Fatigue testing cyclic stress-strain curve

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]. 

One of the most frequently used tests for fatigue-resistance evaluation is the well-known plotting of stress versus the number of cycles, usually referred to as the S-N (curve) relation . Various wave forms of cyclic stresses may be applied to a specimen to test its suitability to withstand prolonged strain. Machine elements are assessed to determine their practical endurance of industrial applications to which they may be exposed. Such tests focus on the nominal stress required to cause fatigue failure at some number of cycles. A logarithmic scale is almost always used for N, the number of cycles to failure. A schematic S-N plot is shown in Fig. 7.1. Note the horizontal line in plot (a), known as a knee , which represents the endurance limit . As implied by its name, at this level of stress, the specimen is characterized by its ability to endure a large number of stress-cycles at the stress level of the horizontal line and below it without failure. In plot (b), no such horizontal line is observed and the curve continues to decrease, indicating that the stress must be reduced for the test specimen to be able to withstand a certain number of cycles. 

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. 

Figure 6.19 (a) Nominal applied stress and time to fracture (steel grade 91, T — 500—700°C) (b) Monkman—Grant curve minimum strain rate and time to fracture. A master curve appears over the entire temperature range [49]. The red star corresponds to a sequential test including pure fatigue test —> pure creep test cyclic loading for 3.3 x 10° cycles under total cyclic strain of 0.15% (Fig. 6.2(b)), followed by a creep under 185 MPa at 550°C until fracture (Fig. 6.5(b)). 

Mechanically, however, the wire did suffer fatigue damage. The above Ic tests had to be terminated, because in each case the specimen fractured. As shown in Fig. 3, the specimen that was cyclically loaded to 540 MN/m (52% of ultimate) fractured after 4x10 load cycles the specimen loaded to 480MN/m (46% of ultimate) fractured after 1.5x10 cycles. In other words, multifilamentary NbTi wears out when cycled at high-enough strain. This is shown in more detail in the S-N curve of Fig. 5, where the vertical axis is the peak stress applied to the wire, and the horizontal axis is the number of cycles at this stress level at which the wire... 

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