Wohler S-N curves

There are data in the literature (Reifsnider, 1991) to suggest that composite repairs follow similar behavior as steels with respect to following the typical Wohler S—N curve, as shown in Figure 7.2. 

Figure 12.17 Typical Wohler or "S-N" curve for polymers (stress amplitude

The dispersion logA- of the individual stress cycles has been selected as a parameter. This representation may be applied in such cases where the spread patterns of the pipes under examination are known. Pipes whose cyclically increasing interior pressures show a behavior in accordance with the indicated Wohler curves S-N curves) demonstrated a spread pattern... 

Fig. 1.5 S -N curve, cyclic stress (T versus number of cycles to failure N, also known as Wohler diagram, showing the three regions that characterize fatigue behavior...

Once the analytical expression of the hysteresis loop has been derived, the following step is to infer the equation of the entire e-N curve or Wohler s curve. To this purpose, it is necessary to separate the two component of deformation, the elastic and the plastic one, as already done in Eq. (6.1). As to the elastic component Eg, it has been said in Sect. 5.2.2 that in 1910 Basquin [7] found a power law relationship with the cycles to failure N, see Eq. (5.22), so that in a log-log scale the elastic component of the S-N curve is a line. Later in the 50, Manson and Coffin [8-11] independently found that also the plastic component of the cyclic strain was related to life cycles N through a power law function, known as the Manson-Coffin relationship... 

The most common published fatigue data chart is the stress-life curve which is commonly called an S-N curve or a Wohler [10] curve. This is a graph of the magnitude of a cyclic stress (S), linear or log scale, against the cycles to failure (N) on a log scale. The cyclic measurement is made under constant oscillatory... 

The common observation that the number of cycles to failure, Np, depends on the stress or strain amplitude is generally represented in the form of Wohler curves (Woh-lerkurven, S — N curves). Such a representation accounts for the fact that stress (or strain) amplitude and number of load cycles are the most important of the many (and closely interacting) parameters which affect the fatigue life. 

Execution of fatigue tests with CRB specimens at different stress levels and with different R-ratios (e g. R=0.1 to 0.7 test frequency IHz if possible) evaluation of data in the form of R-specific Wohler curves (S-N curves). 

The importance of fatigue wear was first recognized by Soviet scientists.(23) Some of extended fatigue wear equations were proposed by Kraghelsky and Nepomnyashchi,(2 ) and recently by 3ain and Bahadur.(23,26) For our discussion, we use the simple relation based on Wohler s a-N curve (27)... 

This time the variation is about 15 %. The question arises now as to the knowledge of the fatigue limit of the plate containing 20 mm hole with respect to that oy of plate without hole. The Wohler s curve of the material is that already assessed in sample problem 4 of Sect. 6.3. It can be seen how the transition point N, between low cycle fatigue and high cycle fatigue, i.e., between plastic and elastic behavior, is at about — 0.0015 so that we can assume that in our case the behavior is mainly elastic and neglect the plastic component and draw the... 

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