By applying a method of analyzing the plastic and quasi-elastic strain energies on the actual fatigue tests of carbon steel, the authors previously made clear the effect of temperature change in the atmosphere around specimens on the change in strain energy of one kind of carbon steel and obtained the ralationships between these energies and the structural changes appearing gradually on the surface of specimen. Furthermore, the dependence of the features of fractographs on the temperature conditions was also investigated.
In the present experiment, the following four cases were adopted as the temperature conditions of atmosphere around the specimen: (1) the constant temperature at +20±2°C up to rupture of the specimen, (2) the constant temperature at -20±2°C, (3) changing the temperature from +20°C to -20°C after a certain number of cycles and (4) changing the temperature in the reverse way of (3).
The main results obtained are as follows:
(1) The circumferential temperature change brought in a certain stage on the fatigue process has important effects upon several fatigue phenomena including the strain energy changes and the failure repetitions of a specimen.
(2) The change in the sum of quasi-elastic strain energy with the progress of fatigue can be expressed generally in Σ
qq=1(
U)
q=
A·qk up to rupture of specimen, where
A is the initial value defined by the ordinary elastic energy calculation,
q is the number of stress repetitions,
k is a constant depending upon the property of test material, the method of experiment and the circumferential temperature condition.
Using this expression and the statistical values for the fatigue life (
N) and Σ
qq=1(
U)
q obtained by the test under the constant temperature condition (+20±2°C), the fatigue lives under other temperature conditions can be inferred approximately.
(3) If the sum of the plastic strain energy, Σ
qq=1 (
Ep)
q, is plotted logarithmically, two nodal points C and D appear. The point C appears to have relation with the occurrence of a slip band, and the energy quantity at this point was a constant about 10kg-m, independent of the stress amplitudes and of the temperature conditions. Therefore, it was found that the critical value of the energy accumulation in a specimen was related probably to the origination of the slip band. On the other hand, the point D appears to have connection with the appearance of intrusions, and the critical energy is considered to have relation to their appearance.
(4) Fractographs collected in the vicinity of the central portion of the fracture surfaces were studied and it was found that their features were dependent upon the temperature conditions after change.
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