Viscoelastic fracture mechanics

There are several important things to note. The first is that elastic deformation is a reversible process, but plastic deformation and brittle fracture are not. More importantly, plastic deformation and viscoelastic behavior are kinetic phenomena time is important, and they can be affected by press speed. In reality, most materials exhibit both plastic and brittle behavior, but specific materials can be classified as primarily plastic or primarily brittle. For example, microcrystalUne cellulose defonns primarily by a plastic deformation mechanism calcium phosphate de-fonns primarily by a brittle fracture mechanism lactose is in the middle [8]. 

During the extrusion of polymers different defects and flow instabilities occur at very low Reynolds numbers. The commonly known ones are sharkskin, melt fracture, slip at the wall and cork flow. These defects are of commercial importance, since they often limit the production rate in polymer processing. Many researchers have been interested in the subject, and thorough reviews on flow stability and melt fracture have been written in the last 30 years [1-4]. More recently, two review papers deahng with viscoelastic fluid mechanics and flow stability, were published by Denn [5] and Larson [6]. However, although much work has been done in the field of extrusion distortions, controversy still exists regarding the site of initiation and physical mechanisms of the instabilities. 

Analysis of Failure Failure of "Flawless" Materials Fracture Mechanics Griffith Theory Stress Intensity Factors Fracture Energy Viscoelastic Effects Examples Fatigue Conclusion... 

More rigorous attempts to include viscoelastic effects into fracture mechanics predictions have recently been made, for example. 

As stated earlier, this chapter has concentrated on the elastic and viscoelastic properties of compact cortical bone and the elastic properties of trabecular bone. At present there is considerable research activity on the fracture properties of the bone. Professor William Bonheld and his associates at Queen Mary and Westfield College, University of London and Professor Dwight Davy and his colleagues at Case Western Reserve University are among those who publish regularly in this area. Review of the literature is necessary in order to become acquainted with the state of bone fracture mechanics. 

Fejes-Kozma, Zs. Karger-Kocsis, J. (1994). Fracture Mechanical Characterization of a Glass Fiber Mat-reinforced Polypropylene by Instrumented Impact Bending. Journal of Reinforced Plastics and Composites, Vol.l3, No.9, pp. 822-834 ISSN 0731-6844 Ferry, J. D. (1980). Viscoelastic Properties of Polymers, 3rd Edition, Wiley Press, ISBN 978-0471048947, New York... 

The deformation characteristic of more ductile polymer materials at ambient temperatures like most thermoplastics or all elastomers is highly non-linear, e.g., either mostly viscoelastic or entropy-elastic or a combination of both. Compared to concepts of LEFM relatively rarely used for polymer materials different concepts of non-linear elastic firacture mechanics such as elastic-plastic fracture mechanics (EPFM) or post-yield fracture mechanics (PYFM) are somewhat widely applied, therefore. One of the most important concepts of EPFM is the J integral concept. Notwithstanding the J integral is primarily defined to be valid... 

Klti Kliippel, M. Evaluation of viscoelastic master curves of filled elastomers and applications to fracture mechanics. J. Phys. Condens. Matter 21 (2009) 035104 (10 pages). 

In adherence of solids, surface effects, rheological effects and fracture mechanics are mixed in an intricate manner, and no experiment can be performed involving only the chemical aspects. The adherence of elastic solids is now clear that of viscoelastics is in progress for other materials, the constitutive equation must be known and taken into account. 

Recall that the whole theory of Griffith has been developed for elastic bodies—what applies to metals within a certain range of imposed stresses. Thus, Eqs. (24.17)-(24.23) form the essence of linear elastic fracture mechanics (LEFM). In Eq. (24.24) a plastic termr p has been added to the elastic term F metals exhibit also plasticity, hence the improvement displayed in Eq. (24.24). If we make a further step and assume that F includes all nonelastic contributions, we shall have an equation usable also for viscoelastic materials. We, therefore, have to use Eq. (24.24) instead of (24.17) while in Eqs. (24.18)-(24.23) we need to put F + F instead... 

For a rubber adhesive, Gent found that he could use the WLF equation to obtain a master curve of breaking stress (see Viscoelasticity - time-temperature superposition). He used a Fracture mechanics analysis to link the breaking stress to critical strain energy density. 

In Table II, the ultimate stresses of materials calculated from their Young s modulus are compared with the measured values of F ax- It is apparent that the measured values for the failure stress are far below the theoretical values. The situation is worse than it looks, however, because the measured values are partially due to viscoelastic dissipation of the applied stress, while the calculated values assume no such viscoelastic dissipation. Needless to say, this indicates that the theoretical values are far in excess of the measured values. An explanation of these discrepancies is presented in the next section, which gives a short introduction to fracture mechanics. 

There are numerous examples of the application of fracture mechanics to structural adhesive systems. Most notable are those of Mostovoy and his coworkers which have already been mentioned. " Bascom and coworkers have made significant contributions to the understanding of the effect of bondline thickness on fracture toughness. Kinloch and Shaw extend the work of Bascom to include rate effects and to develop mathematical models of the fracture resistance of adhesives. Hunston et al have used these methods to study viscoelastic behavior in the fracture process of structural adhesives.Mostovoy and Ripling used these techniques to determine the flaw tolerance of several adhesives,while Bascom and Cottington have studied the effect of flaws caused by air entrapment in structural adhesives." Finally it must be mentioned that one of the most simple, most widely used tests for strucural adhesives, the peel test, is actually a version of the double cantilever beam test. 

Linear elastic fracture mechanics (including the Irwin model of confined plasticity, the line-zone or Dugdale model and viscoelastic effects)... 

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