Fatigue lifetime

Many factors, in addition to stress, strain, amphtude, etc., influence the fatigue life of materials. Here, the effects of (a) stress-based and (b) strain-based evaluations of fatigue life will be considered for low- and high-cycle cases. In the past and even now, the stress-based approach is quite common. 

Many empirical relations have been suggested to predict fatigue lifetimes, one of which is the Manson-Coffin equation. This and other relations have been widely used for metals and alloys (J. Pelleg). The Manson-Coffin relation for lifetime 

Almost no effect of the ratio is observed in SiC/SiC, as seen in the above figure, and the entire damage is caused by the applied tensile stress. In Fig. 7.22, cyclic softening may be observed where the strain amplitude, As/2, is plotted against the 

The reason for the occurrence of strain softening seems to be associated with the initiation and propagation of cracks in the SiC matrix. The effect of frequency in stress-controlled fatigue in SiC/SiC, at a ratio of R = 0.1 is indicated in Fig. 7.23. 

21 Maximum stress versus cycles to failure at different stress ratios [39]. With kind permission of Elsevier 

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Resilin has a remarkably high fatigue lifetime (probably >500 million cycles) and our aim is to reproduce this desirable mechanical property in synthetic materials derived from our studies of resilin structure and function. We believe that recombinant resilin-like materials may be used, in the future, in the medical device field as components of prosthetic implants, including spinal disks and synthetic arteries. Spinal disks, for example, must survive for at least 100 million cycles of contraction and relaxation [30]. 

No results have been acquired to date on the fatigue lifetime of recombinant resilin however, it is informative to consider the performance of natural resilin. 

FIG. 25.6 Stress amplitude vs. fatigue lifetime in number of stress cycles. 

Allen and Bowen40 studied the fatigue behaivor of unidirectional Nicalon SiCf/CAS at room temperature by flexural testing at 10 Hz for / -ratios of 0.1 and 0.5. It was found that the specimens tested at R = 0.1 all failed above a certain stress level. For the specimens fatigued at ft = 0.5 there was considerable scatter in the fatigue lifetime. While some specimens failed lifetimes similar to the specimens with R = 0.1, roughly half the specimens did... 

Figure 15 shows how the average fatigue lifetime of PS depends on frequency for two different stress amplitudes. The variation appears to be a linear one on this log-log plot, with the number of cycles to fracture increasing with increase of frequency, and at essentially the same rate for both stress amplitudes. For the rubber modified HIPS the fatigue endurance is plotted as a function of frequency in Fig. 16. Here too the lifetime increases with increase of test frequency and again the variation is a linear one on a log-lot plot. The slope of these curves is also essentially independent of stress...

When ABS is compared to HIPS at comparable stress amplitudes, fatigue lifetimes are several decades higher. Several factors are involved. First, tensile strength and modulus of the SAN matrix are increased compared to values for the PS matrix. Secondly, the SAN matrix is more resistant to crazing and thirdly, the primary mode of deformation for ABS appears to be shear yielding rather than crazing. 

Fatigues tests have been run on a series of ABS samples under reversed tension-compression at a stress amplitude of 27.6 MPa, at four different frequencies, viz. 0.02, 0.2, 2, and 21 Hz. The average fatigue lifetime as a function of frequency is shown in Fig. 46 and, for comparison purposes, the data for HIPS obtained at 17.2 MPa is also shown. The average fatigue life-time of ABS increases in a linear manner with frequency on this log-log plot. The rate of increase, however, is reduced compared to that of HIPS, or of PS (Fig. 15). The reduced frequency sensitivity is perhaps a result of a reduced magnitude of the p-transition in the SAN copolymer compared to that in PS. A reduced frequency sensitivity of FCP rate in ABS vs. a HIPS-modified PPO has also been noted... 

Fatigue lifetime increases with increase of test frequency, v, according to a power law relationship, for both PS and HIPS and the slope of the lop N-log v plot appears independent of stress ampHtude over the stress range investigated. Fatigue endurance also increases with frequency for ABS but its frequency sensitivity is less than that of HIPS. 

Strength approach a constant value, we find decreased crack propagation rates in pre-cracked specimens and a continued rise in fatigue lifetimes in unnotched specimens. 

R Ratio of minimum stress to maximum stress during fatigue loading S/N Stress/number of cycles (fatigue lifetime plot)... 

Fig. 23. Schematic diagram showing shear fatigue lifetime and craze fatigue lifetime branches with a transition zone...

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