Fatigue failure cycle number

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 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. 

Here is the number of cycles to fracture under the stress cycle in region i, and Nj/Nf is the fraction of the lifetime used up after N, cycles in that region. Failure occurs when the sum of the fractions is unity (eqn. (15.4)). This rule, too, is an empirical one. It is widely used in design against fatigue failure but if the component is a critical one. Miner s Rule should be checked by tests simulating service conditions. 

Case studies in fast fracture and fatigue failure 165 Integration gives the number of cycles to grow the crack from to 02 ... 

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. 

This represents the locus of all the combinations of Ca and Om which cause fatigue failure in a particular number of cycles, N. For plastics the picture is slightly different from that observed in metals. Over the region WX the behaviour is similar in that as the mean stress increases, the stress amplitude must be decreased to cause failure in the same number of cycles. Over the region YZ, however, the mean stress is so large that creep rupture failures are dominant. Point Z may be obtained from creep rupture data at a time equal to that necessary to give (V cycles at the test frequency. It should be realised that, depending on the level of mean stress, different phenomena may be the cause of failure. 

Stainless steels are subject to fatigue failure under dry conditions as are all metallic materials, having distinct fatigue limits where level is dependent on steel type and heat treatment. The limits can be depressed by the simultaneous action of a corrodent, the degree depending upon the nature of the corrodent. Under severe conditions the limit can be displaced to very low values and it is customary to describe resistance by an endurance limit, that is the cyclic stress to give rupture at a specific number of cycles when in contact with a specific corrodent. Some comparative data are in Table 3.25. 

Fatigue data are normally presented as a plot of the stress (S) versus the number of cycles (N) that cause failure at that stress the data plotted defined as an S-N curve (Fig. 2-43). The use of an S-N curve is used to establish a fatigue endurance limit strength. The curve asymptotically approaches a parallel to the abscissa, thus indicating the endurance limit as the value that will produce failure. Below this limit the material is less susceptible to fatigue failure. 

A detailed fatigue analysis is required if any of these conditions is likely to occur to any significant extent. Fatigue failure will occur during the service life of the vessel if the endurance limit (number of cycles for failure) at the particular value of the cyclic stress is exceeded. The codes and standards should be consulted to determine when a detailed fatigue analysis must be undertaken. 

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. 

Fig. 6.11 Changes in the hysteresis behavior during the fatigue of unidirectional SiCf/CAS-II. The number of cycles (in thousands) is shown above each curve. Note that the average modulus, area of the hysteresis loops, and the permanent strain offset all change during fatigue. Failure took place at 3.21 x 106 cycles. After Holmes and Cho.12...

Fatigue failure may occur when a specimen fractured into two parts, was softened or its stiffness significanfly is reduced by thermal heating and/or cracking. Sometimes, for different reasons, a large number of cycles elapses from the first formation of microscopic cracks to complete frac-mre. In this case, the fatigue failure is arbitrarily defined as having occurred when the specimen... 

For the polylmides, at the 5 N load, the wear rates were negatively correlated with the cycles to initiate wear. This result is consistent with the fatigue wear model which states that the wear rate is Inversely proportional to the number of cycles to fatigue failure. Comparing the cycles to initiate at the 5 and 10 N load, it is noted that cycles to initiate significantly Increase for the... 

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