Fatigue cycles to failure

Fig. 4.82 Fatigue cycles to failure versus stress in compression - tension mode (Jctai for unreinforced and reinforced polypropylene specimen taken in different removal directions [98Mai].

Notched fatigue (cycles to failure) of parKake forgings vs amount of work at (a) strength level of 1310 MPa (190 ksO and (b) strength level of 865 MPa (140 ksi) and 1034 MPa (150 ksi). Log average lives and scatterband indkated. 

Figure 6.80 Fatigue cycles to failure versus stress in tension for DR and EF series General Electric Valox poly-cyclohexylenedimethylene terephthalate polyester [3],...

Figure 8.26 Fatigue cycles to failure versus stress in flexure at 23°C for 30% glass fiber-reinforced DSM Stanyl TW200F6 PA 46 resin (ASTM D671,20 Hz) [8],...

Figure 9.5 Fatigue cycles to failure versus stress in flexure for Ipiranga Petrochem Hostalen GM 501OT2 high-density polyethylene (per DIN 53442, at 10 Hz, 23°C) [1],...

Figure 9.25 Fatigue cycles to failure versus stress in tension with a stress ratio of 0.1 at 25°C and 10 Hz for filled and unfilled polyvinyl chloride [11].

Changing the diamond grits strength measure from 32 H to 82 H at constant diamond concentration results in 20-fold increase in fatigue life of the composite. At the same time the decrease in synthetic diamond concentration from 37 5% down to 18.7%, the other parameters being constant, results in 8-fold increase in a number of fatigue cycles to failure of DCM specimens. 

Fig. 4. Fatigue data for high temperature alloys (a) As vs cycles to failure for various alloys tested under strain control. (°) = testing at RT in air ...

Load bend fatigue strength of alloys capable of withstanding 4—5 cycles before failure in 0—90—0 degree cycles, which is above the three-cycles-to-failure minimum in MIL-S l D-883 values pertain to a 0.25-mm thick strip that has been sheared to 0.45-mm width. 

Fatigue is another property of considerable interest to the design engineer. CycHc deflections of a predeterrnined ampHtude, short of giving immediate failure, are appHed to the specimen, and the number of cycles to failure is recorded. In addition to mechanically induced periodic stresses, fatigue failure can be studied when developing cycHc stresses by fluctuating the temperature. 

For low-cycle fatigue of un-cracked components where (imax or iCT inl am above o-y, Basquin s Law no longer holds, as Fig. 15.2 shows. But a linear plot is obtained if the plastic strain range defined in Fig. 15.3, is plotted, on logarithmic scales, against the cycles to failure, Nf (Fig. 15.4). TTiis result is known as the Coffin-Manson Law ... 

Figure 19.7. Fatigue resistance of acetal resin compared with nylon 66 and with polyethylene. Measured as the cycles to failure at a given applied stress. (Du Pont trade literature)...

A distinction must be made regarding the length of service of the pressure reducing systems. Fatigue failure of any mechanical system depends on time, i.e., the number of cycles to failure. Therefore, the treatment required for a continuous service may not be justified for a short term service. A System in short term service is defined as one which operates a total of 12 hours or less during the life of the plant. Pressure relief valves typically meet this limit. Systems in short term service exceeding the screening criteria indicated above should be evaluated. 

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. 

Some viscoelasticity results have been reported for bimodal PDMS [120], using a Rheovibron (an instrument for measuring the dynamic tensile moduli of polymers). Also, measurements have been made on permanent set for PDMS networks in compressive cyclic deformations [121]. There appeared to be less permanent set or "creep" in the case of the bimodal elastomers. This is consistent in a general way with some early results for polyurethane elastomers [122], Specifically, cyclic elongation measurements on unimodal and bimodal networks indicated that the bimodal ones survived many more cycles before the occurrence of fatigue failure. The number of cycles to failure was found to be approximately an order of magnitude higher for the bimodal networks, at the same modulus at 10% deformation [5] ... 

Fatigue tests are normally performed by applying one of the stress cycles described above until the test specimen fractures. The number of cycles to failure, Nf, at a... 

Figure 5.41 Stress amplitude versus cycles to failure illustrating (a) fatigue limit and (b) fatigue life. Reprinted, by permission, from W. Callister, Materials Science and Engineering An Introduction, 5th ed., p. 212. Copyright 2000 by John Wiley Sons, Inc.

The phenomenon involving failure of a material subject to repeated loading is called fatigue. Failure occurs at stress levels below those observed in the "static" tests described above. Lee et al (22) examined the characteristics of some sulphur concretes subject to fatigue. Fatigue lives (the number of cycles to failure) considerably in excess of those for portland cement concretes were observed. Polymerization of the sulphur with di-cyclopentadiene was observed to reduce fatigue life. 

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