Creep rupture tests under interrupted loading were carried out with a low carbon steel at a temperature of 450°C. The load was applied once a day for the period of one tenth of rupture under continuous loading. Transient increase of creep rate was observed at the beginning of every loading interval through all stages (the 1st, 2nd and 3rd stages) of creep. As the results of this increase of strain rate, the rupture life under the interrupted loading was somewhat less than under the continuous loading, although the difference is practically negligible except when it is under high stress level. The increase of creep rate seems to result from the softening of the material during the unloading interval rather than recovery.

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An experiment was made to examine the effect of cold and hot-working on stress corrosion cracking behaviour of 18-8 Cr-Ni stainless steel. After drawing, a wire specimen of 1mm in diameter was heated at 1050°C and then cooled in water. The heat treated wire was cold-worked from 10% to 35% in elongation at room temperature, using the Amsler universal machine, and hot-workings were also done at 400°C, 650°C and 750°C for the heat treated specimen. To investigate the effect of carbide precipitation upon the stress corrosion cracking, cold worked specimens were heated for 50hr at 400°C, 650°C and 750°C.
A hot concentrated aqueous magnesium chloride solution (42wt% MgCl₂) was employed as a corrosive medium, and tensile stresses were imposed on the specimen by a direct weight. The time of fracture was measured, making clock stop by a falling weight when the specimen fractured.
For all the specimens, with or without working, the increase of applied stress caused a decrease in time to fracture, t_f. When the log t_f was plotted as a function of the stress imposed on the specimen, a linear relationship was obtained. This straight line had a knee at which the slope of log t_f-stress curve changed. It seems that the two parts of this curve corresponded approximately to crackings initiated before and after film breakdown. The location of the knee was almost unchanged by cold or hot-working and also by carbide precipitation, even though the mechanical properties of specimen changed significantly.
Resistance against the stress corrosion cracking of the specimen decreased by cold-working when the applied stress was below the knee, while it slightly increased by cold-working when the applied stress was higher than the stress corresponding to the knee. It was true as well with the hot-worked specimens.
It appeares that carbide precipitation did not reveal any appreciable effect on the stress corrosion cracking behaviour of the specimen.
Microscopic examinations confirmed that the cracks propagate across the individual grain and not intergranular crackings.

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By the torsional fatigue testing machine for steel wires mentioned in the 1st report, we studied on the torsional fatigue limits of three steel wires with different carbon contents (0.6% and 0.8%) and reductions in cold drawing (86% and 72%). The diameters of these wires are all 1.7mm. And at the same time we inquired into the effect of the low temperature annealing in the range of 100-500°C for 20min after cold drawing on the torsional fatigue limits. The results obtained are as follows:
i) Steel wires with high carbon content and high reduction in cold drawing have high torsional fatigue limits.
ii) The torsional fatigue limit and the static strength are improved by the low temperature annealing after cold drawing. And the former becomes maximum in the annealing temperature within the range from 200°C to 400°C, but all of the latter at about 200°C.
iii) The low annealing temperatures at which the torsional fatigue limit and static strength become maximum are not always equal. The differences between these temperatures tend to become remarkable in steel wires with high carbon content and high reduction in cold drawing. This seems to be caused by the residual stress in the cold drawn steel wires.
iv) The ratios of the torsional fatigue limit τ_w to the tensile strength σ_B or the torsional strength τ_B in these wires were 0.13-0.17 or 0.17-0.22.

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In order to evaluate the degree to which thermal cracks in a shaft produced from Ni-Cr-Mo steel would lower the fatigue strength, fatigue tests were carried out at room temperature, 300°C and 500°C under rotating bending.
We have obtained the following facts through the experiments:
(1) The strength reduction factor of thermal crack is about 3.5 at room temperature, 3.3 at 300°C and 2.1 at 500°C. As for the explanation of these results, it appears reasonable to assume that notch sensitivity decreases with the increase of temperature and material around the tip of thermal crack is strengthened through work hardning by repeated load and aging.
(2) The fatigue strength of specimens containing thermal cracks at 500°C could be improved by 50per cent through shot-peening.

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A measuring method for temperature dependence of viscoelastic behavior of high polymers by means of creep strain recovery is extended to the penetration case of steel ball in this paper. A general theory of linear viscoelasticity under complex stress state is analyzed mathematically by utlizing Laplace transform and is applied to the Hertz's contact problem for computing the creep recovery in the penetration of steel ball. In the same manner as compression method, the temperature dependence of characteristic relaxation times is predicted from the experimental results on z_or (the recovery amount in penetration creep)-θ (temperature) curves and is compared with Williams-Landel-Ferry's law in regard to its validity. The method by Rockwell Number (RN)-θ curves which are obtained in the constant rate of temperature rise(2°C/min) by conventional Rockwell machine, is recommended as a simple method and is successfully used for various high polymer solids so that the effect of internal molecular structure on transition and flow temperatures can be clearly demonstrated. Further, the merits of these creep recovery methods in both compression and penetration are discussed.

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