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WROUGHT IRON־.
WROUGHT IRON BARS.
EFFECTS OF OVERSTRAINING, FOLLOWED BY INTERVALS OF REST, ON THE TENSILE STRENGTH OF THE IRON.
The experiments were carried on with four kinds of wrought iron, Common Refined, Best Puddled, Burden’s Best, and Norway. One test of each kind was made in the ordinary manner by the application of progressive loads, continued until rupture was reached. With the other specimens of the series an overstraining load was applied, ranging from 25,000 to 45,000 pounds per square inch with the different bars, which was followed by a period of rest under no load, and after different intervals resuming the loading until the tensile strength and rupture was reached. Generally the specimens displayed considerable elongation immediately after passing the primitive elastic limit, and when this rapid stretching had begun it continued for a time under diminished loads, thus making a jog in the stress-strain diagram, a feature quite common to ductile metal. These reduced loads and their elongations were not followed in many of the tests, but where large elongations are shown under loads just beyond the elastic limit, such specimens displayed the jog in their curves. The metal when under loads in the immediate vicinity of the elastic limit appears to be in a critical state as regards a tendency to elongate, and the reapplication of the same stress may cause rapid elongation, or after sustaining a load for a short time rapid elongation may set in.
The well-known effect of overstraining on the exaltation of the elastic limit is shown throughout the series. There is generally a less sharp definition of the secondary elastic limit, and also a less pronounced jog in the overstrained metal, but over a certain range of stresses beyond the primitive elastic limit deferred loads may be applied without causing a measurable increase in the permanent sets. The secondary elastic limits have been critically established as they are found in the “General summary” of the results, and appear lower than an inspection of the diagrams indicate, but higher loads showed a change in the rate of elongation, or repeated stresses caused slightly increased elongations.
The gain in elastic limit by overstraining ranged from 2,000 to 6,000 pounds per square inch. The influence on the results of the magnitude of the overstraining load, the amount of the permanent set, and the interval of rest is less prominently shown, but it appears that there is some gain after the longer periods of rest, and that higher stresses and sets are jointly attended with a gain in the value of the elastic limit. It is difficult also to trace how far beyond the limit of the overstraining load the influence of the rest extends, but an inspection of the stress-strain diagrams indicates that it may be felt throughout the remainder of the test in respect to elongation and tensile strength. The influence, if any, on the contraction of area is not apparent.
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