Tensile Properties

Strength of a metal is usually expressed in the number of pounds

a 1-in. bar will support just before breaking, a term called the

ultimate strength. It has been found that the shape of the test

bar and its method of loading has some effect upon the results,

so it is now usual to turn a rod 5-1/2 in. long down to 0.505 in.

in diameter for a central length of 2-3/8 in., ending the turn

with 1/2-in. fillets. The area of the bar equals 0.2 sq. in., so

the load it bears at rupture multiplied by 5 will represent the

ultimate strength in pounds per square inch.



Such a test bar is stretched apart in a machine like that shown

in Fig. 9. The upper end of the bar is held in wedged jaws by the

top cross-head, and the lower end grasped by the movable head.

The latter is moved up and down by three long screws, driven at

the same speed, which pass through threads cut in the corners of

the cross-head. When the test piece is fixed in position the motor

which drives the machine is given a few turns, which by proper

gearing pulls the cross-head down with a certain pull. This pull

is transmitted to the upper cross-head by the test bar, and can

be weighed on the scale arm, acting through a system of links and

levers.



Thus the load may be increased as rapidly as desirable, always

kept balanced by the weighing mechanism, and the load at fracture

may be read directly from the scale beam.



This same test piece may give other information. If light punch

marks are made, 2 in. apart, before the test is begun, the broken

ends may be clamped together, and the distance between punch marks

measured. If it now measures 3 in. the stretch has been 1 in. in 2,

or 50 per cent. This figure is known as the elongation at fracture,

or briefly, the elongation, and is generally taken to be a measure

of ductility.



When steel shows any elongation, it also contracts in area at the

same time. Often this contraction is sharply localized at the fracture;

the piece is said to neck. A figure for contraction in area is

also of much interest as an indication of toughness; the diameter

at fracture is measured, a corresponding area taken out from a

table of circles, subtracted from the original area (0.200 sq.

in.) and the difference divided by 0.2 to get the percentage

contraction.






Quite often it is desired to discover the elastic limit of the

steel, in fact this is of more use to the designer than the ultimate

strength. The elastic limit is usually very close to the load where

the metal takes on a permanent set. That is to say, if a delicate

caliper (extensometer, so called) be fixed to the side of the

test specimen, it would show the piece to be somewhat longer under

load than when free. Furthermore, if the load had not yet reached the

yield point, and were released at any time, the piece would return

to its original length. However, if the load had been excessive, and

then relieved, the extensometer would no longer read exactly 2.0

in., but something more.



Soft steels give very quickly at the yield point. In fact, if

the testing machine is running slowly, it takes some time for the

lower head to catch up with the stretching steel. Consequently at

the yield point, the top head is suddenly but only temporarily

relieved of load, and the scale beam drops. In commercial practice,

the yield point is therefore determined by the drop of the beam.

For more precise work the calipers are read at intervals of 500 or

1,000 lb. load, and a curve plotted from these results, a curve

which runs straight up to the elastic limit, but there bends off.



A tensile test therefore gives four properties of great usefulness:

The yield point, the ultimate strength, the elongation and the

contraction. Compression tests are seldom made, since the action

of metal in compression and in tension is closely allied, and the

designer is usually satisfied with the latter.





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