Cyclic loading strength

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. 

Cyclic loading significantly reduces the amount of allowable stress a material can withstand. If data are not available on the endurance limit of a material being considered for use, a percentage of its tensile strength can... 

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. 

From the safety point of view, cyclic loading means that fatigue can occur at stress levels far below the limits for static strength. 

Fig. 6.5 Cyclic stress-strain behavior observed during the room temperature fatigue of unidirectional SiQ/LAS-II at a maximum stress below the 105 fatigue limit (loading frequency = 10 Hz, crmin/crmax = 0.1). The cyclic stress-strain curves show very limited, if any, hysteresis. Note that the strength and strain capability is retained after cyclic loading below the proportional limit stress. After Prewo.42...

Fig. 6.20 Relationship between the decay in interfacial shear stress and the fatigue life of fiber-reinforced ceramics, as proposed by Rouby and Reynaud. (a) The interfacial shear stress r is assumed to decrease during cyclic loading, reaching a steady-state value after Ni cycles, (b) The composite strength decreases in parallel to the decrease in r. An infinite fatigue life is predicted if the strength corresponding to t is above the maximum fatigue stress. After Rouby and Reynaud.46...

When thermoplastics are reinforced with short fibres, both stiffness and strength may be increased, but these improvements are accompanied by a reduction of the ultimate strain for high concentrations of fibres [1-5]. This brittle behaviour is incompatible with growing engineering applications in which tlie parts are subjected to cyclic loadings or impacts, as in automotive under-the-hood applications. Thus, very tough nylon thermoplastics with enhanced creep and impact resistances have been produced by the introduction of a rubbery phase to the... 

The response of a Zr stabilized 2014 aluminum alloy to cyclic loading was studied in the present work. The cycles to failure at different stress amplitude levels were compared with those obtained for the unstabilized alloy. The TEM observation of the modified material showed a very fine subgrain structure characterized by the presence of very fine Al3Zr within the grains, the particles play the role of stabilize the structure, inhibit the recrystallization and increase the strength of the alloy in all the tested conditions. Fracture observations were performed by using a scanning... 

There arc various standard methods for determining the tear energy (or the tear strength) and other crack growth parameters for rubbers under both static and cyclic-loading conditions. Some of these standards are di.scussed here. 

Broken fibers, a result of impact damage, cause a reduction in tensile strength and thus shorten the life of composite laminates. Under cyclic loading, fiber fractures begin within the first third of the expected life of cross-ply laminates [6], If carbon or boron fibers are used, these tend to be more brittle, and cracking may occur more easily during onset of impacts, which reduces the structural integrity of composites made with such materials. 

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