Fatigue stress-cycle diagram

Stress cycle diagrams (Woehler diagrams) remain the most commoniy used means for evaluating long-term cyclic, i.e., fatigue, behavior of fiber composite piastics. With this method, totai specimen faiiure, i. e., fracture, is equated with damage = 1, Fig. 1.65 [148]. 

By subjecting a single lap shear specimen to dynamic cyclic load, the fatigue behaviour of adhesive bonds is determined. The test values are plotted in the stress-cycle (Woehler) diagram where the number of load cycles is represented as a function of the component load. An example is shown in Fig. 23. 

Fig. 8.22. Modified Goodman diagram showing fatigue stress amplitude required to cause failure in 10 cycles as a function of mean stress for two carbon-fibre reinforced epoxy resin materials (a) unidirectionally reinforced, stressed parallel to fibres, and (b) cross-plied laminate, with 45% of plies parallel to stress, and 55% transverse. Fibre volume fraction 60% (after R. Tetlow).

Examples of fatigue curves for unreinforced (top) and reinforced (bottom) plastics are shown in Fig. 2-44. The values for stress amplitude and the number of load cycles to failure are plotted on a diagram with logarithmically divided abscissa and English or metrically divided ordinates. 

Fig. 6.6 Fatigue life diagram for the tension-tension fatigue of unidirectional SiCf/Si3N4 at 1000°C and a stress ratio (o /cr, ) of 0.1. Fatigue run-out (5 x 106 cycles) was observed when the maximum stress was below apt. After Holmes et al.43...

Fig.16 S-N fatigue diagram of a bulk diglycidyl ether of bisphenol (DGEBA)/isophoron diamine (IPD) epoxy polymer giving the maximum applied stress as a function of the number of cycles to failure (three-point bending, 25 Hz, stress ratio OminMnax = 0.1) (from [53]). The two dotted lines correspond to theoretical values of the amplitude of the effective tensile stress, Acr, calculated for (a) gross slip condition and (b) under partial slip condition for an imposed displacement ( 10 xm) which corresponds to the experimental contact endurance limit at 105 cycles...

As for the epoxy polymers, a quantitative comparison of the contact fatigue behaviour was attempted on the basis of an estimate of the maximum tensile stress at the edge of the contact. The coefficient of friction of the copolymers increased as the tests proceeded, with a variation which was dependent upon the level of the normal loading. As a first approach, the value of //. at crack initiation was taken into account in the calculation of a . The results are reported in a S-N fatigue diagram giving the maximum applied tensile stress as a function of the number of cycles to crack initiation (Fig. 23). These data show a marked increase in the contact fatigue resistance of the GIM copolymers compared with the MIM material. 

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-... 

The plotted characteristic numbers of cycles to fracture were determined by the Weibull method [21]. Each point represents a test series of ten specimens. In the diagram it can be seen clearly that fretting fatigue leads to a distinct deterioration of the specimen strength and life time. The higher the maximum Hertzian stresses the clearer the decline of the strength and life time is. 

In Figure 10 the number of fracture cycles and the characteristic number of fracture cycles for the respective test series is plotted against the maximum principal stress on the tensile loaded side of the specimens. The characteristic number of fracture cycles was determined by the method developed by Weibull [21]. In the Woehler diagram it can be seen clearly that fretting fatigue leads to a distinct deterioration of the life time. The higher the maximum stresses the clearer the decline of life time is. At Fn = 10 N (Pmax = 2311 MPa) the life time decreases at a maximum base load of OR.max = 210 MPa for about 80%. at Fn = 20 N (pmtx -2912 MPa) the life time decreases for about 91% compared to to life time under the same maximum base loading. 

It is worth restoring the Goodman diagram concept to appreciate its use. Essentially, the emphasis of Goodman s work was on tensile-mean stress with respect to fatigue life. The stress required to produce failures during a specified number of cycles is directly related to tensile strength, as indicated schematically in Fig. 7.47. 

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