Chapter 7

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