Fatigue Threshold

Region II of the da/dn versus AK/ diagram. The same metallurgical effect, in terms of different ultimate strength, can be seen in the titanium alloy type T1-6A1-4 V, as shown in Fig. 11.5 [7]. 

Klesnil e Lukas [11] proposed an empirical relationship between and the R-ratio analogous to that proposed by Walker between AK and R (see Sect. 11.4) 

3 Plastic Zone and /f-Ratio Effect on Fatigue Threshold 

The compressive residual stress state certainly exerts a beneficial effect on the fatigue behavior since in the following reloading it will be necessary to overcome it before the crack opens and restarts to grow. This actually means that it will not be the entire load excursion Aa to cause fatigue, but rather only the part of it that wiU open the crack, This will reduce the effective applied AK. Let s follow 

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A significant improvement in the administration of a lipid-containing emulsion was achieved with a special additive to PC. As shown in Fig. 22b, the improved behavior of the new polycarbonate PC2 proved in practice to be verifiable by fracture mechanical fatigue crack growth experiments. In the presence of the fat emulsion the more lipid-resistant PC2 shows a higher fatigue threshold value AK as well as an improvement by a factor of two in AKcf in comparison to PCI used so far. 

The cyclic load form is important. The crack growth rate increases, and the fatigue threshold decreases as compared with that in air. The crack growth rate obeys a Paris Law with increase in crack growth rate and a decreased fatigue threshold compared with the behavior in air (73). 

With the increase in testing temperature, the fatigue thresholds for both microstructures decrease slightly up to 1100°C. At 1200°C, which is above the brittle-to-plastic transition (see below), the da/dN versus AK curves showed three distinctive regions that were more pronounced for the fine-grained samples [132]. 

In the case of structural joints, cohesive failure of the adhesive is often observed. Indeed, in many instances, the requirement will be to engineer failure within the bulk of the adhesive layer. Modelling of the adhesive response as a function of water uptake with temperature is carried out to represent the influence of environmental exposure. With fatigued structural joints, by combining the use of fracture mechanics and appropriate modelling tools, the resultant crack propagation rates and consequently durability levels can be predicted as a function of various environmental and mechanical test parameters, such as frequency. Fatigue threshold values can be determined, which are used to predict durability performance. " ... 

Table 7.3 lists the relevant parameters determined for some ceramic materials. In the table AKxh is the fatigue threshold, defined as the maximum value of AK at which growth rates did not exceed 10 ° m/cycle (ASTM E 647 procedure). Fig. 7.28 shows micrographs where a comparison is made between monotonic (monotonicaUy increasing loads) and fatigue fractures. The lengths of selected microcracks, obtained by monitoring the top surface of the cantilever-beam specimens (Fig. 7.27b) at various maximum applied stress levels and plotted as a function of the number of stress cycles, are shown in Fig. 7.28a, b at load ratios R of 0.05 and —1, respectively (Fig. 7.29). 

D. L. Chen, B. Weiss, R. Stickler The effective fatigue threshold Significance of the loading cycle below the crack opening load, Internet. J. Fatigue 16, A85 (199A)... 

Table 3. Fatigue Threshold Stress Intensity Factor and Paris Regime or UHMWPE Subjected to Different Processing Conditions ...

In figure 4 the output of moving difference filter ARi f) is plotted and the estimate of fatigue threshold is... 

Table 1 shows the estimated of fatigue threshold obtained from times series of bioelectric impedance nd... 

Ai info/ we hypothesized that the fatigue threshold is associated to the time (where the series presents the... 

Fig. 4 Time series of J/ i,%.along the time of isometric contraction of the biceps brachii muscle. The estimate of fatigue threshold ) is...

Table 1 Estimated of fatigue thresholds obtained by the bioelectric impedance ( , >) and by the force (r,, ) times series for each volunteer.

A comparison between CF and SCC based on a detailed analysis of micromechanisms near the fatigue threshold... 

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