material that is difficult to
bend or otherwise deform is stiff; one that is easily bent or
otherwise deformed is _flexible_. Flexibility is not the exact
counterpart of stiffness, as it also involves toughness and
pliability.
If successively larger loads are applied to a body and then
removed it will be found that at first the body completely
regains its original form upon release from the stress--in other
words, the body is ~elastic~. No substance known is perfectly
elastic, though many are practically so under small loads.
Eventually a point will be reached where the recovery of the
specimen is incomplete. This point is known as the ~elastic
limit~, which may be defined as the limit beyond which it is
impossible to carry the distortion of a body without producing a
permanent alteration in shape. After this limit has been
exceeded, the size and shape of the specimen after removal of
the load will not be the same as before, and the difference or
amount of change is known as the ~permanent set~.
Elastic limit as measured in tests and used in design may be
defined as that unit stress at which the deformation begins to
increase in a faster ratio than the applied load. In practice
the elastic limit of a material under test is determined from
the stress-strain diagram. It is that point in the line where
the diagram begins perceptibly to curve.[2] (See Fig. 1.)
[Footnote 2: If the straight portion does not pass through the
origin, a parallel line should be drawn through the origin, and
the load at elastic limit taken from this line. (See Fig. 32.)]
~Resilience~ is the amount of work done upon a body in deforming
it. Within the elastic limit it is also a measure of the
potential energy stored in the material and represents the
amount of work the material would do upon being released from a
state of stress. This may be graphically represented by a
diagram in which the abscissae represent the amount of deflection
and the ordinates the force acting. The area included between
the stress-strain curve and the initial line (which is zero)
represents the work done. (See Fig. 1.) If the unit of space is
in inches and the unit of force is in pounds the result is
inch-pounds. If the elastic limit is taken as the apex of the
triangle the area of the triangle will represent the ~elastic
resilience~ of the specimen. This amount of work can be applied
repeatedly and is perhaps the best measure of the toughness of
the wood as a working quality, though it is not synonymous with
toughness.
Permanent set is due to the ~plasticity~ of the material. A
perfectly plastic substance would have no elasticity and the
smallest forces would cause a set. Lead and moist clay are
nearly plastic and wood possesses this property to a greater or
less extent. The plasticity of wood is increased by wetting,
heating, and especially by steaming and boiling. Were it not for
this property it would be impossible to dry wood without
destroying completely its cohesion, due to the irregularity of
shrinkage.
A substance that can undergo little change in shape without
breaking or rupturing is ~brittle~. Chalk and glass are common
examples of brittle materials. Sometimes the word _brash_ is
used to describe this condition in wood. A brittle wood breaks
suddenly with a clean instead of a splintery fracture and
without warning. Such woods are unfitted to resist shock or
sudden application of load.
The measure of the stiffness of wood is termed the ~modulus of
elasticity~ (or _coefficient of elasticity_). It is the ratio of
stress per unit of area to the deformation per unit of
{ unit stress }
length. { E = ------------- } It is a number indicative of
{ unit strain }
stiffness, not of strength, and only applies to conditions
within the elastic limit. It is nearly the same whether derived
from compression tests or from tension tests.
A large modulus indicates a stiff material. Thus in green wood
tested in static bending it varies from 643,000 pounds per
square inch for arborvitae to 1,662,000 pounds for longleaf pine,
and 1,769,000 pounds for pignut hickory. (See Table IX.) The
values derived from tests of small beams of dry material are
much greater, approaching 3,000,000 for some of our woods. These
values are small when compared with steel which has a modulus of
elasticity of about 30,000,000 pounds per square inch. (See
Table I.)
|------------------------------------------------------------------------------|
| TABLE I |
|------------------------------------------------------------------------------|
| COMPARATIVE STRENGTH OF IRON, STEEL, AND WOOD |
|------------------------------------------------------------------------------|
| | Sp. | Modulus of | Tensile | Crushing | Modulus |
| MATERIAL | gr., | elasticity | strength | strength | of |
| | dry | in bending | | | rupture |
|-------------------------+----- +------------+----------+----------+----------|
| | | Lbs. per | Lbs. per | Lbs. per | Lbs. per |
| | | sq. in. | sq. in. | sq. in. | sq. in. |
| | | | | | |
| Cast iron, cold blast | | | | | |
| (Hodgkinson) | 7.1 | 17,270,000 | 16,700 | 106,000 | 38,500 |
| Bessenger steel, | | | | | |
| high grade (Fairbain) | 7.8 | 29,215,000 | 88,400 | 225,600 | |
| Longleaf pine, | | | | | |
| 3.5% moisture (U.S.) | .63 | 2,800,000 | | 13,000 | 21,000 |
| Redspruce, | | | | | |
| 3.5% moisture (U.S.) | .41 | 1,800,000 | | 8,800 | 14,500 |
| Pignut hickory, | | | | | |
| 3.5% moisture (U.S.) | .86 | 2,370,000 | | 11,130 | 24,000 |
|------------------------------------------------------------------------------|
| NOTE.--Great variation may be found in different samples of metals as well |
| as of wood. The examples given represent reasonable values. |
|------------------------------------------------------------------------------|
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