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

so would exist just the same. Moreover, it would be found in the latter case that the bow would be much harder to bend than where the pieces were not glued together--in other words, the _stiffness_ of the bow would be materially increased. ~Stiffness~ is the property by means of which a body acted upon by external forces tends to retain its natural size and shape, or resists deformation. Thus a material that is difficult to bend or otherwise deform is stiff; one that is easily bent or otherwise deformed is _flexible_. Flexibility is not the exact counterpart of stiffness, as it also involves toughness and pliability. If successively larger loads are applied to a body and then removed it will be found that at first the body completely regains its original form upon release from the stress--in other words, the body is ~elastic~. No substance known is perfectly elastic, though many are practically so under small loads. Eventually a point will be reached where the recovery of the specimen is incomplete. This point is known as the ~elastic limit~, which may be defined as the limit beyond which it is impossible to carry the distortion of a body without producing a permanent alteration in shape. After this limit has been exceeded, the size and shape of the specimen after removal of the load will not be the same as before, and the difference or amount of change is known as the ~permanent set~. Elastic limit as measured in tests and used in design may be defined as that unit stress at which the deformation begins to increase in a faster ratio than the applied load. In practice the elastic limit of a material under test is determined from the stress-strain diagram. It is that point in the line where the diagram begins perceptibly to curve.[2] (See Fig. 1.) [Footnote 2: If the straight portion does not pass through the origin, a parallel line should be drawn through the origin, and the load at elastic limit taken from this line. (See Fig. 32.)] ~Resilience~ is the amount of work done upon a body in deforming it. Within the elastic limit it is also a measure of the potential energy stored in the material and represents the amount of work the material would do upon being released from a state of stress. This may be graphically represented by a diagram in which the abscissæ represent the amount of deflection and the ordinates the force acting. The area included between the stress-strain curve and the initial line (which is zero) represents the work done. (See Fig. 1.) If the unit of space is in inches and the unit of force is in pounds the result is inch-pounds. If the elastic limit is taken as the apex of the triangle the area of the triangle will represent the ~elastic resilience~ of the specimen. This amount of work can be applied repeatedly and is perhaps the best measure of the toughness of the wood as a working quality, though it is not synonymous with toughness. Permanent set is due to the ~plasticity~ of the material. A perfectly plastic substance would have no elasticity and the smallest forces would cause a set. Lead and moist clay are nearly plastic and wood possesses this property to a greater or less extent. The plasticity of wood is increased by wetting, heating, and especially by steaming and boiling. Were it not for this property it would be impossible to dry wood without destroying completely its cohesion, due to the irregularity of shrinkage. A substance that can undergo little change in shape without breaking or rupturing is ~brittle~. Chalk and glass are common examples of brittle materials. Sometimes the word _brash_ is used to describe this condition in wood. A brittle wood breaks suddenly with a clean instead of a splintery fracture and without warning. Such woods are unfitted to resist shock or sudden application of load. The measure of the stiffness of wood is termed the ~modulus of elasticity~ (or _coefficient of elasticity_). It is the ratio of stress per unit of area to the deformation per unit of { unit stress } length. { E = ------------- } It is a number indicative of { unit strain } stiffness, not of strength, and only applies to conditions within the elastic limit. It is nearly the same whether derived from compression tests or from tension tests. A large modulus indicates a stiff material. Thus in green wood tested in static bending it varies from 643,000 pounds per square inch for arborvitæ to 1,662,000 pounds for longleaf pine, and 1,769,000 pounds for pignut hickory. (See Table IX.) The values derived from tests of small beams of dry material are much greater, approaching 3,000,000 for some of our woods. These values are small when compared with steel which has a modulus of elasticity of about 30,000,000 pounds per square inch. (See Table I.) |------------------------------------------------------------------------------| | TABLE I | |------------------------------------------------------------------------------| | COMPARATIVE STRENGTH OF IRON, STEEL, AND WOOD | |------------------------------------------------------------------------------| | | Sp. | Modulus of | Tensile | Crushing | Modulus | | MATERIAL | gr., | elasticity | strength | strength | of | | | dry | in bending | | | rupture | |-------------------------+----- +------------+----------+----------+----------| | | | Lbs. per | Lbs. per | Lbs. per | Lbs. per | | | | sq. in. | sq. in. | sq. in. | sq. in. | | | | | | | | | Cast iron, cold blast | | | | | | | (Hodgkinson) | 7.1 | 17,270,000 | 16,700 | 106,000 | 38,500 | | Bessenger steel, | | | | | | | high grade (Fairbain) | 7.8 | 29,215,000 | 88,400 | 225,600 | | | Longleaf pine, | | | | | | | 3.5% moisture (U.S.) | .63 | 2,800,000 | | 13,000 | 21,000 | | Redspruce, | | | | | | | 3.5% moisture (U.S.) | .41 | 1,800,000 | | 8,800 | 14,500 | | Pignut hickory, | | | | | | | 3.5% moisture (U.S.) | .86 | 2,370,000 | | 11,130 | 24,000 | |------------------------------------------------------------------------------| | NOTE.