Cyclic Fatigue Testing

The action imposed on the sample may not necessarily be harmonic but may have the form of a pulse with a smaller or larger duty cycle. Various devices can be used to generate a cyclic regime. 

FIGURE 5.15 Schematic illustration of cyclic fatigue testing. 

Physical-Chemical Mechanics of Disperse Systems and Materials 

The Nffd) dependence is typically a linear decaying function when plotted on a log-linear scale, that is, log Nf = a - bo. As one can see, this expression is similar to the one established in the static durability testing. This similarity has a particular physical meaning (in a statistical aspect). Furthermore, the extrapolation to a 0 is not valid here either upon the lowering of the applied stress or strain to a particular value, the value of increases rapidly, that is, there is a cyclic fatigue limit similar to the static fatigue limit. 

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TABLE 7. Tensile Cyclic Fatigue Tests at 566°C/1050°F of SiOC Nextel 312 BN 2-D Composites... 

FIGURE 8. Short beam shear test to determine the yield shear strength (Tei of WHIPOX CMC (a) and cyclic fatigue test showing degradation if the toad exceeds (Tei (b). 

Evidence for elastic-ligaments bridging and frictional pull-outs similar to those observed in other structural ceramics were found in all samples after fracture toughness and cyclic-fatigue testing. However, the excellent JGc and JQh values can be mostly attributed to the unique KB-induced lamellae (Figure 7.18b) that serve as very tenacious crack bridges, somewhat reminiscent of wood. 

An interesting comparison exists between static and cyclic fatigues for Ce-TZP, in which, as is known, a t m transformation takes place. Figure 7.16 shows a comparison of the measured results of static and cyclic fatigue tests for three Ce-TZP materials. The time-to-failure of the statically-loaded specimens is longer... 

Fig. 7.85 Scanning electron micrographs of the fracture surfaces of a, c, and e Ce-TZP-IV and b, d, and f Ce-TZP-V as observed after a and b short-term strength and after cyclic fatigue tests with C and d R = 0.2 and e and f R = — 1 [8]. With kind permission of John Wiley and Sons...

Cyclic fatigue tests were conducted on commercial PZT-SA stacks using a semi-bipolar 100-Hz sine wave that had an equivalent field range of +4.5/-0.9 kV/mm and a 20 Iv a preload. More than lO cycles were accumulated and the performance of tested stacks was measured and evaluated. The following conclusions can be drawn ... 

Table 1. Results of a cyclic fatigue test on CERCOM SIC whisker-reinforced...

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. 

The loads and strain rates are controllable cyclic fatigue testing is possible accessories are available to... 

Fig. 8. Transmission electron micrograph of a cross-section of the interphase region of a PAA-preireated joint from a dry cyclic-fatigue test 110. ...

This information is contained in X-ray scattering patterns. It is evidenced by a complex transformation. The pathway has already been pointed out by Debye in 1931. Because of the mathematical complexity of the corresponding CDF-method it is only mastered by few research groups in the world. Dr. Zeinolebadi has studied various materials by straining and relieving them cyclically (fatigue test). Simultaneously the samples have been irradiated and monitored by the X-rays. 

Figure 1. Applied stress for a. Static fatigue test b. Cyclic fatigue test ACOUSTIC EMISSION Acquisition System...

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