SAMPLING AND TESTING HIGHWAY MATERIALS. 87 
(&) This bar shall be annealed or otherwise treated so as 
to eliminate internal or unequal stress in the material, and to 
insure its elastic modulus being uniform for successive tests. 
(c) The extensometer shall be permanently attached to the bar 
and shall measure the elongation oh two opposite sides. 
a!) The extensometer shall be preferably of the indicating or 
direct-reading type and shall indicate to ten-thousandths of an 
inch ( or less. 
(e) The method of securing the bar in the drawheads of the 
machine shall be positive and without slip, and shall insure its 
axial location. 
(/') The length of the bar measured by the extensometer shall 
be sufficient so that the smallest extensometer division — that 
is, 0.0001 inch — shall correspond to a difference in loading of 
0.2 per cent of the capacity of the machine or less. 
(g) The extensometer shall be protected from injury by a 
permanently attached case with cover removable for reading 
the scale. 
(h) The apparatus shall itself be calibrated either by the 
United States Bureau of Standards or in a manner that will 
insure equally trustworthy results. 
(i) The apparatus shall be plainly marked with the maximum 
load that can be applied safely without injury. 
Methods of Gripping Test Specimens. 
22. It is recommended that for specimens of rolled material, 
serrated grips, flat and V-shaped, be adopted, the former for 
rectangular and the latter for round specimens. Serrated grips 
with curved faces appear to have no advantage, and to cause 
crushing of the material. 
23. Wedges with ball and socket do not seem to be necessary, 
and for commercial testing their use has. been generally discon- 
tinued. 
24. Specimens of turned form, with threaded ends, should be 
secured in such a manner that side bending stresses are avoided. 
25. It is considered important for correct results that the 
specimen be located in the exact center of the heads, and to better 
secure this condition the openings in the beads should be lined 
up with each other by means of a plumb bob and should be 
tested for parellelism with a spirit level. Each pair of pack- 
ing pieces and wedges that are to be used together in the 
same head should correspond exactly in thickness and other 
dimensions, and the wedges should be inserted the same distance 
when the specimen is in place. 
Selection and Preparation of Specimen. 
26. Specimens representative of steel castings may be cut from 
the bottom of a sink head or riser, or from a coupon attached to 
the casting. In either case the parr from which the specimen is 
taken should be relatively large in proportion to the size of the 
casting and should be annealed with it. 
27. Workmanship on specimens shall be of the most careful 
nature, and surfaces should be free from nicks and tool marks. 
All wire edges should be removed and corners generously rounded. 
28. If specimens of rolled material are sheared in the rough 
from sections, at least one-eighth inch of the material should be 
removed from the sheared edges in machining. 
General Requirements for the Measubing of Elongation. 
29. In determining the modulus of elasticity and the elastic 
limit, it is recommended that when practicable the elongation 
be measured in a length not less than 8 inches, and thai the fol- 
lowing requirements be provided for: 
(a) The specimen shall be round in section, finished a* smooth 
as possible, and shall he provided with threaded ends for attach- 
ment to the drawheads of the machine. 
