Fatigue testing 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)... 

Dynamic mechanical measurements are performed at very small strains in order to ensure that linear viscoelasticity relations can be applied to the data. Stress-strain data involve large strain behavior and are accumulated in the nonlinear region. In other words, the tensile test itself alters the structure of the test specimen, which usually cannot be cycled back to its initial state. (Similarly, dynamic deformations at large strains test the fatigue resistance of the material.)... 

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. 

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]. 

The lower temperature is limited by the creep behavior of the solder as outlined previously. At —40 °C (—40 °F) for example, creep is so slow that the dwell time necessary to induce a strain, which is comparable to the real world of most applications, is so long that there is no such thing as an acceleration in the test. Additionally, cycling at low temperature causes large stresses that have to be borne by the components, and consequently, the destruction of components due to the fatigue test of the solder joints and brittle fracture of the solder becomes a threat. 

Walker, E.K. Multiaxial stress-strain approximation for notch fatigue behavior. J. Test. 

Macroscopic behavior of the studied materials is characterized by stress-strain curves. Accordingly, two types of mechanical tests are designed. The first one is a continuous stretching test which characterizes the mechanical properties of the studied materials. The second one is a slow load-cycling test that tries to shed some light on the less investigated fatigue mechanisms. 

Low-Cycle Fatigue. For the low-cycle fatigue region (N< 10 cycles), tests are conducted wifii controlled cycles of elastic plus jdastic (total) strain range, rather than with controlled load or stress cycles. Under controlled-strain testing, fiitigue life behavior is rqite-... 

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