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
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