Fatigue cyclic loading

In view of these considerations, it is considered fruitless to attempt to define a precise point for any value between LL and KN. Fatigue, cyclic load, and environmental allowables will have to be determined in other ways. 

The cyclic loading applied to hull structure will eventually lead to fatigue cracking. 

The use of fatigue data and crack length measurements to predict the remaining service life of a stmcture under cyclic loading is possibly the most common application of fracture mechanics for performance prediction. In complex stmctures the growth of cracks is routinely monitored at intervals, and from data about crack growth rates and the applied loadings at that point in the stmcture, a decision is made about whether the stmcmre can continue to operate safely until the next scheduled inspection. 

Data on fatigue crack propagation are gathered by cyclically loading specimens containing a sharp crack like that shown in Fig. 15.7. We define... 

In the case of a pressure vessel subjected to cyclic loading (as here) cracks can grow by fatigue and a vessel initially passed as safe may subsequently become unsafe due to this crack growth. The probable extent of crack growth can be determined by making fatigue tests on pre-cracked pieces of steel of the same type as that used in the pressure vessel, and the safe vessel lifetime can be estimated by the method illustrated in Case Studv 3. 

Material Properties. Materials possess various mechanical and chemical properties, and, therefore, it is possible to select materials appropriate for severe corrosion conditions. For example, if the equipment is under cyclic loading, a material with high fatigue strength is desired. Similarly, it is desirable to have corrosion-resistant materials for the corrosive environments. There are several sources for obtaining information on materials properties. Some are listed in Table 4-173. 

Ward et a/."" have shown that, under cyclical loading, the oxidation rate of steels is similar to that under unstressed isothermal conditions, provided the fatigue stress is below the stress required to exceed the scale failure strain. If, however, the failure strain is exceeded, the oxidation rate is accelerated due to repetitive scale failure, and linear kinetics are observed. 

McIntyre, P., Hydrogen-steel interactions during cyclic loading , Proc. of UK/USSR Seminar on Corrosion Fatigue of Metals, Lvov, USSR, 19-22 May 1980, The Metals Society pp. 62-73 (1983)... 

Under a repeated applied cyclic load, fatigue cracks begin somewhere in the product and extend during the cycling. Eventually the crack will expand to such an extent that... 

Several criticisms of these parameters have recently been pointed out. First, they have no specific association with a material plane (i.e., they are scalar parameters), despite the fact that cracks are known to nucleate on specific material planes. With traditional parameters it is difficult to account for the effects of crack closure under compressive loading. Traditional parameters have not been successful at unifying experimental results for simple tension and equibiaxial tension fatigue tests. Finally, a nonproportional loading history can always be constmcted for a given scalar equivalence parameter that holds constant the value of the scalar parameter, but which results in cyclic loading of material planes. For such histories, scalar parameters incorrectly predict infinite fatigue life. 

In conclusion, it may be mentioned that the characterization of the mechanical behaviour of materials has many facets. Different methods of testing pertain to different aspects and conditions. The tensile properties, as determined by the tensile test, correspond to slowly applied single load applications. Rapidly applied and cyclic load applications respectively provide the impact and the fatigue properties. Hardness is an analog of the tensile strength which a tensile test measures. The creep test pertains to mechanical behaviour under long term loading at elevated temperatures. 

Fatigue failure is likely to occur in equipment subject to cyclic loading for example, rotating equipment, such as pumps and compressors, and equipment subjected to pressure cycling. A comprehensive treatment of this subject is given by Harris (1976). 

The analytical solutions derived in Sections 4.3 and 4.4 for the stress distributions in the monotonic fiber pull-out and fiber push-out loadings are further extended to cyclic loading (Zhou et al., 1993) and the progressive damage processes of the interface are characterized. It is assumed that the cyclic fatigue of uniform stress amplitude causes the frictional properties at the debonded interface to degrade... 

Cyclic Effects Fatigue due to pressure cycling, thermal cycling, and other cyclic loading shall be considered in the design. 

Baste, U. (1993), Thermal shock and cyclic loading of ceramic parts in stationary gas turbines , in Schneider G. A. and Petzow G. (editors), Thermal Shock and Thermal Fatigue Behavior of Advanced Ceramics, Dordrecht Kluwer Academic, 87-97 Becher, P.F. (1981), Transient thermal stress behavior in Zr02-toughened A1203 , J. Am. Ceram. Soc., 64(1), 37-39. 

Stiffness, resistance to deformation under constant applied load (creep resistance), resistance to damage by cyclical loading (fatigue resistance), and excellent lubricity are mechanical properties for which acetal resins are perhaps best known and which have contributed significantly to their excellent commercial success. General-purpose acetal resins are substantially stiffer than general-purpose polyamides (nylon-6 or -6,6 types) when the latter have reached equilibrium water content. 

Fatigue Manifestation of a cumulative process leading to progressive fracture of a material subjected to cyclic loading. 

Failure may be mechanical, due to wear, abrasion and erosion, britle fracture, surface deterioration, cyclic loading, embrittlement, thermal or pressure shock, or fatigue. Failure may also be chemical, in essence due to corrosion. 

In a shear field, a particle experiences rotation because of velocity differences in addition to the load created by the shear forces. This produces cyclical loading. The cyclical application of a load promotes dispersion, since solid particles often undergo fatigue fracture [1],... 

Fatigue Test. Fatigue testing places a given load repeatedly on a bonded joint. Lap shear or other specimens are tested on a machine capable of inducing cyclic loading... 

---