Fatigue S-N diagram

Figure 5.104 Fatigue S-N diagram for SMC-R65 composite. Reprinted, by permission, from Composite Materials Technology, P. K. Mallick and S. Newman, eds., p. 52. Copyright 1990 by Carl Hanser Verlag.

Figure 1.13 Typical fatigue curve (S-N diagram for a material having an endurance limit, oe).

For the characterization of the fatigue properties of polymers and polymer composites, Wohler experiments to establish S-N diagrams are usually employed (Fig. 3). 

Permanent structural changes that occur in a material subjected to fluctuating stress and strain, which cause decay of mechanical properties. See S-N diagram. The ability of a material to plastically deform before fracturing in constant strain amplitude and low-cycle fatigue tests. See S-N diagram. ... 

Plot of stress, S, vs. number of cycles, N, required to cause failure of similar specimens in fatigue test. Data for each curve on the S-N diagram are obtained by determining fatigue life of a number of specimens subjected to various amounts of fluctuating stress. The stress axis may represent stress amplitude, maximum stress, or minimum stress. A log scale is usually used, especially for the N-axis. 

The fatigue limit is the stress below which a material can be stressed cyclically for an infinite number of times without failure. The fatigue strength is the cyclic stress a material can withstand for a given number of cycles before failure. The S-N diagram is the plot of stress (S) against the number of cycles (N) required to cause failure of similar specimens in a fatigue test of exactly the same conditions. 

Fatigue curves are used to determine the number of allowable cycles. The fatigue curve is also known as the S - N diagram, because one axis represents stress, S, and the other axis represent number of cycles, N. Each material group has their own fatigue curve based on test results and are shown in ASME Section VIII, Division 2, Annex 3-F. 

S-N Diagram AKA fatigue curve. A plot of alternating stress, Sa, against the maximum number of allowable of cycles, Na-... 

Fig. 10.23. S-N diagram of polyoxymethylene (polyacetal) at different loading frequencies (after [102]). The solid line corresponds to mechanical fatigue at higher frequencies (dashed lines), the material fails by thermal fatigue. If the load is reduced, the thermal fatigue curves join with the curve for mechanical fatigue...

Figure 1.73 compares approaches to determine fatigue properties and service life using S-N diagrams and damage curves [148]. This also indicates the considerable difference in costs and time, if part design has to be done from scratch without data banks and information systems. 

The fatigue resistance of nano- and microcomposites was also compared by Mallick and co-workers [46,56]. In particular, th performed stress-controlled tension-tension fatigue tests on PP nanocomposites reinforced with 5 wt% of organoclay and on 40 wt% talc-filled PP. As reported in Figure 9.15, the S-N diagram of nanocomposites was slightly better than that of neat PP and much higher than that of talc-filled composites. 

S-N diagram Plot of stress (S) against the number of cycles (N) required to cause failure of similar specimens in a fatigue test. 

Fig. 5.25 Fatigue limit at 10° cycles inferred by staircase test method, shown on the previously determined S-N diagrams...

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