Fatigue testing cyclic stress-strain behavior

Leis, B.N., Gowda, C.V.B., Topper, T.H. Cyclic inelastic deformation and the fatigue notch factor. Cyclic stress-strain behavior—analysis, experimentation and failure prediction. Am. Soc. Test. Mater., ASTM STP 519, 133-159 (1973)... 

Low-Cycle Fatigue Properties. Results of low-cycle fatigue experiments under strain control on as-worked W plate material at 815 °C are shown in Fig. 3.1-172. Low-cycle fatigue tests of pure W were performed in the temperature range between 1650 °C and 3300 C [1.184]. A relationship Afaiiure = exp(—aT) was found to be valid up to test temperatures of 2700 °C [1.185]. In all cases the failure mode was intercrystalline. Similar results were also obtained at a test temperature of 1232 °C [ 1.186]. The deformation behavior of Nb and Nb IZr under plastic-strain control at room temperature was investigated and cyclic stress-strain curves published [1.182]. 

Exploratory creep tests were performed on tensile specimens of NT-154 silicon nitride at 1300 and 1370 C, These specimens were made from the same lot of the material, designated as CP-serles, used In the cyclic fatigue tests discussed In the last progress report, Creep strain was measured using the laser diffraction strain extensometer described elsewhere,2 Figure 7 shows the creep curves of specimens CP-28 and CP-10 tested at 1300 C under applied stress of 160 and 180 MPa, respectively. Both tests were shutdown Inadvertently due to equipment adjustments. The problems have been corrected since then. Therefore, the results of the tests do not represent the creep life of the specimens. However, there were sufficient data points to delineate the essential features of the creep behavior up to the steady-state phase which was clearly definable. Both curves showed a reversed Inflection at the transition between the primary and secondary creep phases, resulting In forming a bump In the otherwise smooth creep curves. 

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