Tensile deformations, repeated

As demonstrated in Fig. 11 for material B, most of the tensile deformation was reversible, even at strains of over 40%. In Fig. 11 for each cycle a new sample was employed. In cases where the same specimen had been subjected to repeated load cycling, the authors observed a substantial amount of strain hardening. 

The strain rates and temperatures prevailing in abrasion are very different from those used in routine laboratory testing of tensile or tear strength. Because of friction, local temperatures may far exceed those of the test track or of the bulk of the rubber (229-230). Even at small sliding velocities the effective strain rate is very large, as small volume elements of rubber are deformed repeatedly to high strains by the many surface... 

An important aspect of the tensile deformation of fibers of semicrystalline and crystalline polymers is the recovery after unloading. When the first extension of an oriented fiber reaches well into the second stage of the tensile curve, i.e. beyond the yield point, then the recovery is not complete. The permanent extension or set is approximately equal to the extension at which yielding occurs. The recoverable extension shows a spontaneous and latent recovery corresponding to elastic and viscoelastic or delayed elastic contributions. Further repeated extension of the fiber up to the same maximum extension hardly increases the permanent deformation but is still accompanied by a little hysteresis, as shown in Fig. 6.19. 

Machine components ate commonly subjected to loads, and hence stresses, which vary over time. The response of materials to such loading is usually examined by a fatigue test. The cylinder, loaded elastically to a level below that for plastic deformation, is rotated. Thus the axial stress at all locations on the surface alternates between a maximum tensile value and a maximum compressive value. The cylinder is rotated until fracture occurs, or until a large number of cycles is attained, eg, lO. The test is then repeated at a different maximum stress level. The results ate presented as a plot of maximum stress, C, versus number of cycles to fracture. For many steels, there is a maximum stress level below which fracture does not occur called the... 

Particularly in automotive applications, the design engineer selects materials that will endure periodic fluctuations of both tensile and flexural type loads for the service lifetime of the molded part. Under-the-hood automotive conditions include extremes in ambient temperature combined with possible localized vibrations from the engine and rotation of molded parts (cooling fans) under continuous load. Although deformation will occur due to creep, there is concern regarding fatigue type failure after repeated fluctuations of the applied load. 

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