Cycle Fatigue

Otnax = maximum stress level of the fatigue cycle... 

Fig. 8.75 Relation between (k — l)/n) and tj in 1% NaCl, where k is the ratio of fatigue strength in air to that in a corrosive environment, the notch sensitivity factor on fatigue strength, the corrosion current density at start of fatigue cycling, and jy the total life in...

Fatigue corrosion occurring as Thermal fatigue cracking (thermal effect corrosion Corrosion fatigue Cycles of thermally induced stress leads to metal failure. Results from a combination of thermal cycling stress and SCC or other corrosion process. 

In this fracture mechanics approach, the rate of crack growth is a function of the maximum value of tearing energy attained during the fatigue cycle. For a strip with an edge or central cut cycled in tension, a relation between cycles to failure and the initial cut size for the case where that cut is relatively very small can be derived8,9 ... 

Material Tensile strength, a (MPa) Max. fatigue stress, amax (MPa) max Fatigue cycles Residual strength (MPa)... 

Cho et al.52 developed an analytical model that relates the temperature rise during fatigue to the interfacial frictional sliding stress, as elaborated later. Holmes and Cho12 used the model to show that in SiCf/CAS-II, the interfacial shear stress, r, decreases from a value of around 15 MPa to 5 MPa within the first 25 000 fatigue cycles (Fig. 6.13). The approach used to determine r from temperature rise data is described in greater detail in the following section. 

A comparison of the crack velocities measured under static and cyclic loads is illustrated in Fig. 7.2. For this purpose, the crack velocity under cyclic loads, da/dt = da/dN x vc, plotted against the maximum stress intensity factor of the fatigue cycle, Kmax = AA7(1 — R), from the results shown in Fig. 7.1. The static crack velocity da/dt is also plotted against the stress intensity factor Kj corresponding to the applied load. In the intermediate range of crack growth, the static crack velocity generally follows the power-law relationship... 

Early work (e.g., Refs. 44 and 45) on silicon nitride ceramics for a limited range of high temperature cyclic loading conditions led to the hypothesis that the mechanisms of cyclic and static fracture at elevated temperature are identical, and that the cyclic crack growth rates can be predicted on the basis of static fracture data. One of the techniques commonly used to derive cyclic crack growth rates solely on the basis of static load fracture data involves integration of the relationship in Eqn. (13) over the duration of the fatigue cycle such that... 

The increase of fatigue cycle could result from multiple factors, such as wettability of the surface with matrix polymerizing dough, the extent of air bubble entrapment, interfacial voids, and potential formation of chemical bonds between the particle surface and the polymer matrix. What we are seeing is the net cumulative effect of all possible factors. The correlation figure is used to indicate the rough trends to see the contribution of the selected factor. 

The comparison of (a) and (b) shows the effect of adding untreated milled carbon fibers, which is no improvement. The comparison of (a) and (c) shows the effect of the surface treatment of X-ray-opaque powders on fatigue cycles, which is a significant (nearly threefold) increase. The comparison of (c) and (d) shows the effect of adding untreated carbon fibers on bone cement with treated Zr02, which is an appreciable negative effect. The comparison of (d) and (e) shows the effect of the surface treatment of carbon fibers, which is a roughly twofold increase. 

From these comparisons it is clear that every quantum improvement in the fatigue cycles can be attributed to the plasma surface modifications of fillers. Among the improvement of fatigue properties by the plasma treatments on the carbon fibers, as shown in Figure 30.28, the oxygen plasma posttreatment showed a better effect that was already seen in the plasma treatment on the X-ray opaque powder. There were reports that the oxygen plasma-treated surface showed a better adhesion to PMMA than the argon plasma-treated surface [46 9]. 

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