The effect of temperature on the static and dynamic strength properties of metals.

A survey is presented of earlier investigations concerning the effect of temperature on the static and dynamic strength properties of iron and steel. Results are given of static and dynamic tests performed on a medium-carbon steel at temperatures ranging from +15°C to -183°C. Also investigated is the effect of various amounts of permanent dynamic strain on the subsequent static and dynamic properties of the steel. A correlation of the results of the above tests with the various mechanisms of deformation is permitted by photomicrographic examination of the tested specimens. Finally the results of the dynamic tests are compared with a delay-time theory.
In order to be able to measure the true specimen strains in the low temperature static tests, a low temperature strain gauge was developed. This gauge, which incorporates wire resistance strain gauges, was used to find static stress-strain curves at a number of temperatures. From the variation of static upper yield stress with test temperature an estimate of σ0, the yield stress at absolute zero, is obtained. This value of σ0 is used in comparing the results of the dynamic tests with a delay-time theory. It is also shown that as the test temperature is lowered the slip by which the ferrite deforms in the static tests becomes finer.
The dynamic apparatus employed in this investigation was such that eccentricity of loading was reduced to a minimum. A falling weight was used to apply compressive stress waves of the order of 105 lb./in.2 and duration 110 microseconds to the test specimens. Fire resistance strain gauges were used to obtain stress time records at points on a weighbar and on an anvil bar, just above and just below the specimen faces. From these records a method of wave analysis gives stress against time and particle velocity against time curves for both faces of the specimen. Using these derived curves dynamic stress-strain curves are found at various impact velocities and test temperatures. These stress-strain curves indicate the large increase in strength of the steel under such loading conditions – the dynamic stress-strain curves being from 100 to 200 higher than the room temperature static stress-strain curve. A further comparison of the above curves shows that for identical velocities of impact, as the test temperature is lowered the time to yield, and upper and lower yield stresses increase, whereas the permanent deformation, the ratio of dynamic upper yield stress to static upper yield stress at the same temperature, and the maximum and average strain rates all decrease. It is also found that in dynamic tests deformation occurs primarily by very fine slip. However, in some of the low temperature impacts twinning also occurs. Results indicate that the stress at which twins are initiated is approximately independent of temperature and strain rate. It is thought that for the medium carbon steel under test, this stress for twin formation is in the range 121,000 lb./in.2 to 128,000 lb./in2.
The yield stress, delay-time results obtained in the dynamic tests are compared with Campbell's delay-time theory. This theory, which assumes that yield will occur when a critical number of source dislocations have been released from their atmospheres of carbon and nitrogen, is found to be in fair agreement with the experimental results. The greatest discrepancy between the theory and the results obtained occurs at the lower test temperatures. This is attributed mainly to twin formation.
It is found that the effect of deforming the steel dynamically is to render it softer for subsequent static deformation of the same type. In particular the static upper yield stress is reduced to a minimum after about 1 permanent dynamic strain. Also it is found that the effect of 'pulsing' steel, i.e. subjecting it to a rapidly applied stress of insufficient magnitude and duration to cause yielding, is to reduce the upper yield stress obtained during static reloading. The effect of dynamic deformation at a low temperature on the subsequent static stress-strain curve is also considered. It is found that the static stress-strain curve obtained after an impact is little affected by lowering the test temperature, provided that the magnitude and duration of the impact stress are increased so as to obtain the same permanent strain.
It is also shown that 'pulsing' the steel lowers the dynamic upper yield stress obtained in a subsequent impact test. It is found that the effect of a given stress pulse is greater on the subsequent dynamic upper yield stress than it is on the subsequent static upper yield stress. Also, whereas after a small amount of permanent dynamic strain an upper yield point is still evident in a static retest, in a dynamic retest the yield point has almost been eliminated.
Finally, repeated impact tests appear to indicate that Campbell's criterion for yielding, i.e. that a critical number of dislocations must be released from their atmospheres, is approximately valid whether or not the dislocations are freed in one or several impacts, provided that no strain ageing occurs between tests.