S-N diagrams

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

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.

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

S-N diagram See fotigue S-N diagram, snow, artificial A plastic copolymer of butyl and isobutyl methacrylate often dispersed from an aerosol atomizing package. It is used in decorative window displays. 

Fig. 4. S-N Diagram of Double Overlap Joint with Adhesive KH-225 at 100 Hz and R = 0.1.

Figure 17.59 S-N diagrams for representative unidirectional DMC composite materials. Source Reprinted with permission from Curtis PT, RAE Technical Report TR82031, FtAE, Farnborough (now DFIA), 1982, Curtis PT, RAE Technical Report TR86021, RAE, Farnborough, 1986, Curtis PT, RAE Technical Report TR87031, RAE, Farnborough, 1987. Copyright 1987, QinetiQ Ltd.

Maximum fluctuating stress a material can endure for infinite number of cycles - determined firom the S-N diagram. Function the part or assembly was originally designed and manufactured to perform. 

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. ... 

The stress below which a material can be stressed cyclically for an infinite number of times without failure. 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. 10.22. S-N diagram of Si3N4 at different temperatures (measured in bending at ii = — 1) [113]. The dashed line is a fit according to the Basquin equation, common to temperatures of 20°C and 1000°C, whereas the dotted line is valid at 1200°C...

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...

The S-N diagram plots the life time of a material at constant stress amplitude and R ratio. It is, however, not possible to assert the life time, using the diagram, if the load amplitude changes. The most obvious way to determine the life time in this case is to simulate the service load history in the laboratory. Unfortunately, this is a rather involved procedure that is not feasible in most cases. It would be helpful if it were possible to estimate the life time directly from the S-N curves. One way to do this is to use Miner s rule (also known as Palmgren-Miner rule) [99] that will be explained now. The rule is rather simple and thus easy to employ, but it has some disadvantages, discussed at the end of this section. 

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