Part 4 (1/2)
[Ill.u.s.tration: FIG. 17.--Characteristic failures of simple beams.]
(2)~Cross-grained tension,~ in which the fracture is caused by a tensile force acting oblique to the grain. (See Fig. 17, No. 2.) This is a common form of failure where the beam has diagonal, spiral or other form of cross grain on its lower side. Since the tensile strength of wood across the grain is only a small fraction of that with the grain it is easy to see why a cross-grained timber would fail in this manner.
(3)~Splintering tension,~ in which the failure consists of a considerable number of slight tension failures, producing a ragged or splintery break on the under surface of the beam. (See Fig. 17, No. 3.) This is common in tough woods. In this case the surface of fracture is fibrous.
(4)~Brittle tension,~ in which the beam fails by a clean break extending entirely through it. (See Fig. 17, No. 4.) It is characteristic of a brittle wood which gives way suddenly without warning, like a piece of chalk. In this case the surface of fracture is described as brash.
~Compression failure~ (see Fig. 17, No. 5) has few variations except that it appears at various distances from the neutral plane of the beam. It is very common in green timbers. The compressive stress parallel to the fibres causes them to buckle or bend as in an endwise compressive test. This action usually begins on the top side shortly after the elastic limit is reached and extends downward, sometimes almost reaching the neutral plane before complete failure occurs. Frequently two or more failures develop at about the same time.
~Horizontal shear failure,~ in which the upper and lower portions of the beam slide along each other for a portion of their length either at one or at both ends (see Fig. 17, No. 6), is fairly common in air-dry material and in green material when the ratio of the height of the beam to the span is relatively large. It is not common in small clear specimens. It is often due to shake or season checks, common in large timbers, which reduce the actual area resisting the shearing action considerably below the calculated area used in the formulae for horizontal shear. (See page 98 for this formulae.) For this reason it is unsafe, in designing large timber beams, to use shearing stresses higher than those calculated for beams that failed in horizontal shear. The effect of a failure in horizontal shear is to divide the beam into two or more beams the combined strength of which is much less than that of the original beam. Fig. 18 shows a large beam in which two failures in horizontal shear occurred at the same end. That the parts behave independently is shown by the compression failure below the original location of the neutral plane.
[Ill.u.s.tration: FIG. 18.--Failure of a large beam by horizontal shear. _Photo by U. S, Forest Service._]
Table XI gives an a.n.a.lysis of the causes of first failure in 840 large timber beams of nine different species of conifers. Of the total number tested 165 were air-seasoned, the remainder green.
The failure occurring first signifies the point of greatest weakness in the specimen under the particular conditions of loading employed (in this case, third-point static loading).
|-----------------------------------------------------------| | TABLE XI | |-----------------------------------------------------------| | MANNER OF FIRST FAILURE OF LARGE BEAMS | | (Forest Service Bul. 108, p. 56) | |-----------------------------------------------------------| | | Total | Per cent of total failing by | | COMMON NAME | number |---------+-------------+-------| | OF SPECIES | of | Tension | Compression | Shear | | | tests | | | | |------------------+--------+---------+-------------+-------| | Longleaf pine: | | | | | | green | 17 | 18 | 24 | 58 | | dry | 9 | 22 | 22 | 56 | | Douglas fir: | | | | | | green | 191 | 27 | 72 | 1 | | dry | 91 | 19 | 76 | 5 | | Shortleaf pine: | | | | | | green | 48 | 27 | 56 | 17 | | dry | 13 | 54 | | 46 | | Western larch: | | | | | | green | 62 | 23 | 71 | 6 | | dry | 52 | 54 | 19 | 27 | | Loblolly pine: | | | | | | green | 111 | 40 | 53 | 7 | | dry | 25 | 60 | 12 | 28 | | Tamarack: | | | | | | green | 30 | 37 | 53 | 10 | | dry | 9 | 45 | 22 | 33 | | Western hemlock: | | | | | | green | 39 | 21 | 74 | 5 | | dry | 44 | 11 | 66 | 23 | | Redwood: | | | | | | green | 28 | 43 | 50 | 7 | | dry | 12 | 83 | 17 | | | Norway pine: | | | | | | green | 49 | 18 | 76 | 6 | | dry | 10 | 30 | 60 | 10 | |-----------------------------------------------------------| | NOTE.--These tests were made on timbers ranging in cross | | section from 4” x 10” to 8” x 16”, and with a span of 15 | | feet. | |-----------------------------------------------------------|
TOUGHNESS: TORSION
Toughness is a term applied to more than one property of wood.
Thus wood that is difficult to split is said to be tough. Again, a tough wood is one that will not rupture until it has deformed considerably under loads at or near its maximum strength, or one which still hangs together after it has been ruptured and may be bent back and forth without breaking apart. Toughness includes flexibility and is the reverse of brittleness, in that tough woods break gradually and give warning of failure. Tough woods offer great resistance to impact and will permit rougher treatment in manipulations attending manufacture and use.
Toughness is dependent upon the strength, cohesion, quality, length, and arrangement of fibre, and the pliability of the wood. Coniferous woods as a rule are not as tough as hardwoods, of which hickory and elm are the best examples.
