Failure mechanisms in polymers

Before entering into mathematical descriptions of creep yielding and rupture, it is instructive to describe several of the physical deformation 

The behaviour of linear polymers depends largely on the temperature and the stress state e.g. the response in tension may differ markedly from that in compression. 

In Chaps. 13 and 15 we have already discussed several mechanisms of failure, so that we may confine ourselves to a short summary. This will be based on a very clear survey by Bin Ahmad and Ashby (1988). 

By increase of the temperature the mode of failure is changed from brittle fracture to ductile fracture, characterised by the appearance of a yield point prior to fracture, and sometimes by indications of necking. 

A further temperature rise leads to necking, with the possibility of cold drawing the latter phenomenon is dependent on the stability of the neck, and is governed by the level of adiabatic heating and strain hardening. In this case the extensions can be very large. 

Finally, at a still higher temperature the polymer starts to deform homogeneously by viscous flow. For amorphous polymers the stress levels are very low in this case. 

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FIG. 25.1 Failure mechanisms in polymers. From Bin Ahmad and Ashby (1988) Courtesy of Chapman and Hall. 

Failure mechanisms in polymer matrix composites Criteria, testing and industrial applications (ISBN 978-1-84569-750-1)... 

Friedrich, K. and Karger-Kocsis, J. (1989). Unfilled and short fiber reinforced semi-crystalline thermoplastics. In Fractography and Failure Mechanisms of Polymers and Composites, (A.C. Roulin-Moloney ed.), Elsevier Appl. Science, London, pp. 437-494. 

Fagan, A. F., Bell, J. M., and Briggs, G. A. D. (1989). Acoustic microscopy of polymers and polymer composites. In Fractography and failure mechanisms of polymers and composites (ed. A. C. Roulin-Moloney), pp. 213-30. Elsevier Applied Science, London. [204]... 

The 20th Europhysics Conference on Macromola ular Physics in Lausanne (Sept. 88), devoted to Physical Mechanisms in Polymers Failure offered the chance of bringing together practically all of the authors of this volume with each other, with other leading experts, and with an interested audience this volume might therefore also be considered as partial Proc ings of that Conference. 

A final area of direct practical relevance for AFM on polymers is the analysis of defects and failure mechanisms. In fact this application is natural owing to the direct visualization of surface structures and morphologies of polymers and polymer products and therefore it is omnipresent. 

Bucknall, 1977]. The impact modified materials are usually the blends of a rigid matrix polymer with an elastomer. The composition of the constituents, their miscibility and the morphology influence the deformation and failure mechanism in the blend. Particle size of the elastomer, its dispersion and its adhesion with matrix are also the important factors determining the toughness. 

Qayyum MM, White JR. Effect of stabilizers on failure mechanisms in weathered polypropylene. Polym Degrad Stab 1993 41 163-172. 

A.M. Donald, Failure mechanisms in polymtaic materials, in Rubber Toughened Engineering Plastics, ed. by A.A. Collyer (Chapmann and Hall, London, 1994), pp. 1-28 A.M. Donald, Crazing, in The Physics of Glassy Polymers, ed. by R.N. Haward, R.J. Young, 2nd edn. (Chapman and Hall, London, 1997), pp. 295-341... 

It is significantly accelerated by the presence of hydrolyzable ionic contaminants (for example, hahdes and acids from flux residues or extracted from polymers). Delaminations or voids that promote the accumulation of moisture or contaminants can promote dendritic growth. Conductive anodic filament growth (discussed later) is a special case of dendritic growth. Time to failure is inversely proportional to spacing squared and voltage. The failure mechanisms in accelerated tests have been reviewed. ... 

Pomies F, Carlsson LA, Gillespie JW Jr (1995) Marine environmental effects on polymer matrix composites. In Martin RH (ed) Composite materials fatigue and fracture, vol 5, ASTM STP 1230. American Society for Testing and Materials, Philadelphia, pp 28-303 Price JN (1989) Stress corrosion cracking in glass reinforced composites. In Roulin-Moloney AC (ed) Fractography and failure mechanisms of polymers and composites. Elsevier Applied Science, New York, pp 495-531... 

Minor morphological differences, as between MR and NATSYN show up by their thermomechanical responses. Work on polymer chain conformations in bulk, 1968-to date, especially on fluorinated acrylics with B. McGarvay and W. Lee, on elastomeric dienes with H. Mark and K. Sato, and of entanglements in GRS witn R. Meyers, followed. These experiences were reviewed, with emphasis on the structural factors in rubber-like behavior at all phases of extension, in a chapter coauthored by T. L. Smith, on the Rupture of Elastomers in Liebowitz s treatise "Fracture , while later analyses focused on the effect of fillers in elastomers, the role of interfaces in composites, failure mechanisms in plastics as a function of morphology, and on resulting lessons for polymer engineering. 

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