Life Fatigue Diagrams

FIGURE 13. Life prediction diagram constmcted from the dynamic fatigue results at 1000°C for 10-YSZ, lO-YSZ/30 mol% alumina particulate composite, and lO-YSZ/30 mol% alumina platelet composite. The prediction represents at a failure probability of 50%. 

To ensure that blade stress levels are within the fatigue life requirements of the eompressor, it is usual praetiee to strain-gauge the blading on one or two prototype maehines, measure the stress levels, and generate a Campbell diagram showing the plotted test data. To measure data, an impeller ean also be mounted on a shaker table with a variable frequeney output (0-10,000 Hz). Aeeelerometers ean be mounted at various positions on the... 

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. 6.7 Fatigue life diagram for the tension-tension fatigue of unidirectional SiQ/1723 at room temperature (10 Hz, VmJ max — 0.1). The 106 cycle fatigue limit of 440 MPa is higher than cr. and

Tahreja R. Fatigue of composite materials — damage mechanisms and fatigue-life diagrams. Proc R Soc Lond Ser A Math Phys Eng Sci 1981 378(1775) 461-75. 

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. 

Towo AN, Ansell MP (2008) Fatigue of sisal fibre reinforced composites constant-life diagrams and hysteresis loop capture. Compos Sci Technol 68 915-1924... 

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. 

One of the key limitations of the S-N curve is its inabUity to predict lifetimes at stress ratios different from those under which the curve was developed. To predict the lifetime of a certain component, a more useful presentation of fatigue life test data is the modified Goodman diagram. 

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. 

In the review paper Lee and Barr (2004) have shown that in the elements subjected to cyclic compression, the influence of fibre reinforcement on the fatigue life cannot be expected, while it is significant in the elements under flexural loading. This is shown in Figures 11.9 and 11.10 where diagrams of S-N type are given here S is the dimensionless term - the percentage of... 

The cylinder is subjected to a repetitive stress by the hydraulic pressure. Approx. 40 strokes are performed per hour so that after an assumed life of the press of 30 years, and two 8-hour shifts per working day, and 300 working days per year, the number of load alternations = 30 X 300 X 2 X 8 X 40 = 5,760,000 is attained. Therefore, the fatigue limit has to be calculated as per Wohler s diagram. 

Fig. 7.5 Fatigue-life diagram for unidirectional composites under loading parallel to fibers (adapted from Talreja 1981)...

Talreja R (1981) Fatigue of composite materials damage mechanisms and fatigue-life diagrams. 

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. 

Figore 17.10 (a) Fatigue life reduction factor as a function of creep holding time for different 316L(N) plates [32], (b) Creep-fatigue interaction diagram in RCC-MRx and ASME codes [21]. 

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