Curve Wohler

The repeated mechanical loading of a polymer leads to a speedier failure than an instantaneous loading. The Wohler curves (or SN curves) plot the level of stress or strain (S) leading... 

It should be remembered [39], that when evaluating data about fatigue limits in the pulsating or alternating mode which have been determined with inert fluid environments, that the corrosion fatigue may not only effect remarkably the reduction of the admissible stresses but also depends on the number of cycles (a continuous small slope of the Wohler-curve). 

Figure 11-25. Wohler curve for the alternating flexural stress applied to a high-impact-strength poly(styrene) (SAN), a humid polyamide (PA), and a poly(styrene) (PS). The flexural stressa is measured as a function of the number N of alternations in the load. Read PA instead of PAN.

FIGURE 20.45 Wohler curves in air after precorrosion and under corrosion fatigue conditions. 

Table la.26 High cycle fatigue strength Op of various stainless steels (Wohler curves) and rotating fatigue strength OR(Ref. 20, 21, 22)... 

Table lc.20 High cycle fatigure strength Og and rotating fatigue strength Or of titanium and titanium alloys (Wohler curves) (Ref. 10, 21, 22, 23, 24)... 

Figure 11.44 Wohler curve in inert and in corrosive environment (schematic). The figure shows (a) a metal with a fatigue limit (b) a metal without a fatigue limit and (c) a metal undergoing corrosion fatigue.

As mentioned above, the fatigue behavior with damage development should be assessed by the Wohler curve and the fatigue endurance limit. The latter value is the most important parameter for design and construction purposes. For the description of the Wohler curves, different methods can be selected according to the plotting (semi- or double logarithmic scale). 

Figure 2 Wohler curve (S -Nf) for a unilaterally compression-molded GMT-PP with 40wt.% glass swirl mat reinforcement. Note the following testing conditions were set R = 0.1, /= 5 Hz, T = 30°C (DFG Ne 546/5-1, 17]).

Fig. 3. Wohler curves for different thermoplastics, generated via flexural fatigue loading at 10 Hz (12). To convert MPa to psi, multiply by 145.

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... 

During the dynamical fatigue, the variation of material resistance with the stress or deformation amplitude is usually determined by drawing of Wohler curves (variation of O vs. N ), Figure 3.336, [914]. On these curves, three distinct regions can be delimited ... 

The fatigue process that characterises the regions II and III of Wohler curves is composed by at list two steps, namely - the crack initiation step due to the homogeneous response of polymer and a second one heterogeneous that evolves through the appearance of some discontinuities of material, which continuously evolve. 

Figure 3.336. Wohler curves of (a) crosslinked polyurethane (M= 6000 g/mole V = 4 Hz) (b) linear polyurethane, Estane, (v = 4 Hz) [208].

Fatigue data is usually published in the form of Wohler curves where stress or strain amplitude is plotted against the number of cycles to failure on a logarithmic scale. 

Compared to the Wohler curve determined in exposure to air, the fatigue strength curve under CF conditions is steeper and the materials no longer reach a fatigue... 

Figure 1-19. Schematic Wohler curves under different conditions.

With a 10% stress reduction on various load levels selected as a criterion for service limits, Wohler curves can be derived. Figure 5.280. Here, too, it is readily apparent that damage occurs faster under the influence of sulfuric acid, and that the load limit progresses clearly more steeply than in air. 

Fig. 1.55 Appearance of persistent slip bands broken line) in a specimen of mild steel 0.09 % C tested in reci nocating bending. Wohler curve is also shown full line) [78]...

Therefore, the Basquin line approximate the Wohler curve only in the high cycle fatigue regime, as shown in Fig. 5.15. When the plastic component of the strain cannot be neglected and this happens in general below 10" -10 cycles, the Basquin component must be integrated with the Mason-Cofi n component, as it will be shown in the next chapter. 

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