Brittle fracture of polymers

Elastic-plastic fiacture mechanics has been extensively used in studies on plastics, but to data mainly tty research laboratories. The plastics industry continues to rely on impact testing, and in particular the notched Izod test, as the principal method for assessing toughness. One of the most important contributions of fracture mechanics to potymer engineering has been to provide a theoretical basis for understanding these impact test data. 

17 Relationship between crack length and type ol failuie in a bar of polymer of width W= 10 mm. The brittle fracture cun is calculated for a of 2.0 MPa m. Straight line shows ductile stress for o -40 MPa. Note that brittle fracture occurs before the material is able to yield, except at short crack lengths or very long crack lengths. 

It is clear that ductile failure occurs both at very short and at very long crack lengths, although the yield zone will be limited in extent in the latter case. The bar fails by brittle fracture at intermediate crack lengths. In this geometry, Ki increases with a, so there is no possibility that the crack will stop as a result of the tip becoming unloaded as it propagates fracture is catastrophic. 

A particularly awkward feature of fracture in polymers, as in some other materials, is that under certain conditions small cracks grow very slowfy under sub-critical stresses. As a crack grows Ki increases. Finally, Kj may 

18 Relationship between and crack speed in PMMA. Data from many different authors (after W. Ooell). 

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As indicated in section 8.1, fracture can take place in essentially two ways, either following macroscopic yield, when the fracture is said to be ductile fracture, or without macroscopic yield, when it is called brittle fracture. The present section is concerned with the brittle fracture of polymers. Experimental studies of brittle fracture can be divided broadly into two types, those that are undertaken with a view to understanding the details of what happens during fracture and those that are aimed at providing engineering data about a polymer. Experiments of the former type are often designed to test the predictions of a theoretical model, whereas experiments... 

Micro plastic zones occur even in the brittle fracture of polymers in front of the crack tip. Crazes are localized bands of plastically deformed polymer material, which always appear perpendicular to the stretching direction. They are constituted hy polymer fibrils of about 5 -15 nm diameter, which are stretched in the loading direction and separated by elongated voids with diameters up to about 50 nm. The craze-bulk interface is relatively sharp and only about 10 nm thick. Crazing is connected with volume increase of the material. In Part II, Figs. 1.4 and 1.5 and those figures that follow show typical examples of crazes in PS. Crazes in other polymers can also possess a coarser internal structure. 

Unfortunately, various groups who study brittle fracture of polymers throughout the world choose to express their results in different ways. A related parameter used by some groups to describe the dependence of stress at fracture to crack size is the critical stress intensity factor... 

Styrene polymers brittle fracture of, 23 363 burning of, 23 403 extrusion of, 23 398 glass-reinforced, 23 311 tensile strengths of, 23 359 Styrene product, factors in the quality of, 23 338-339 Styrene vapors, 23 403 Styrenic block copolymers, 24 102, 703-704... 

No detectable signals can be obtained in the brittle fracture of isotropic polymers (even crystalline ones), but once plastic flow is induced (eg by small amounts of orientation above Tg before testing below Tg), radicals are readQy obtained during deformation. 

Applications of linear elastic fracture mechanics (primarily) to the brittle fracture of solid polymers is discussed by Professor Williams. For those not versed in the theory of fracture mechanics, this paper should serve as an excellent introduction to the subject. The basic theory is developed and several variants are then introduced to deal with weak time dependence in solid polymers. Previously unpublished calculations on failure times and craze growth are presented. Within the framework of brittle fracture mechanics and testing this paper provides for a systematic approach to the faOure of engineering plastics. 

LeGrand, D.G. Crazing, yielding, and fracture of polymers 1. Ductile-brittle transition in polycarbonate. J. Appl. Polym. Sci. 1969, 13, 2129-2147. 

Figure 11.6. Schematic illustrations of brittle fracture, (a) Idealized limiting case of perfectly uniaxially oriented polymer chains (horizontal lines), with a fracture surface (thick vertical line) resulting from the scission of the chain backbone bonds crossing these chains and perpendicular to them. This limit is approached, but not reached, in fracture transverse to the direction of orientation of highly oriented fibers, (b) Isotropic amorphous polymer with a typical random coil type of chain structure. Much fewer bonds cross the fracture surface (thick vertical line), and therefore much fewer bonds have to break, than for the brittle fracture of a polymer whose chains are perfectly aligned and perpendicular to the fracture surface, (c) Illustration of a defect, such as a tiny dust particle (shown as a filled circle), incorporated into the specimen during fabrication, which can act as a stress concentrator facilitating brittle fracture.

The most common trends which determine the preferred mode of failure of a polymer are depicted schematically in Figure 11.9, which shows the stresses required for brittle fracture (of),... 

One reason for such a wide use of impact tests is the fact that materials ductile at slow strain rates exhibit brittle fracture under conditions of impact loading. The brittle nature of polymer fracture during an impact event, combined with the above notions of the dynamic effects during such an event, lead naturally to an... 

The cause of brittle fracture in polymers is the inability of the material to quickly dissipate by molecular relaxation processes the internal stresses generated as a result of the imposed deformation. Brittle fracture occurs when the time to failure is the same order of magnitude (or faster) than the speed of the relaxation process that dominates the mechanical behavior in the temperature range of interest. The relevant relaxation processes are the first T < Tg secondary relaxation (p or y). A qualitative criterion for determining whether the relaxation... 

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