The torsion or twisting test is useful in determining the toughness of wood. If the ends of a shaft are turned in opposite directions, or one end is turned and the other is fixed, all of the fibres except those at the axis tend to a.s.sume the form of helices. (See Fig. 19.) The strain produced by torsion or twisting is essentially shear transverse and parallel to the fibres, combined with longitudinal tension and transverse compression. Within the elastic limit the strains increase directly as the distance from the axis of the specimen. The outer elements are subjected to tensile stresses, and as they become twisted tend to compress those near the axis. The elongated elements also contract laterally. Cross sections which were originally plane become warped. With increasing strain the lateral adhesion of the outer fibres is destroyed, allowing them to slide past each other, and reducing greatly their power of resistance. In this way the strains on the fibres nearer the axis are progressively increased until finally all of the elements are sheared apart. It is only in the toughest materials that the full effect of this action can be observed. (See Fig.
20.) Brittle woods snap off suddenly with only a small amount of torsion, and their fracture is irregular and oblique to the axis of the piece instead of frayed out and more nearly perpendicular to the axis as is the case with tough woods.
[Ill.u.s.tration: FIG. 19.--Torsion of a shaft.]
[Ill.u.s.tration: FIG. 20.--Effect of torsion on different grades of hickory. _Photo by U. S. Forest Service._]
HARDNESS
The term _hardness_ is used in two senses, namely: (1) resistance to indentation, and (2) resistance to abrasion or scratching. In the latter sense hardness combined with toughness is a measure of the wearing ability of wood and is an important consideration in the use of wood for floors, paving blocks, bearings, and rollers. While resistance to indentation is dependent mostly upon the density of the wood, the wearing qualities may be governed by other factors such as toughness, and the size, cohesion, and arrangement of the fibres. In use for floors, some woods tend to compact and wear smooth, while others become splintery and rough. This feature is affected to some extent by the manner in which the wood is sawed; thus edge-grain pine flooring is much better than flat-sawn for uniformity of wear.
|-------------------------------------------------------------------| | TABLE 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 | | (Forest Service Cir. 213) | |-------------------------------------------------------------------| | COMMON NAME OF SPECIES | Average | End | Radial | Tangential | | | | surface | surface | surface | |------------------------+---------+---------+---------+------------| | | Pounds | Pounds | Pounds | Pounds | | | | | | | | Hardwoods | | | | | | | | | | | | 1 Osage orange | 1,971 | 1,838 | 2,312 | 1,762 | | 2 Honey locust | 1,851 | 1,862 | 1,860 | 1,832 | | 3 Swamp white oak | 1,174 | 1,205 | 1,217 | 1,099 | | 4 White oak | 1,164 | 1,183 | 1,163 | 1,147 | | 5 Post oak | 1,099 | 1,139 | 1,068 | 1,081 | | 6 Black oak | 1,069 | 1,093 | 1,083 | 1,031 | | 7 Red oak | 1,043 | 1,107 | 1,020 | 1,002 | | 8 White ash | 1,046 | 1,121 | 1,000 | 1,017 | | 9 Beech | 942 | 1,012 | 897 | 918 | | 10 Sugar maple | 937 | 992 | 918 | 901 | | 11 Rock elm | 910 | 954 | 883 | 893 | | 12 Hackberry | 799 | 829 | 795 | 773 | | 13 Slippery elm | 788 | 919 | 757 | 687 | | 14 Yellow birch | 778 | 827 | 768 | 739 | | 15 Tupelo | 738 | 814 | 666 | 733 | | 16 Red maple | 671 | 766 | 621 | 626 | | 17 Sycamore | 608 | 664 | 560 | 599 | | 18 Black ash | 551 | 565 | 542 | 546 | | 19 White elm | 496 | 536 | 456 | 497 | | 20 Ba.s.swood | 239 | 273 | 226 | 217 | | | | | | | | Conifers | | | | | | | | | | | | 1 Longleaf pine | 532 | 574 | 502 | 521 | | 2 Douglas fir | 410 | 415 | 399 | 416 | | 3 Bald cypress | 390 | 460 | 355 | 354 | | 4 Hemlock | 384 | 463 | 354 | 334 | | 5 Tamarack | 384 | 401 | 380 | 370 | | 6 Red pine | 347 | 355 | 345 | 340 | | 7 White fir | 346 | 381 | 322 | 334 | | 8 Western yellow pine | 328 | 334 | 307 | 342 | | 9 Lodgepole pine | 318 | 316 | 318 | 319 | | 10 White pine | 299 | 304 | 294 | 299 | | 11 Engelmann pine | 266 | 272 | 253 | 274 | | 12 Alpine fir | 241 | 284 | 203 | 235 | |-------------------------------------------------------------------| | NOTE.--Black locust and hickory are not included in this table, | | but their position would be near the head of the list. | |-------------------------------------------------------------------|
Tests for either form of hardness are of comparative value only.
Tests for indentation are commonly made by penetrations of the material with a steel punch or ball.[16] Tests for abrasion are made by wearing down wood with sandpaper or by means of a sand blast.
[Footnote 16: See articles by Gabriel Janka listed in bibliography, pages 151-152.]