--Great variation may be found in different samples of metals as well | | as of wood. The examples given represent reasonable values. | |------------------------------------------------------------------------------| 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 the remainder, the ability to do so depending upon the property of cohesion. The result is an elongation or stretching of the material in the direction of the applied force. The action is the opposite of compression. Wood exhibits its greatest strength in tension parallel to the grain, and it is very uncommon in practice for a specimen to be pulled in two lengthwise. This is due to the difficulty of making the end fastenings secure enough for the full tensile strength to be brought into play before the fastenings shear off longitudinally. This is not the case with metals, and as a result they are used in almost all places where tensile strength is particularly needed, even though the remainder of the structure, such as sills, beams, joists, posts, and flooring, may be of wood. Thus in a wooden truss bridge the tension members are steel rods. The tensile strength of wood parallel to the grain depends upon the strength of the fibres and is affected not only by the nature and dimensions of the wood elements but also by their arrangement. It is greatest in straight-grained specimens with thick-walled fibres. Cross grain of any kind materially reduces the tensile strength of wood, since the tensile strength at right angles to the grain is only a small fraction of that parallel to the grain. |--------------------------------------------------------------| | TABLE II | |--------------------------------------------------------------| | RATIO OF STRENGTH OF WOOD IN TENSION AND IN COMPRESSION | | (Bul. 10, U. S. Div. of Forestry, p. 44) | |--------------------------------------------------------------| | | Ratio: | A stick 1 square inch in | | | | cross section. | | | Tensile | | | KIND OF WOOD | strength | Weight required to-- | | | R = ----------- +----------------------------| | | compressive | Pull apart | Crush endwise | | | strength | | | |---------------+-----------------+------------+---------------| | Hickory | 3.7 | 32,000 | 8,500 | | Elm | 3.8 | 29,000 | 7,500 | | Larch | 2.3 | 19,400 | 8,600 | | Longleaf Pine | 2.2 | 17,300 | 7,400 | |--------------------------------------------------------------| | NOTE.--Moisture condition not given. | |--------------------------------------------------------------| Failure of wood in tension parallel to the grain occurs sometimes in flexure, especially with dry material. The tension portion of the fracture is nearly the same as though the piece were pulled in two lengthwise. The fibre walls are torn across obliquely and usually in a spiral direction. There is practically no pulling apart of the fibres, that is, no separation of the fibres along their walls, regardless of their thickness. The nature of tension failure is apparently not affected by the moisture condition of the specimen, at least not so much so as the other strength values.[3] [Footnote 3: See Brush, Warren D.: A microscopic study of the mechanical failure of wood. Vol. II, Rev. F.S. Investigations, Washington, D.C., 1912, p. 35.] Tension at right angles to the grain is closely related to cleavability. When wood fails in this manner the thin fibre walls are torn in two lengthwise while the thick-walled fibres are usually pulled apart along the primary wall. |--------------------------------------------| | TABLE III | |--------------------------------------------| | TENSILE STRENGTH AT RIGHT ANGLES TO THE | | GRAIN OF SMALL CLEAR PIECES OF 25 WOODS IN | | GREEN CONDITION | | (Forest Service Cir. 213) | |--------------------------------------------| | | When | When | | COMMON NAME | surface of | surface of | | OF SPECIES | failure is | failure is | | | radial | tangential | |------------------+------------+------------| | | Lbs. per | Lbs. per | | | sq. inch | sq. inch | | | | | | Hardwoods | | | | | | | | Ash, white | 645 | 671 | | Basswood | 226 | 303 | | Beech | 633 | 969 | | Birch, yellow | 446 | 526 | | Elm, slippery | 765 | 832 | | Hackberry | 661 | 786 | | Locust, honey | 1,133 | 1,445 | | Maple, sugar | 610 | 864 | | Oak, post | 714 | 924 | | red | 639 | 874 | | swamp white | 757 | 909 | | white | 622 | 749 | | yellow | 728 | 929 | | Sycamore | 540 | 781 | | Tupelo | 472 | 796 | | | | | | Conifers | | | | | | | | Arborvitæ | 241 | 235 | | Cypress, bald | 242 | 251 | | Fir, white | 213 | 304 | | Hemlock | 271 | 323 | | Pine, longleaf | 240 | 298 | | red | 179 | 205 | | sugar | 239 | 304 | | western yellow | 230 | 252 | | white | 225 | 285 | | Tamarack | 236 | 274 | |--------------------------------------------| 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 entire area of the specimen, and (2) with a load concentrated over a portion of the area. (See Fig. 2.) The latter is the condition more commonly met with in practice, as, for example, where a post rests on a horizontal sill, or a rail rests on a cross-tie. The former condition, however, gives the true resistance of the grain to simple crushing. [Illustration: FIG. 2.