The Mechanical Properties of Wood Including a Discussion of the Factors Affecting the Mechanical Properties, and Methods of Timber Testing

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Title: The Mechanical Properties of Wood

       Including a Discussion of the Factors Affecting the Mechanical

              Properties, and Methods of Timber Testing

Author: Samuel J. Record

Release Date: May 8, 2004 [EBook #12299]

Language: English

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THE MECHANICAL PROPERTIES OF WOOD

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.

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

INDEX

FOOTNOTES

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, page 5) 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.

Stress is measured in pounds (or other unit of weight or force). A unit stress is the stress on a unit of the sectional area.

For instance, if a load (P) of one 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 strain is

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 abscissæ. 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.

Figure 1

Stress-strain diagrams