THE MECHANICAL PROPERTIES OF WOOD INCLUDING A DISCUSSION OF THE FACTORS AFFECTING THE MECHANICAL PROPERTIES, AND METHODS OF TIMBER TESTING TEXT

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 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. [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