Creep phenomenology

By 1969, when a major survey (Thompson 1969) was published, the behaviour of point defeets and also of dislocations in crystals subject to collisions with neutrons and to the eonsequential collision cascades had become a major field of researeh. Another decade later, the subjeet had developed a good deal further and a highly quantitative body of theory, as well as of phenomenological knowledge, had been assembled. Gittus (1978) published an all-embracing text that eovered a number of new topics chapter headings include Bubbles , Voids and Irradi-ation(-enhanced) Creep . 

We tried to compare the phenomenological equations (81) and (82) with the experimental results obtained in studies on the slow (creeping) flow of highly-concentrated disperse systems containing a polyfractional filler [98]. 

Researchers have examined the creep and creep recovery of textile fibers extensively (13-21). For example, Hunt and Darlington (16, 17) studied the effects of temperature, humidity, and previous thermal history on the creep properties of Nylon 6,6. They were able to explain the shift in creep curves with changes in temperature and humidity. Lead-erman (19) studied the time dependence of creep at different temperatures and humidities. Shifts in creep curves due to changes in temperature and humidity were explained with simple equations and convenient shift factors. Morton and Hearle (21) also examined the dependence of fiber creep on temperature and humidity. Meredith (20) studied many mechanical properties, including creep of several generic fiber types. Phenomenological theory of linear viscoelasticity of semicrystalline polymers has been tested with creep measurements performed on textile fibers (18). From these works one can readily appreciate that creep behavior is affected by many factors on both practical and theoretical levels. 

The secondary transition has long been recognized as controlling creep. Craze fibril growth is some kind of creep phenomenon, and therefore the influence of the secondary peak seems quite reasonable. Conclusion The phenomenology of the craze at a propagating crack-tip changes fundamentally for a particular temperature which seems to be the secondary relaxation p peak temperature. 

Time constants are related to the relaxation times and can be found in equations based on mechanical models (phenomenological approaches), in constitutive equations (empirical or semiempirical) for viscoelastic fluids that are based on either molecular theories or continuum mechanics. Equations based on mechanical models are covered in later sections, particularly in the treatment of creep-compliance studies while the Bird-Leider relationship is an example of an empirical relationship for viscoelastic fluids. 

A second fundamental theme that will be taken up in this chapter in which there is an interplay between the various types of defects introduced earlier is that of mass transport assisted deformation. Our discussion will build on the analysis of diffusion at extended defects. We will begin with an examination of the phenomenology of creep. 

This situation becomes simplified even further if the creep behavior of the nonreacting network obeys the well known phenomenological equation (9-12) suggested by Thirion... 

Results of these experiments are shown in Figures 2.48-2.50. The obtained experimental data creep in RubCon samples were analyzed with the phenomenological theory of structural diagrams (Figure 2.51a-c). 

Extending on the concepts of Krafft [4,5], and Landes and Wei [2], an analytical model was proposed by Yin et al. [3] to explore the crack growth response over a broader range of K levels. In this model, the phenomenological model of creep proposed by Hart [6] is used. 

In summary, it is concluded that the creep behavior of intermetallic phases can be described as that of the conventional disordered alloys given by the familiar phenomenological constitutive equations. 

Bae Beake, B. Modelling indentation creep of polymers a phenomenological approach. J. Phys. D 39 (2006) 4478 85. 

The theory of linear viscoelasticity is phenomenological there is no attempt to discover the time and frequencty response of the solid in an altogether a priori fashion. The aim is to predict behaviour under certain circumstances, having observed it under others for example, to correlate creep, stress relaxation, and (fynamic properties so that if one of these has been determined then all the others are known. This is closety related to electrical network theory, both in aim and, as will soon be apparent, in method. 

The reader will note that the definition of a phenomenological theory given at the start of this section has been satisfied by the Zener model. Measurements in creep, stress relaxation, or dynamic response are interrelated by three of the model parameters, say /r, and r. ... 

The creep behavior is often analyzed in terms of the phenomenological equation... 

Figure 13.16 Phenomenological model of the cavitation processes during tensile creep deformation in silicon nitride [15, 27].

A phenomenological model has been proposed for the non-linear viscoelastic behaviour of thermorheologjcally complex polymer glasses prior to and including yield. The approach was based upon stress additivity. A linear viscoelastic material will exhibit stress-strain additivity. The molecular processes modelled were resolved into two parallel processes, each with a characteristic relaxation time spectrum. The model described the yield behaviour and creep experiments at increasing stress. " ... 

In this chapter we describe the common forms of viscoelastic behaviour and discuss the phenomena in terms of the deformation characteristics of elastic solids and viscous fluids. The discussion is confined to linear viscoelasticity, for which the Boltzmann superposition principle enables the response to multistep loading processes to be determined from simpler creep and relaxation experiments. Phenomenological mechanical models are considered and used to derive retardation and relaxation spectra, which describe the time-scale of the response to an applied deformation. Finally we show that in alternating strain experiments the presence of the viscous component leads to a phase difference between stress and strain. 

The models discussed here, which are phenomenological and have no direct relation with chemical composition or molecular structure, in principle enable the response to a complicated loading pattern to be deduced from a single creep (or stress-relaxation) plot extending over a long time interval. Interpretation depends on the assumption in linear viscoelasticity that the total deformation can be considered as the sum of independent elastic (Hookean) and viscous (Newtonian) components. In essence, the simple behaviour is modelled by a set of either integral or differential equations, which are then applicable in other situations. 

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