Fracture mechanics viscoelastic effects

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. 

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. 

Loos and co-workers [64] studied the effect of CNT on the mechanical and viscoelastic properties of epoxy matrices. Bisphenol A based epoxy resin nanocomposites were prepared with various small proportions of single-walled carbon nanotubes (SWCNT) and then investigated using acetone as a diluent to reduce the resin viscosity, and the products after removal of the solvent were characterised by FT-IR, Raman spectroscopy, thermogravimetric analysis (TGA), DSC, DMA, tensile, compression, flexural and impact testing, and SEM of the fracture surfaces. The effects of small amounts of SWCNT on mechanical and viscoelastic properties of the nanocomposites are discussed in terms of structural changes in the epoxy matrix. 

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)... 

Only recently, work involving the time-dependent fracture characteristics of adhesively bonded joints has been under way. Francis et discussed the effects of a viscoelastic adhesive layer, geometry, mixed-mode fracture response, mechanical load history, environmental history, and processing variations on the fracture processes of adhesively bonded joints. However, their finite-element analysis includes only linear elastic fracture mechanics. 

Thus the characteristic times at which viscoelastic relaxation will influence deformation in the bulk (to) and at the edge of contact (r are very different. Consequently we may treat the two effects independently, in the same way that crack tip stresses and bulk stresses are separated in fracture mechanics. We will consider bulk deformation first and ignore the presence of adhesion. 

Barquins and Maugis (6) have approached this problem from the view point of fracture mechanics for the special case that viscoelastic losses are localized at the crack tip. They completely ignore effects of creep. Viscoelastic response is assumed to be proportional to the thermodynamic work of adhesion in analogy with the results of peel experiments. Specifically, F -w=w0(flj.v) where T is the strain release rate and (t> ajV) is assumed to be a universal function of crack speed V for a viscoelastic material and Ct is the WLF shift factor (). For macroscopic glass-pol3mrethane contacts (a = 50-250 pm), Maugis and Barquins showed experimentally that with n = 0.6 for 10 m/s < V < 10 m/s and a T) . Most SFM experiments are at V much smaller... 

Equations (9.14) to (9.18) do not constitute a fracture criterion since they do not specify whether and when material separation in the plastically deformed zone occurs. In order to use fracture mechanics to predict the stability of an elastic-plastic crack, limits of the plastic deformation have to exist and must be known explicitly or implicitly. Understandably the determination, interpretation, and application of critical stress intensity factors for viscoelastic solids is particularly complicated due to the pronounced time and temperature dependence of the mechanical properties of these materials. The fact that a material shows a ductile behavior does not preclude the use of fracture mechanics but it reduces — or even eliminates — the independence of the fracture mechanics functions (G, K, R) from the geometrical parameters [6]. The occurrence of plastic deformation in a material has a threefold effect ... 

The third part of the book deals with the properties and applications of polymers. It starts with a discussion of polymer solution properties through the mechanical properties of polymers and concludes with an overview of the various applications of polymer materials solids. The viscoelastic nature of polymers is also treated. This section also includes a discussion of polymer fracture. The effects of various molecular and environmental factors on mechanical properties are examined. 

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