The cyclic creep properties of Cr-Mo-V rotor steel have been explored at room temperature and elevated temperature by multiple-step tests with increasing maximum stress ( R = 0), and with subsequent tests in partial unloading (0 < R < I). Cyclic creep acceleration is observed at room temperature and cyclic creep retardation at elevated temperature. This behavior is explained in terms of cyclic softening at low temperature and by creep-dominated deformation at high temperature with a ferritic material considered especially prone to retardation because of the high diffusivity of b.c.c. material. Other interesting effects, such as strain burst phenomena and the observation of anomalously high values of the ratio of diametral strain to axial strain, are also reported.
Measurements of loading arm behavior on compact tension type, transversely wedge-loaded crack arrest specimens have been made. A crack-opening measurement point close to the crack initiation site was selected in addition to face or load-line position. Conclusions concerning acceleration, vibration, and damping behavior of the specimen arms were reached and differences in the behavior of two alternative specimen types are indicated. The influence of fracture morphology on specimen behavior is reported in detail. Suggestions for further work to clarify crack run-arrest behavior of tentative test specimens are presented.
Tests of a Cr-Mo-V rotor steel have been carried out at 550°C using continuous cycling and cycling with tensile dwells. A special feature of these tests was that they were carried out in control by the axial total strain but that diametral strain was simultaneously measured. Comparison of the diametral and axial strain ranges showed that “Poisson's ratio” initially increased but subsequently declined steadily with accumulating cycles. While these results do not challenge the traditional view of Poisson's ratio which is defined for homogeneous material, they indicate that numerical values of the ratio must be used with caution. The results on diametral strain response are carefully analyzed and defended as a true material response; they indicate that the ratio of diametral to axial strain is sensitive to the accumulated fatigue damage of the specimen in a rather unpredictable fashion.
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