Fracture creep

So far we have concentrated on mechanical properties at room temperature. Many structures - particularly those associated with energy conversion, like turbines, reactors, steam and chemical plant - operate at much higher temperatures. 

At room temperature, most metals and ceramics deform in a way which depends on stress but which, for practical purposes, is independent of time  

As the temperature is raised, loads which give no permanent deformation at room temperature cause materials to creep. Creep is slow, continuous deformation with time  

The point, then, is that the temperature at which materials start to creep depends on their melting point. As a general rule, it is found that creep starts when 

Polymers, too, creep - many of them do so at room temperature. As we said in Chapter 5, most common polymers are not crystalline, and have no well-defined melting point. For them, the important temperature is the glass temperature, Tq, at which the Van der Waals bonds solidify. Above this temperature, the polymer is in a leathery or rubbery state, and creeps rapidly under load. Below, it becomes hard (and 

Cavern-type pores form by diffusion processes in which the material increases its length in the direction of the tensile stress by moving atoms from 

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It is not surprising - since creep causes creep fracture - that the time-to-failure, tf, is described by a constitutive equation which looks very like that for creep itself ... 

In Chapter 17 we showed that, when a material is loaded at a high temperature, it creeps, that is, it deforms, continuously and permanently, at a stress that is less than the stress that would cause any permanent deformation at room temperature. In order to understand how we can make engineering materials more resistant to creep deformation and creep fracture, we must first look at how creep and creep-fracture take place on an atomic level, i.e. we must identify and understand the mechanisms by which they take place. 

There are two mechanisms of creep dislocation creep (which gives power-law behaviour) and diffusiona creep (which gives linear-viscous creep). The rate of both is usually limited by diffusion, so both follow Arrhenius s Law. Creep fracture, too, depends on diffusion. Diffusion becomes appreciable at about 0.37 - that is why materials start to creep above this temperature. 

The strength of a fibre is not only a function of the test length, but also of the testing time and the temperature. It is shown that the introduction of a fracture criterion, which states that the total shear deformation in a creep experiment is bounded to a maximum value, explains the well-known Coleman relation as well as the relation between creep fracture stress and creep fracture strain. Moreover, it explains why highly oriented fibres have a longer lifetime than less oriented fibres of the same polymer, assuming that all other parameters stay the same. 

R. L. Tsai and R. Raj, Creep Fracture in Ceramics Containing Small Amounts of a Liquid Phase, Acta Metall., 30,1043-1058 (1982). 

H. E. Evans, Mechanisms of Creep Fracture, Elsevier Applied Science Publishers, London, U.K., 1984. 

Microcracks are also known to occur in the elevated temperature creep of alumina with little or no preexisting glass phase. For example, the development of microcracks during creep fracture of two hot-pressed aluminas, which were free of grain boundary glass films, was studied by Wilkinson et al.52 who employed tensile and four-point bend specimens. They found that the concentrations and morphology of the cavities and microcracks were strongly... 

Fig. 10.12 Curves showing the creep crack growth rate as a function of crack tip fracture process zone size for different values of crack length. The abscissa is plotted as a percentage of frontal creep fracture process zone size to the crack length. See text for discussion.

Gittus, J. Creep, Viscoelasticity, and Creep Fracture in Solids Wiley New York, 1975. 

The theoretically estimated and experimentally determined CGRs agree well over a considerable temperature range after calibration at one temperature and specification of an appropriate activation energy for the crack-tip strain rate (Fig. 10). The numerical solution employed in the version of the CEFM described here yields very reasonable results for the environmentally assisted and creep fracture of sensitized... 

The need for fundamental and applied research on lifetime under service conditions is great. As a result, the subjects of dielectric breakdown, fatigue, wear, creep, fracture, oxidation, photodegradation, additive migration, etc., are increasingly important to the eflFective use of synthetic polymers. The development of a scientific basis for tests that predict accurately the useful service life is a primary need. 

Figure 7. The creep fracture surfaces after testing at 473 K (a) the AZ 91 composite (stress 70 MPa), (b) the QE 22 composite (stress 120 MPa), SEM.

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