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The Mechanical Properties Of Wood | by Samuel J. Record



Including a discussion of the factors affecting the mechanical properties, and methods of timber testing

TitleThe Mechanical Properties Of Wood
AuthorSamuel J. Record
PublisherJ. Wiley & Sons, Inc.
Year1914
Copyright1914, J. Wiley & Sons, Inc.
AmazonThe Mechanical Properties Of Wood

The Mechanical Properties Of Wood.

By. Samuel J. Record, M.A., M.F. Assistant Professor Of Forest Products, Yale University

Frontispiece

Frontispiece.

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. × 35. Photo by 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 refer...
-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...
-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 i...
-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...
-Tensile Strength
Tension results when a pulling force is applied to opposite ends of a body. This external pull is communicated to the interior, so that any portion of the material exerts a pull or tensile force upon ...
-Compressive Or Crushing Strength
Compression across the grain is very closely related to hardness and transverse shear. There are two ways in which wood is subjected to stress of this kind, namely, (1) with the load acting over the e...
-Compressive Or Crushing Strength. Continued
The first is typical of any dry thin-walled cells, as is usually the case in seasoned white pine and spruce, and in the early wood of hard pines, hemlock, and other species with decided contrast betwe...
-Shearing Strength
Whenever forces act upon a body in such a way that one portion tends to slide upon another adjacent to it the action is called a shear.8 In wood this shearing action may be (1) along the grain, or (2)...
-Transverse Or Bending Strength: Beams
When external forces acting in the same plane are applied at right angles to the axis of a bar so as to cause it to bend, they occasion a shortening of the longitudinal fibres on the concave side and ...
-Beams
There are three common forms of beams, as follows: (1) Simple beam - a bar resting upon two supports, one near each end. (See Fig. 16, No. 1.) (2) Cantilever beam - a bar resting upon one suppor...
-Stiffness Of Beams
The two main requirements of a beam are stiffness and strength. The formul for the modulus of elasticity (E) or measure of stiffness of a rectangular prismatic simple beam loaded at the centre ...
-Strength Of Beams
The measure of the breaking strength of a beam is expressed in terms of unit stress by a modulus of rupture, which is a purely hypothetical expression for points beyond the elastic limit. The formul&a...
-Kinds Of Loads
There are various ways in which beams are loaded, of which the following are the most important: (1) Uniform load occurs where the load is spread evenly over the beam. (2) Concentrated load occu...
-Application Of Loads
There are three12 general methods in which loads may be applied to beams, namely: [Footnote 12: A fourth might be added, namely, vibratory, or harmonic repetition, which is frequently serious in th...
-Failures In Timber Beams
If a beam is loaded too heavily it will break or fail in some characteristic manner. These failures may be classified according to the way in which they develop, as tension, compression, and horizonta...
-Toughness: Torsion
Toughness is a term applied to more than one property of wood. Thus wood that is difficult to split is said to be tough. Again, a tough wood is one that will not rupture until it has deformed consider...
-Hardness
The term hardness is used in two senses, namely: (1) resistance to indentation, and (2) resistance to abrasion or scratching. In the latter sense hardness combined with toughness is a measure of the w...
-Cleavability
Cleavability is the term used to denote the facility with which wood is split. A splitting stress is one in which the forces act normally like a wedge. (See Fig. 21.) The plane of cleavage is parallel...
-Part II. Factors Affecting The Mechanical Properties Of Wood. Introduction
Wood is an organic product - a structure of infinite variation of detail and design.17 It is on this account that no two woods are alike - in reality no two specimens from the same log are identical. ...
-Rate Of Growth
To understand the effect of variations in the rate of growth it is first necessary to know how wood is formed. A tree increases in diameter by the formation, between the old wood and the inner bark, o...
-Rate Of Growth. Continued
An analysis of tests on large beams was made to ascertain if average rate of growth has any relation to the mechanical properties of the beams. The analysis indicated conclusively that there was no s...
-Heartwood And Sapwood
Examination of the end of a log of many species reveals a darker-colored inner portion - the heartwood, surrounded by a lighter-colored zone - the sapwood. In some instances this distinction in color ...
-Weight, Density, And Specific Gravity
From data obtained from a large number of tests on the strength of different woods it appears that, other things being equal, the crushing strength parallel to the grain, fibre stress at elastic limit...
-Color
In species which show a distinct difference between heartwood and sapwood the natural color of heartwood is invariably darker than that of the sapwood, and very frequently the contrast is conspicuous....
-Cross Grain
Cross grain is a very common defect in timber. One form of it is produced in lumber by the method of sawing and has no reference to the natural arrangement of the wood elements. Thus if the plane of t...
-Knots
Knots are portions of branches included in the wood of the stem or larger branch. Branches originate as a rule from the central axis of a stem, and while living increase in size by the addition of ann...
-Frost Splits
A common defect in standing timber results from radial splits which extend inward from the periphery of the tree, and almost, if not always, near the base. It is most common in trees which split readi...
-Shakes, Galls, Pitch Pockets
Heart shake occurs in nearly all overmature timber, being more frequent in hardwoods (especially oak) than in conifers. In typical heart shake the centre of the hole shows indications of becoming holl...
-Insect Injuries37
The larv of many insects are destructive to wood. Some attack the wood of living trees, others only that of felled or converted material. Every hole breaks the continuity of the fibres and impa...
-Marine Wood-Borer Injuries
Vast amounts of timber used for piles in wharves and other marine structures are constantly being destroyed or seriously injured by marine borers. Almost invariably they are confined to salt water, an...
-Fungous Injuries
Fungi are responsible for almost all decay of wood. So far as known, all decay is produced by living organisms, either fungi or bacteria. Some species attack living trees, sometimes killing them, or m...
