TIMBER 289 



In this case the line of action of the force is parallel to the grain. 

 The intensity of stress is obtained by dividing the force by the area 

 of the sheared surface. 



241. Indentation tests. Indentation tests are intended to show 

 the crushing strength of timber perpendicular to the grain. A rec- 

 tangular piece of the tim- 

 ber is usually chosen, 



and a metal block whose 



width equals the width 



of the specimen is pressed 



into it by ' an ordinary 



testing machine. Con- FIG 166 



venient load increments 



are taken, and these, together with the corresponding compressions, 



give sufficient data for a load compression curve from which the 



elastic properties may be determined. Fig. 166 illustrates a specimen 



that has been tested in compression perpendicular to the grain. 



242. Tension tests. Tension tests of timber are seldom made on 

 account of the difficulty of obtaining satisfactory test pieces. The 

 specimens to be tested must be much larger at the ends than in the 

 middle in order to provide for attachment in the heads of the testing 

 machine, and for this reason the piece is likely to fail by the shearing 

 off of the enlarged ends, or by the pulling out of the fastenings. 

 This test, therefore, is little used, the flexure test being relied upon 

 to lurnish information regarding the tensile strength of timber. 



243. European tests of timber. As early as the middle of the 

 eighteenth century tests to determine the strength of timber were 

 made in France. This work was done for the most part from a scien- 

 tific standpoint. The most important European tests were carried out 

 by Bauschinger in his laboratory at Munich, from 1883 to 1887. The 

 object of these tests was to determine the effect of the time of felling 

 and conditions of growth upon the strength of Scotch pine and spruce. 

 I 'iom these tests Bauschinger drew the following conclusions. 



1. Stems of spruce or pine which are of the same age at equal diameters, 

 sin.l in which the rate of growth is about equal, have the same mechanical 

 |.ro,,,.rti.-s (when reduced to the same moisture contents), irrespective of local 

 < Mid it ions of growth. 



