Brittle polymers crack propagation

The slow growth of cracks in poly(methyl methacrylate) is an ideal application of linear elastic fracture mechanics to the failure of brittle polymers. Cracks grow in a very well-controlled manner when stable test pieces such as the double-torsion specimen are used. In this case the crack will grow steadily at a constant speed if the ends of the specimen are displaced at a constant rate. The values of Kc or % at which a crack propagates depends upon both the crack velocity and the temperature of testing, another result of the rate- and temperature-dependence of the mechanical properties of polymers. This behaviour is demonstrated clearly... 

When a crack propagates in polystyrene at low crack velocities, the craze ruptures close to its median plane by a mechanism having the approximate characteristics of viscous flow. Each fracture surface is then covered by a thin layer of craze. At higher crack velocities, however, failure occurs along the boundaries between the craze and the adjacent bulk polymer by practically brittle fracture (I). The change in fracture... 

It is worth noting that this semi-ductile behavior has been found in other polymers Newmann and Williams [4] observed stable crack propagation before brittle fracture in ABS over the temperature range from —40 to 0°C Bernal and Frontini also observed this type of behavior in a rubber-modified thermoplastic at room temperature [12]. 

Several cautions are, however, in order. Polymers are notorious for their time dependent behavior. Slow but persistent relaxation processes can result in glass transition type behavior (under stress) at temperatures well below the commonly quoted dilatometric or DTA glass transition temperature. Under such a condition the polymer is ductile, not brittle. Thus, the question of a brittle-ductile transition arises, a subject which this writer has discussed on occasion. It is then necessary to compare the propensity of a sample to fail by brittle crack propagation versus its tendency to fail (in service) by excessive creep. The use of linear elastic fracture mechanics addresses the first failure mode and not the second. If the brittle-ductile transition is kinetic in origin then at some stress a time always exists at which large strains will develop, provided that brittle failure does not intervene. 

This paper addresses three topics of current scientific and technological interest poly(vinyl chloride) (PVC) toughening of brittle or ductile but notch-sensitive polymers by the inclusion of rubbery phases and the kinetics of fatigue crack propagation (FCP). 

The achievement of enhanced toughness in a brittle polymer such as an epoxy resin is vital in applications such as adhesives, for which peel strength and resistance to interfacial debonding through crack propagation are important criteria for performance. This may be achieved by incorporation of elastomeric or other phase-separated particles into the network. These may be present before reaction or they may form during the process of... 

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