Most tensile tests at cryogenic temperatures have been carried out under constant displacement rate. However, in cryogenic structures such as superconducting magnets and pressure vessels, the structural materials experience unrestricted forces. It is, therefore, desirable to characterize the load-controlled tensile properties at cryogenic temperatures to simulate operating conditions of the cryogenic structures.
At 4 K, discontinuous yielding occurs, which arises mainly from the extremely low specific heat and thermal conductivity of the metals at low temperatures, and it affects the load-displacement or stress-strain curves. So, it is supposed that the discontinuous yieldings may affect tensile properties such as yield or ultimate tensile strengths.
To evaluate the mechanical properties at cryogenic temperatures, a testing facility which can be exclusively used at cryogenic temperatures is necessary. The facility and testing procedures used in this study were described in Chapter III. It was shown that the precision and accuracy of the facility met the requirements of standards for materials testing machines
In Chapter IV, tensile deformation behavior of AISI 304, 316, 310S austenitic stainless steels under constant displacement rate was described. Discontinuous yieldings were observed in all alloys at 4 K. The stress-displacement curves at 4 K and 77 K showed different tendency from those at 298 K. As the testing temperature decreased, the ultimate tensile strengths of 304 and 316 stainless steels largely increased in comparison to the increase of yield strengths, but the increase of ultimate tensile strength of 310S stainless steel was almost the same to that of yield strength. 310S stainless steel had the highest yield strength and the lowest ultimate tensile strength at all temperatures. These tensile characteristics were strongly affected by austenite stability.
In Chapter V, the effect of load rate (5, 50, 500 and 5,000 N/s) and testing temperature (4 K and 77 K) on the tensile properties of three, austenitic stainless steels (AISI 304, 316 and 310S) were investigated. No change of the yield strength with load rate was observed at 4 K and 77 K. A transition of the ultimate tensile strength was observed under load control at 4 K, which had ever not been observed under displacement control. A model was proposed, which can explain the relationship between discontinuous yielding and ultimate tensile strength. According to this model, the ultimate tensile strength obtained under load control is equal to or less than that obtained under displacement control.
In Chapter VI, the effects of specimen diameter on the load-controlled tensile properties was investigated using AISI 304, 316 and 310S austenitic stainless steels. The diameters of the specimens were 4.5, 6.35 and 8 mm. The loading rates were 0.154, 1.54, 15.4 and 154 MPa/s. Both the discontinuous yielding stress and strain decreased with loading rate and specimen diameter. The effect of loading rate on the onset of discontinuous yielding was larger than that of specimen diameter. It was shown that the discontinuous yielding stress should be measured because the ultimate tensile strengths depend on the discontinuous yielding stress. Finally, the validity of the model proposed in Chapter V was approved by showing the fact that the relationship between discontinuous yielding stress and ultimate tensile strength was independent of the load rate and specimen size.