Fatigue cyclic

At and near room temperature, metals have well-defined, almost constant, moduli and yield strengths (in contrast to polymers, which do not). And most metallic alloys have a ductility of 20% or better. Certain high-strength alloys (spring steel, for instance) and components made by powder methods, have less - as little as 2%. But even this is enough to ensure that an unnotched component yields before it fractures, and that fracture, when it occurs, is of a tough, ductile, type. But - partly because of their ductility - metals are prey to cyclic fatigue and, of all the classes of materials, they are the least resistant to corrosion and oxidation. 

Ceramic-matrix fiber composites, 26 775 Ceramics mechanical properties, 5 613-638 cyclic fatigue, 5 633-634 elastic behavior, 5 613-615 fracture analysis, 5 634-635 fracture toughness, 5 619-623 hardness, 5 626-628 impact and erosion, 5 630 plasticity, 5 623-626 strength, 5 615-619 subcritical crack growth, 5 628—630 thermal stress and thermal shock, 5 632-633... 

Cyclic enone, 12 185 Cyclic ethers, 10 567, 569 12 663 polymerization, 14 271 Cyclic fatigue, in ceramics, 5 633-634 Cyclic gas generators, 6 786-787, 789, 827 Cyclic halides, 19 56 Cyclic hexakis(thio-l,4-phenylene), melt polymerization of, 23 705 Cyclic hydrocarbons, 13 687 Cyclic hydroxyalkyl alkyl peroxide, 18 454 Cyclic ion exchange operation, 14 408-413 Cyclic ketones, 12 176, 177 14 590-592. See also Cyclic 1,2-diketones physical properties of, 14 591t hydroxyalkyl hydroperoxides from, 18 450... 

The analytical solutions derived in Sections 4.3 and 4.4 for the stress distributions in the monotonic fiber pull-out and fiber push-out loadings are further extended to cyclic loading (Zhou et al., 1993) and the progressive damage processes of the interface are characterized. It is assumed that the cyclic fatigue of uniform stress amplitude causes the frictional properties at the debonded interface to degrade... 

In particular, the techniques based on the termination of certain plies within the laminate has also shown promise. Static tensile tests of [30°/-30°/30°/90°]s carbon-epoxy laminates containing terminals of [90°] layers at the mid-plane show that premature delamination is completely suppressed with a remarkable 20% improvement in tensile strength, compared to those without a ply terminal. Cyclic fatigue on the same laminates confirms similar results in that the laminate without a ply terminal has delamination equivalent to about 40% of the laminate width after 2x10 cycles, whereas the laminates with a ply terminal exhibit no evidence of delamination even after 9x10 cycles. All these observations are in agreement with the substantially lower interlaminar normal and shear stresses for the latter laminates, as calculated from finite element analysis. A combination of the adhesive interleaf and the tapered layer end has also been explored by Llanos and Vizzini, (1992). 

Cyclic fatigue measurements require the specimen to be subjected to cyclic stress or strain of a higher amplitude than that employed for the simple dynamic test just described. The deformation must be of sufficient intensity to bring about specimen failure after a certain number of cycles, N. The value of stress leading to failure for a given N is typically 20% to 40% of the static tensile strength. 

Another important factor for tensile testing is cyclic testing or applying force and releasing (or relaxing) force on a test specimen to evaluate its ability to endure cyclic fatigue. A typical stress vs. strain relationship is shown in Fig. 3.16 and a cyclic stress relationship in Fig. 3.17. 

Lin CKJ, Jenkins MG, Ferber MK (1993) Evaluation of Tensile Static, Dynamic and Cyclic Fatigue Behaviour for a HIPed Silicon Nitride at Elevated Temperatures. In Chen IW, Becher PF, Mitomo M, Petzow G, Yen TS (eds) Silicon Nitride Ceramics -Scientific and Technological Advances. Symp Proc 287, Mat Res Soc, Pittsburgh, p 455... 

M. J. Reece, F. Guiu, and M. F. R. Sammur, Cyclic Fatigue Crack Propagation in Alumina under Direct Tension-Compression Loading, J. Am. Ceram. Soc., 72[2], 348-352 (1990). 

T. Fett, G. Himsolt, and D. Munz, Cyclic Fatigue of Hot-Pressed Si3N4 at High Temperatures, Adv. Ceram. Mater., 1[2], 179-184 (1986). 

Examination of the crack profiles after cyclic fatigue revealed extensive branching at the crack tip and a substantial damage zone in the vicinity of the (macro)crack tip. Increases in test temperature caused an increase in the size of the zone. Close examination revealed bifurcation of cracks and ligaments extended across the crack faces. An example of a typical fatigue crack profile appears in Chapter 7 by Suresh. 

Fig. 8.16 Variation of static and cyclic fatigue crack velocity, daldt, with the applied (maximum) stress intensity factor, KIy for fatigue tests on A CVSiCw composites conducted at 1400°C. The inset shows a schematic of the change in crack velocity for a change from static- cyclic-> static loading at fixed Kt.51...

Fig. 8.17 Cavitation at GBI junctions in the vicinity of a crack tip damage zone created by cyclic fatigue loading in AI2O3/S1C composite tested at 1400°C in air.S7...

E-3 spectrometer. A servo-hydraulic system was built which allowed loading of the samples in a wide variety of modes, and permitted tensile tests to be conducted at constant stress rate, at constant stress (creep), in cyclic fatigue, at constant strain rate, at constant strain or step strain. Provision was made for the simultaneous recording of stress-strain data and ESR spectra and a variable temperature control unit was employed. 

Biomechanical compliance or impedance matching of the tissue/material interface is not important for short-term implant experiments. However, for long implanting periods, cyclic fatigue failure of the tissue/material interface is caused by compliance mismatching. 

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