February 10. 1917 



The Elasticity of Wood 



Hu Maxwell 



Editor's Note 



Tbe property ut elastkity in wood is a matter of eoinmou knowledge with most users, but it is 

 ilifficult to measure. The results of scientific tests arc expressed iu terms wbicli mean little^ t^ 

 those who are not engineers, and the practical user generally depends upon his own experience or tl 

 of other.s for what he knows of this property which is always present in wood an. I is frequentl 

 great importance. 



ARTICLE THREE 



Less is heard of the elasticity or stififuess of wood tbau of its 

 strengtli, weight, liardness, and figure. It is a property not so easily 

 understood as the others and it is not so often observed or discussed 

 by workers iu or users of wood, yet in many ways it is as important 

 as the others. The fact may be lost sight of that much of the smooth- 

 ness of a railroad journey is due to the elasticity of the wooden ties 

 over which the train glides; or that much of the noiseless character 

 of a wooden pavement, or still more of the ease with which vehicles 

 travel over it, is on account of the elasticity which absorbs and dis- 

 tributes jolts. The same can be claimed of the wooden floor when it 

 is compared with the unyielding floor of cement. Much of the smooth- 

 ness with which a buggy or carriage skims along the highway is owing 

 to the springiness of the spokes, felloes, running gear, and other 

 wooden parts. If wood were not so highly elastic, the game of cro- 

 quet would not be played, the golf course would be deserted, and most 

 of the sport would disappear from angling. There are scores of ways 

 in which we use or enjoy wood's elasticity, and often withotit giving 

 a thought to the fine quality wliich means so much to us. 



Elasticity and stiffness in wood are practically the same thing, but 

 the two words do not always convey the same idea. Most persons who 

 are acquainted with the use of wood readily understand what is meant 

 by stiffness. To them it means that property in wood which makes 

 a stick hard to bend, and when bent, but not bent too far, causes it 

 to spring back to its original form when the force is removed. Every 

 wood has that property, but some show it more plainly than others. 

 Persons who are acquainted with the common uses of hickory and 

 ash, for example, know that both are hard to bend, and that both will 

 recover their shape when the pressure is released; but it is difficult 

 by the eye alone, or by the ordinary means of observation, to deter- 

 mine which of these woods bends with greatest resistance, or which 

 flies back with the more energy. 



A Measuring Apparatus 



Machines have been devised to measure the force required to bend a 

 stick and to make a record of it. Figures prepared from such data 



show how much stifter one wood is than another, and the figures thus 

 evolved are used as a measure of elasticity or stiffness. In technical 

 books it is called "modtdus of elasticity," which means the measure 

 of elasticity — sometimes called ' ' factor of stiffness. " It is expressed 

 in hundreds of thousands, or even mUlions, of pounds per square inch 

 of cross section. White oak's modulus of elasticity is 1,318,000 

 pounds. This figure is so large that it seems unreasonable, and unless 

 it is explained, it is misleading or meaningless. At best, it is more 

 or less theoretical and is based on. the ability of wood to stretch under 

 tension. 



Wood will stretch a little if the force is applied as a pull lengthwise, 

 and machines have been devised to measure what force is necessary to 

 produce a certain amount of stretching in a stick of a certain size and 

 length. The force calculated is sufiicient to stretch the stick one inch ; 

 but since no wood will stretch an inch, under the conditions specified, 

 the actual stretch is only one-thousandth of an inch. In the case of 

 white oak, as the figures are given above, it is found that a pull of 1,318 

 pounds stretches the stick the thousandth part of an inch, and from 

 that it is calcidated that a thousand times that force, or 1,318,000 

 poim.ds, would be required to i^roduce a stretching of one inch ; hence, 

 it is said that the modulus of elasticity of which oak is 1,318,000. 

 This explanation is not strictly scientific in all respects, and it is 

 intended to show in a general way only what the large figures mean 

 which are commonly employed to express the modulus of elasticity of 

 different woods. No wood in the world, in the form of a stick an inch 

 square and suspended lengthwise, will hold up a weight of a million 

 pounds in a lengthwise or stretching pull — nor one-tenth of it — yet 

 it is assumed that such weights are so sustained in calculating the 

 elasticity of wood. The figures are, therefore, theoretical, and are 

 useful chiefly for comparison, one wood with another. Below is a list 

 of twenty woods, selected from Sargent 's list of 405. They range from 

 the least elastic to the most elastic of our timbers. 



Modulus of 

 Species elasticity 



Golden lig 270.000 



Black willow 576,000 



Arbor vitai 750.000 



White elm ; 1.069.000 



Modulus of 

 Species elasticity 



Yew ; . . 1,078,000 



Red gum 1,194,000 



Bnsswood 1,198,000 



White pine 1,208.000 



TEST IN STATIC BENDING 



This is a machine used in the government laboratory at Madison. Wis. 



i'ressure can be applied slowly and an accurate record kept of 



the behavior of the beam up to the time it breaks 



OSAGE ORANGE FENCE POSTS 



The modern rancher, by stringing wire on these posts, makes fences to 



turn cattle. The Indians whittled bows of the wood, strung 



them with sinew, and used them to kill buffalo 



—15— 



