Cyclic stress amplitude

Thus, Ea of the polymer is proportional to the slope of the stress-temperature curve in thermal-expansion dominated regimes. A decrease in Ea results in a lower thermal-expansion-mismatch induced stress. Thus, a lower Ea should also endow the material with the ability to sustain a greater number of thermal cycles before fatigue-induced fracture because of a smaller cyclical stress amplitude. 

In a typical fatigue test, a sample is subjected to an alternating stress of a given amplitude and frequency. The cyclic stress amplitude is defined as... 

Fig. 8.45. Linear log-log relationship between cyclic stress amplitude and fatigue life of lanthanum in rotating beam tests at three different cyclic frequencies. From Shibarov et al. (1971).

Crack initiation testing is used extensively for testing fatigue in terms of the number of cycles it takes for brittle material to fracture completely. The data are interpreted in the form of a cyclic stress amplitude vs. cycles to failure (S-N) curve. The same concept has been applied to study the crack initiation stress of gluteraldehyde fixed heart valve tissues (Vesely et al., 1995). First the crack initiation stress for a monofilament of a particular material is determined on the virgin suture before the barbs are inserted. Once the number of cycles it takes to initiate a crack in the virgin monofilament is known, we can then compare this result with the number of cycles it will take to initiate a crack at the cut point in the same barbed suture. This will provide evidence of the life expectancy of such a barb in vivo. This is crucially important, given that the loads exerted by different tissues are different, and therefore the number of cycles it takes to initiate a crack in a barbed suture will differ from tissue to tissue. Obviously, the in vivo performance will also depend on the amount... 

Fig. 11. Differential Scanning Calorimetry enthalpy overshoot of samples of different previous physical aging histories plotted as a function of cyclic stress amplitude for a fixed 8000 cycles (43). A aged at 130°C for 24 h aged at 130°C for 12 h aged at 130°C for 6 h. To convert MPa to psi, multiply by 145.

Conducting, over one hundred years ago, the first fatigue experiments on iron and steel specimens, Wohler [4] arrived to the conclusion that the fatigue resistance of a material depended on the applied cyclic stress amplitude (7a, conventionally indicated also by the symbol S... 

Fig. 5.42 Schematic of Shake-down effect in an elastic-ideally plastic material subjected to cyclic stress amplitude and mean stress

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