Fracture of polymer

Figore 15.9 Schematic drawings of (a) a craze showing microvoids and fibrillar bridges and (b) a craze followed by a crack. 

Principles of fracture mechanics developed in Section 8.5 also apply to brittle and quasi-brittle polymers the susceptibility of these materials to fracture when a crack is present may be expressed in terms of the plane strain fracture toughness. The magnitude of Kjc depends on characteristics of the polymer (molecular weight, percent crystallinity, etc.), as well as on temperature, strain rate, and the external environment. Representative values of for several polymers are given in Table 8.1 and Table B.5, Appendix B. 

Other mechanieal properties that are sometimes influential in the suitability of a polymer for some partieular appheation inelude tear resistance and hardness. The abOity to resist tearing is an important property of some plastics, especially those used for thin films in packaging. Tear strength, the meehanical parameter measured, is the energy required to tear apart a eut speeimen that has a standard geometry. The magnitude of tensile and tear strengths are related. 

As with metals, hardness represents a material s resistanee to seratehing, penetration, marring, and so on. Polymers are softer than metals and eeramies, and most hardness tests are condncted by penetration teehniques similar to those deseribed for metals in Section 6.10. Rockwell tests are frequently used for polymers. Other indentation techniques employed are the Durometer and Barcol.  

As with other engineering solids, the large-strain deformation of polymers is terminated by fracture. Since fracture defines the range of use of polymers in industrial practice, the understanding of its various forms and the factors governing it is of paramount technological interest. 

We start by demonstrating that polymers are intrinsically brittle solids and fracture in a brittle manner at low temperatures and/or high strain rates. However, in common with many other intrinsically brittle solids, polymers also exhibit a brittle-to-ductile transition in their fracture behavior at a certain temperature and strain rate, often well below room temperature. 

Finally, we consider in some detail a selection of prominent fracture mechanisms in polymers and finish with a table of the measured initial fracture-instability criteria. 

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An alternative energy approach to the fracture of polymers has also been developed on the basis of non-linear elasticity. This assumes that a material without any cracks will have a uniform strain energy density (strain energy per unit volume). Let this be IIq. When there is a crack in the material this strain energy density will reduce to zero over an area as shown shaded in Fig. 2.65. This area will be given by ka where )k is a proportionality constant. Thus the loss of elastic energy due to the presence of the crack is given by... 

McGarry, F.J. (1969). The fracture of polymers and fiber reinforced polymer composites. In Proc. AlAAj ASME lOlh Structures, Structural Dynamics Mater. Conf., New Orleans, pp. 456-471. 

Andrews, E. H. Fracture of Polymers American Elsevier New York 1968. 

Breach, C.D. Donald, A.M. Jones, R.A.L. In 9th International Conference on Deformation, Yield, and Fracture of Polymers (Conference Papers). The Institute of Materials London, 1994. 

Cook N, Dawson D, Thomas K. Deformation Yield and Fracture of Polymers, Cambridge, Proceedings, 1991, pi 7/1... 

Brule B, Monnerie L, Halary J-L (2003) In Blackman BRK, Pavan A, Williams JG (eds) Fracture of polymers, composites and adhesives II. Elsevier and ESIS... 

Halary J-L, Monnerie L (2003) In Proceedings of the 12th international conference on deformation, yield and fracture of polymers. IOM communications ed 25... 

EPR spectroscopy is also frequently used in polymer science. Applications involve the detection of free radicals in cross-linking, high-energy irradiation, photochemical degradation and oxidation, and mechanical fracture of polymer chains. 

Sue HJ, Earls JD, Hefner RE Jr, Proceedings Deformation Yield and Fracture of polymers, Cambridge, 1997, p.129-132. 

Beguelin P, Kausch H-H (1995)Techniques for high speed testing. In Williams JG, Pavan A (eds) Impact and dynamic fracture of polymers and composites. Mechanical Engineering Publications, London... 

The macroscopic properties such as mechanical behavior of block copolymers or polymer blends depend directly on the relative concentrations of different constituents and their meso-structures. How to predict the exact macroscopic properties of polymer blends or block copolymers with meso-phase separation structures from pure component properties remains a big challenge. Some theoretical efforts have been explored. For example, Buxton et al. found that the deformations and fractures of polymer blends can be described by the... 

Grein C, Beguelin P, Plummer CJG, Kausch HH, Teze L, Germain Y (2000) Influence of the morphology on the impact fracture behaviour of iPP/EPR blends. In Williams JG, Pavan A (eds) Fracture of Polymers, Composites and Adhesives. ESIS Pulication 27. Elsevier Science, Kidlington (Oxford, UK), p 319... 

Suresh S, Pruitt L (1991) Fatigue crack growth in polymers and organic composites under cyclic compressive loads. In Proceedings of the 8th International Conference on Deformation, Yield and Fracture Of Polymers, 32 1-4... 

Dubourg MC, Chateauminois A (2002) In Williams JG (ed) Fracture of polymers, composites and adhesives (ESIS TC4) (ESIS publication 32). Elsevier, Les Diablerets, Switzerland, p 51... 

It is well known that ESR is a sensitive eiqierimental technique to detect selectively free radicals having broken bonds or unpaired electrons. Therefore, one obtains direct experimental evidence for chemical bond scission, whenever an ESR spectrum was observed from a fractured polymer material. ESR studies on mechanical destruction give us a decisive answer to the question whether the macroscopic fracture of polymers results in breakage of main chains of the polymer molecules. And also ESR may provide interesting information which is helpful for crarelating phenomraiolopcal results with breaking of chemical bonds. 

Since crazing represents a cavitational form of plasticity, it is clear that crazes play an important role in the fracture of polymers. Crazing is, generally, involved when PC fails under plane strain conditions, e.g. in fracture mechanics tests on thick samples with sharp notches where high triaxial stresses are built... 

Barenblat, G-1. Methods ofthe Combustion Theory in the Mechanics of Deformation, Flow and Fracture of Polymers, in Deformation and Fracture of Polymers (ed. Kausch, H. H., Hassell, J. A., Jaffee, R. I.), New York—London, Plenum 1973, p. 91... 

Doll, W., Schinker, M. G., Konczol, L. in Deformation, Yield and Fracture of Polymers, p. 20.1, Cambridge/London, Plastics and Rubber Institute 1982... 

Attenburrow, G. E., Bassett, D. C., in Proc. 4th Int. Conf. on Deformation, Yield and Fracture of Polymers, Cambridge, U.K., April 1979, London, Plastics and Rubber Inst., paper 13.1... 

Miller P, Kramer EJ (1988) Proc. of 7th International Conference on Yield. Deformation and Fracture of Polymers, The Plastics and Rubber Institute, London, p 81.1... 

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