Crack propagation poly

The isothermal/adiabatic transition argument was originally applied to poly(me-thyl methacrylate) in which crack propagation was observed to become unstable above a certain fast crack velocity . This instability was attributed to the transition of the crack tip deformation from an isothermal process to an adiabatic one, resulting in substantial softening at the crack tip. The applicability of this process to epoxies is questionable based on the observation that it is slow, not fast, rates at which the epoxy stable to unstable transition occurs. 

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). 

Hristov [123] studied the impact fracture behavior of WPG modified by MAPP as compatibilizer and poly (butadiene styrene) rubber as impact modifier using Gharpy impact testing. They demonstrated that the crack propagation energy compared to the crack initiation energy is much more influenced by the morphology. 

Stress-relief additives additives Up to 25 Inhibit crack propagation Reduce crack initiation Reduce CTE Silicones, acrylonitrile-butadiene elastomers, poly(butyl acrylate)... 

R. W. Lang, J. A. Manson, and R. W. Hertzberg, Craze Development in Poly(Methyl-Methacrylate) during Stable Fatigue Crack Propagation , Polym. Eng. Sci. 24, 833-842(1984). 

MPa.m. The bigger its value then the more resistant is the material to crack propagation. The value of for poly(methyl methacrylate) (PMMA) is about 1 MPa.m while brittle thermosets, for example some epoxies, may have values of below 0.5 MPa.m. Polycarbonate has a value of of about 2 MPa.m. ... 

The fracture toughness of poly(vinyl chloride), with a low degree of crystallinity, cannot be readily explained by simple fracture mechanic theories, since there is a marked yielded zone, as well as craze-crack propagation involved in all fractures. 

Figure 13.11 Paris diagrams for fatigue crack propagation of poly(dimethyl-siloxane)-modified epoxy networks. (Rey et al., 1999 with kind permission from Kluwer Academic Publisher.)...

An example of a study conducted using a tensile stage in the SEM is the evaluation of the ductile failure of poly(vinyl chloride). Smith et al. [316] stamped dumbbell shaped pieces of polymer from 1 mm thick sheets and extended them to a neck in an Instron tester. The prestrained pieces were then strained in the SEM. Low accelerating voltage was used for imaging of the uncoated specimens. These experiments showed that, after neck formation, fracture occurs by crack propagation from a flaw or cavity within the surface craze. 

Plastics with stress-strain curves of the type in Figure 18.1a are rigid and brittle. The former term refers to the high initial modulus. The latter refers to the area under the stress-strain curve, which represents the energy per unit volume required to cause failure. These materials usually fail by catastrophic crack propagation at strains in the order of 2%. Since hardness correlates well with tensile modulus, which is another valuable property of this type of plastic. Examples of this class are polystyrene, poly(methyl methacrylate), and most thermosets. 

Figure 7.17 Master relationship for adhesive fracture energy, Gc, versus effective rate, aax, of crack propagation. The points for each curve represent data from various test rates and temperatures [16] O, Gc for cohesive fracture of crosslinked SBR , Gc for fracture of crosslinked SBR/etched fluorinated ethylene-propylene copolymer joint , Gc for fracture of crosslinked SBR/treated fluorinated ethylene-propylene copolymer joint T, Gc for fracture of crosslinked SBR/poly(ethylene terephthalate) joint. 

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... 

The mechanisms of crack propagation in poly(methyl methacrylate) eure particularly amenable to analysis. The situation is not so simple for other polymers. For example, in polystyrene there is usually multiple crazing in the vicinity of the crack tip. In tougher polymers such as polycarbonate shear yielding as well as crazing often takes place at the crack tip. In these cases crack propagation does not occur in such a well-controlled manner as in poly (methyl methacrylate) and it is more difficult to analyse. 

Toughened or rubber modified polystyrene blends are prepared by incorporating up to 10 percent by weight of poly butadiene or styrene-butadiene copolymer rubbers into the resin. The dispersed rubber particles decrease the plastic s brittleness by interrupting the crack propagation process. Unlike the unblended polymer, rubber modified polystyrene is translucent. The volume of rubber modified polystyrene made is roughly the same as that of general purpose polystyrene. 

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