--Compression across the grain.] The first effect of compression across the grain is to compact the fibres, the load gradually but irregularly increasing as the density of the material is increased. If the specimen lies on a flat surface and the load is applied to only a portion of the upper area, the bearing plate indents the wood, crushing the upper fibres without affecting the lower part. (See Fig. 3.) As the load increases the projecting ends sometimes split horizontally. (See Fig. 4.) The irregularities in the load are due to the fact that the fibres collapse a few at a time, beginning with those with the thinnest walls. The projection of the ends increases the strength of the material directly beneath the compressing weight by introducing a beam action which helps support the load. This influence is exerted for a short distance only. [Illustration: FIG. 3.--Side view of failures in compression across the grain, showing crushing of blocks under bearing plate. Specimen at right shows splitting at ends.] [Illustration: FIG. 4.--End view of failures in compression across the grain, showing splitting of the ends of the test specimens.] When wood is used for columns, props, posts, and spokes, the weight of the load tends to shorten the material endwise. This is ~endwise compression~, or compression parallel to the grain. In the case of long columns, that is, pieces in which the length is very great compared with their diameter, the failure is by sidewise bending or flexure, instead of by crushing or splitting. (See Fig. 5.) A familiar instance of this action is afforded by a flexible walking-stick. If downward pressure is exerted with the hand on the upper end of the stick placed vertically on the floor, it will be noted that a definite amount of force must be applied in each instance before decided flexure takes place. After this point is reached a very slight increase of pressure very largely increases the deflection, thus obtaining so great a leverage about the middle section as to cause rupture. [Illustration: FIG. 5.--Testing a buggy spoke in endwise compression, illustrating the failure by sidewise bending of a long column fixed only at the lower end. _Photo by U. S. Forest Service_] The lateral bending of a column produces a combination of bending with compressive stress over the section, the compressive stress being maximum at the section of greatest deflection on the concave side. The convex surface is under tension, as in an ordinary beam test. (See Fig. 6.) If the same stick is braced in such a way that flexure is prevented, its supporting strength is increased enormously, since the compressive stress acts uniformly over the section, and failure is by crushing or splitting, as in small blocks. In all columns free to bend in any direction the deflection will be seen in the direction in which the column is least stiff. This sidewise bending can be overcome by making pillars and columns thicker in the middle than at the ends, and by bracing studding, props, and compression members of trusses. The strength of a column also depends to a considerable extent upon whether the ends are free to turn or are fixed. [Illustration: FIG. 6.--Unequal distribution of stress in a long column due to lateral bending.] |-------------------------------------------------------| | TABLE IV | |-------------------------------------------------------| | RESULTS OF COMPRESSION TESTS ACROSS THE GRAIN ON | | 51 WOODS IN GREEN CONDITION, AND COMPARISON WITH | | WHITE OAK | | (U. S. Forest Service) | |-------------------------------------------------------| | | Fibre stress | Fiber stress | | COMMON NAME | at elastic | in per cent | | OF SPECIES | limit | of white oak, | | | perpendicular | or 853 pounds | | | to grain | per sq. in. | |-----------------------+---------------+---------------| | | Lbs. per | | | | sq. inch | Per cent | | | | | | Osage orange | 2,260 | 265.0 | | Honey locust | 1,684 | 197.5 | | Black locust | 1,426 | 167.2 | | Post oak | 1,148 | 134.6 | | Pignut hickory | 1,142 | 133.9 | | Water hickory | 1,088 | 127.5 | | Shagbark hickory | 1,070 | 125.5 | | Mockernut hickory | 1,012 | 118.6 | | Big shellbark hickory | 997 | 116.9 | | Bitternut hickory | 986 | 115.7 | | Nutmeg hickory | 938 | 110.0 | | Yellow oak | 857 | 100.5 | | White oak | 853 | 100.0 | | Bur oak | 836 | 98.0 | | White ash | 828 | 97.1 | | Red oak | 778 | 91.2 | | Sugar maple | 742 | 87.0 | | Rock elm | 696 | 81.6 | | Beech | 607 | 71.2 | | Slippery elm | 599 | 70.2 | | Redwood | 578 | 67.8 | | Bald cypress | 548 | 64.3 | | Red maple | 531 | 62.3 | | Hackberry | 525 | 61.6 | | Incense cedar | 518 | 60.8 | | Hemlock | 497 | 58.3 | | Longleaf pine | 491 | 57.6 | | Tamarack | 480 | 56.3 | | Silver maple | 456 | 53.5 | | Yellow birch | 454 | 53.2 | | Tupelo | 451 | 52.9 | | Black cherry | 444 | 52.1 | | Sycamore | 433 | 50.8 | | Douglas fir | 427 | 50.1 | | Cucumber tree | 408 | 47.8 | | Shortleaf pine | 400 | 46.9 |