-Parasitic Plant Injuries
The most common of the higher parasitic plants damaging timber trees are mistletoes. Many species of deciduous trees are attacked by the common mistletoe (Phoradendron flavescens). It is very prevalen...
-Locality Of Growth
The data available regarding the effect of the locality of growth upon the properties of wood are not sufficient to warrant definite conclusions. The subject has, however, been kept in mind in many of...
-Season Of Cutting
It is generally believed that winter-felled timber has decided advantages over that cut at other seasons of the year, and to that cause alone are frequently ascribed much greater durability, less liab...
-Water Content
Water occurs in living wood in three conditions, namely: (1) in the cell walls, (2) in the protoplasmic contents of the cells, and (3) as free water in the cell cavities and spaces. In heartwood it oc...
-Water Content. Continued
Progress of drying throughout the length of a chestnut beam, the black spots indicating the presence of free water in the wood. The first section at the left was cut one-fourth inch from the end, the ...
-Temperature
The effect of temperature on wood depends very largely upon the moisture content of the wood and the surrounding medium. If absolutely dry wood is heated in absolutely dry air the wood expands. The ex...
-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 t...
-Part III. Timber Testing. 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, ...
-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 ...
-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 us...
-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 a...
-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 inte...
-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 strai...
-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.) Figure ...
-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.) ...
-Endwise Compression
Apparatus: An ordinary static testing machine and a compressometer are required. (See Fig. 33.) Figure 33 Endwise compression test, showing method of measuring the deformation by means ...
-Compression Across The Grain
Apparatus: An ordinary static testing machine, a bearing plate, and a deflectometer are required. (See Fig. 35.) Figure 35 Compression across the grain. Note method of measuring the def...
-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 s...
-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 ...
-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 garn...
-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 ca...
-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...
-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...
-Torsion Test
Apparatus: The torsion test is made in a Riehle-Miller torsional testing machine or its equivalent. (See Fig. 49.) [Footnote 64: Wood is so seldom subjected to a pure stress of this kind that the t...
-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, a...
-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 substitut...
-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...
-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 th...
-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 w...
-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 f...
-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 wi...
-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 ot...
-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)...
-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...
-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...
-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 cond...
-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 s...
-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 test...
-Methods Of Test
All tests will be made according to Circular 38 except in case of conflict with the instructions given below: Static bending: The tests will be on specimens 2 2 30 on 28-inch spa...
-Moisture Determinations
Moisture determinations will be made on all specimens tested except those to be photographed or kept for exhibit. A 1-inch disk will be cut from near the point of failure of bending and compression pa...
-Radial And Tangential Shrinkage. Specimens
Preparation: Two specimens 1 inch thick, 4 inches wide, and 1 inch long will be obtained from near the periphery of each d bolt. These will be cut from the sector-shaped sections left after securing...
-Volumetric Shrinkage And Specific Gravity. Specimens
Selection and preparation: Four 2 2 6 specimens will be cut from the mechanical test sticks of each d bolt; also from each of the composite bolts used in getting a comparison of g...
-Strength Values For Structural Timbers. (From Cir. 189, U.s. Forest Service)
The following tables bring together in condensed form the average strength values resulting from a large number of tests made by the Forest Service on the principal structural timbers of the United St...
-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: Pub...
-Bibliography Part II. Publications And Articles On The Mechanical Properties Of Wood, And Timber Testing
ABBOT, ARTHUR V.: Testing machines, their history, construction and use. Van Nostrand's Eng. Mag., Vol. XXX, 1884, pp. 204-214; 325-344; 382-397; 477-490. ADAMS, E.E.: Tests to determine the streng...
-Publications And Articles On The Mechanical Properties Of Wood, And Timber Testing. Part 2
GOTTGETREU, RUDOLPH: Physische und chemische Beschaffenheit der Baumaterialien. 3d ed., Berlin, 1880. GREEN, A.O.: Tasmanian timbers: their qualities and uses. Hobart, Tasmania, 1903, pp. 63. GR...
-Articles On The Mechanical Properties Of Wood, And Timber Testing. Part 3
LABORDRE, P., and ANSTETT, F.: Contribution to the study of means for improving the strength of wood for pavements. Proc. Int. Assn. Test. Mat., 1912, XXIII, pp. 12. LANZA, GAETANO: An acco...
-Bibliography Part III. Publications Of The U.s. Government On The Mechanical Properties Of Wood, And Timber Testing. Miscellaneous
House Misc. Doc. 42, pt. 9, 47th Cong., 2d sess., 1884. (Vol. IX, Tenth Census report.) Report on the forests of North America (exclusive of Mexico). Part II, The Woods of the United States. House ...
-Forest Service
Cir. 7 - The Government timber tests [189-], pp. 4. Cir. 8 - Strength of boxed or turpentine timber. 1892, pp. 4. Bul. 6 - Timber Physics. Pt. I. Preliminary report. 1. Need of the investiga...
-Reports Of Tests On The Strength Of Structural Material, Made At The Watertown Arsenal, Mass
House Ex. Doc. No. 12, 47th Cong., 1st sess., 1882. Strength of wood grown on the Pacific slope, pp. 19-93. Senate Ex. Doc. No. 1, 47th Cong., 2d sess., 1883. Resistance of white and yellow pines t...







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previous page: Carpentry for Boys. How-To-Do-It Series | by J. S. Zerbe
  
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