Time-dependent properties fatigue

An important aspect of the mechanical properties of fibers concerns their response to time dependent deformations. Fibers are frequently subjected to conditions of loading and unloading at various frequencies and strains, and it is important to know their response to these dynamic conditions. In this connection the fatigue properties of textile fibers are of particular importance, and have been studied extensively in cycHc tension (23). The results have been interpreted in terms of molecular processes. The mechanical and other properties of fibers have been reviewed extensively (20,24—27). 

In this book no prior knowledge of plastics is assumed. Chapter 1 provides a brief introduction to the structure of plastics and it provides an insight to the way in which their unique structure affects their performance. There is a resume of the main types of plastics which are available. Chapter 2 deals with the mechanical properties of unreinforced and reinforced plastics under the general heading of deformation. The time dependent behaviour of the materials is introduced and simple design procedures are illustrated. Chapter 3 continues the discussion on properties but concentrates on fracture as caused by creep, fatigue and impact. The concepts of fracture mechanics are also introduced for reinforced and unreinforced plastics. 

Fatigue is an example of the influence of time on the mechanical properties of a material. Another example of a time-dependent mechanical property is creep. Creep, sometimes called viscoplasticity, is defined as time-dependent deformation nnder constant stress, usually at elevated temperatures. Elevated temperatures are necessary because creep is typically important only above Tmp % where T p is the absolute melting point of the material. 

The result of most fatigue testing is the generation of S-N curves. These curves plot stress S against the number of cycles to failure N. The data are obtained by testing many specimens to failure at different stress levels. As with any type of time-dependent strength property, the life to failure increases as the stress is decreased. Also, an endurance limit is reached, i.e., a maximum stress above which the material will not withstand cyclic stress indefinitely. For experimental convenience and time required, fatigue tests are usually discontinued if failure has not occurred by 10 million cycles. 

Of course, environmental factors as well as fatigue do influence mechanical properties as a function of time but this degradation of properties due to accumulated damage is quite separate from the inherent time dependence of viscoelasticity considered here. 

A.M. Ermi, J. Moteff, Correlation of substructure with time-dependent fatigue properties of AlSl 304 stainless steel, Metall. Trans. A 13A (1982) 1577-1588. 

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