TENSILE STRENGTH. 



The experiments wore especially directed to the comparison of the 

 tensile strength and stiffness of portions of the same stick, in different 

 positions relatively to the heart. 



In designing the form of the test-piece, it was of importance to make 

 the head of such a depth as would prevent the central portions from 

 being pulled through tlie head by shearing along the surface BO, and 

 it was also necessary that the depth should not be inconveniently great. 

 Wedge shaped holders (Fig. H) were adopted which would grip the 



specimen along ihe faces AB. This form of holder was intended to 

 increase the resistance to shear which is always much less than the tensile 

 strength. As the tension on the test-piece increases, so also does the 

 normal pressure upon the faces AB, Fig. K.and, therefore, so also does the 

 resistance to shear along the surface BC. At first, the faces of the 

 holders in contact with the specimen were left rough, but it was found 

 that the roughness prevented the specimen from sliding in far enough 

 to be gripped along the whole of the face AB, so [that the bearing 

 surface was practically limited to a comparatively small area near the top 

 of the head. Thus it often happened that the specimen still failed by 

 shearing along the surface BC. This difficulty was obviated by 

 j.laning the (aces of the holders. 



The test-pieces were prepared from the uninjured portions of the 

 beams, which had already been fractured transversely. The extensions 

 of a length of ten inches of the specimen under gradually increased 

 loads were measured by means of Uuwin's extensometer until the 

 total extension exceeded about one-eightieth of an inch. After this the 

 extensometer WMS ri moved, and in many cases additional extension read- 

 ing*, up to the point of fracture, of a length of sixteen inches of the 

 specimen, were mea.-und by means of a steel rule and indicator clamped 

 to the specimen at points 16-inches apart and allowed to slide over one 

 another. 



The results obtained are given in the following Tables and an exam- 

 ination of these will show : 



1st. '1 hat the increments of extension up to the point of fracture ate 

 almost directly piopoitionnl to the increments of load; 



2nd. That the presence of knots is most detrimental both to the-. 

 strength and to the stiffness, inasmuch as they practically diminish 

 the effective sectional area, and also produce a curvature in the grain ; 



3rd. That wood near the heart possesses much less strength and much 

 less stiffness than that more distant from the heart ; 



80 



