Crazing

7 Failure envelope (heavy continuous line) for PMMA under biaxial stress ( t33=0) at room temperature, shovring intersection of crazing and shear yielding envelopes (after S. Stemstein and L Ongchin). 

Crazing and fracture caused by absorbed liquids and vapours is one of the most serious limitations on the use of plastics in engineering applications. The terms solvent crazing and environmental stress cracki are 

Most practical problems are concerned not with short-term faQure in active environments, but with long-term crazing in relatively mild environments. Modification of the chemical structure is often sufficient to overcome this problem. For example, pofys ene crazes at low strains in air and at even lower strains in a range of liquid environments, whereas styrene-aciylonitrile copolymers (SAN) are more resistant. The nitrile group (—C=N) carries an electric dipole which provides additional intermolecular attractions, thereby both increasing the stresses necessary to cause cavitation, and at the same time reducing the absorption of non-polar liquids (S.N.3). 

From the engineering standpoint, crazing itself is of minor importance. There are a few applications in which a certain level of craze formation renders the component unserviceable, e.g. PMMA helicopter cabins, where visibility is reduced, and ABS pipes, in which porosity can be a problem. 

8 Stress-strain curves for polystyrene (PS) and high-impact polystyrene (HIPS) virhich is a mixture of polystyrene with extremely small rubber particles. 

Eventually, crazes break down to form cracks, and when the cracks grow to critical size the sample fails. Although crazes lead to failure in this way they can be useful, because if many crazes are produced before failure occurs, energy is absorbed by the material as local yielding takes place. The impact strength of modified blends is due to the large number of crazes formed. Even when there is no macroscopic indication of crazing, microscopy may 

Real-time cryodeformation of PP and impact modified PP has been conducted in the TEM as a function of temperature using a commercially available cooling/straining holder in conjunction with a copper deformation cartridge 

Preparation of crazed polymers for TEM is quite difficult. First, the whole specimen must be stressed to failure, resulting in crazes that 

Kramer et al. [519] developed a TEM method to observe and measure deformation and frac- 

The microstructure observed for thick films shows fibrils, about 4-10 nm in diameter for PS, in agreement with SAXS measurements on the crazes in the bulk polymer. Very thin films of PS (100 nm) show modification in the craze structure as there is no plastic restraint normal to the film [525]. Deformation zones have also been studied in polycarbonate, polystyrene-acrylonitrile, and other polymers [526]. Crazes in thermosets can be studied in thin films spun onto NaCl substrates, which can be washed away when the film has been cured. Mass thickness measurements are difficult to make in radiation sensitive materials that is why most TEM work has been done on PS and least on 

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. 

The in situ deformation of amorphous polymers by shear deformation and craze growth has been observed in optical microscope studies by Donald and Kramer [317]. Grids with thin films of various polymers and polymer blends were prepared on copper grids which were strained in air on a strain frame held in an optical microscope. The films were precracked in an electron microscope by a method more fully described by Lauterwasser and Kramer [318]. Many crazing studies are evaluated by in situ methods, and optical microscopy plays a major role in providing an overview of the deformation structure. Crazing studies will be more fully explored in the next section. 

Crazing is the first stage of fracture in many glassy polymers and also in blends and semicrystalline polymers where there is a glassy matrix. Crazing is a localized tensile yielding process that produces thin sheets of deformed, crazed , material with the sheets perpendicular to the principal stress axis. Within a craze the 

A quantitative analysis of craze shape and mass thickness contrast within the craze allowed Lauterwasser and Kramer [318] to derive the stress profile existing along a polystyrene craze. Kramer and his coworkers have extended this study to many other polymers, relating the mean density of craze material to entanglement density in the polymer glass and to toughness [329] without a basic change of preparation technique. 

The equations derived by Irwin for the stress distribution near the tip of a crack show that the stress becomes infinite at the tip itself and is extremely high just beyond this. This cannot correspond to reality the region just beyond the crack tip must yield and thus reduce the stress. For thick specimens under conditions of plane strain, a region called a craze often forms ahead of the crack. By viewing the interference fringes formed in the crack and in the craze in reflected light it can be deduced that the craze is a region of lower density than that of the bulk polymer and that it extends typically a few tens of micrometres from the tip of the crack. The profile of the craze, i.e. its variation in thickness with distance from the crack tip, can 

These measurements show that the shape of the craze is well described by the Dugdale plastic zone model originally proposed for metals. The tip of the craze is approximated by a straight line perpendicular to the direction of advance of the crack and the thickness profile can be deduced from the assumption that there is a constant craze stress normal to the plane of the craze. Provided that cr cTcr, where a is the applied tensile stress causing the crack, the length R of the craze ahead of the crack tip is given by 

The thickness profile is given by a rather complicated expression involving R, (Tci- and E and is shown in fig. 8.9. 

Measurements of the critical angle for reflection at a craze yield a value for the refractive index of the craze and show that it must consist of approximately 50% polymer and 50% void. Investigations by electron microscopy, electron diffraction and small-angle X-ray scattering show 

Unfortunately, the initiation and evolution of crazes do not concern only the majority of thermoplastic glassy polymers, which exhibit brittle behavior. Crazes usually also constitute the dominant micromechanism for failure when many polymers generally considered tough are subjected 

Some criteria have been proposed for craze initiation. The earliest criterion states that crazing occurs when the uniaxial tensile stress reaches a critical value (27). Since the crazing stress depends on the strain rate and 

Sternstein and Ongchin (28) considered that if cavitation occurs in crazes the criterion for crazing initiation should include the dilative stress component. They proposed the criterion to fit the experimental data for surface craze initiation in PMMA when the polymer is subjected to biaxial tension. The segmental mobility of the polymer will increase due to dilative stresses, thus provoking cavitation and the orientation of molecular segments along the maximum stress direction. 

The Sternstein-Ongchin criterion is expressed in terms of the stress such 

The effect of polymer structure on crazing has been explained in terms of molecular entanglements. It has been suggested that molecular entangle- 

Dompus and Groeninckx [71,72] recently developed a criterion for internal cavitation which has been treated as an energy balance between strain energy relieved by cavitation and surface energy required to create a new surface. It is given by the following equation  

Where K, A, d, and y are the rubber bulk modulus, relative volume strain, rubber particle diameter and surface energy per unit area, respectively. 

When a triaxial stress is applied to a brittle polymer, microvoiding is initiated in the matrix because it cannot easily deform. The localised yielded region therefore consists of an interpenetrating system of voids and polymer fibrils (1-pm diameter) called a 

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 [380] stamped dumbbell shaped pieces of polymer from 1 mm thick sheets and extended them 

They prepared thin films c. 0.5 fim thick by spin casting diblock and triblock copolymers of PS and PVP from benzene onto a rock salt substrate [393]. The dry films were exposed to benzene vapor for 24 h and the glassy films were floated off the rock salt onto a water bath surface where they were picked up onto a grid coated with a thin film of PS. The film adhered to the grid after a brief exposure to the solvent. An SEM beam was used to bum a thin slit in the material and cracks 50-100 /xm long by 10 //m wide were introduced in the center of each grid square. Grids were deformed as described above and examined by OM to locate areas of interest for TEM study and measurement of craze fibril extension ratios [382]. 

For which polymers and under which conditions do crazes occur Crazes form primarily in amorphous polymers, for molar masses above the entan- 

Crazes under a constant stress are not stable and grow in length and thickness with a constant rate. The direction of growth is well-defined with 

It is possible to provide an estimate for the draw ratio reached. It may be identified with the point where strain hardening sets in and, as for an entangled polymer melt, the hardening is caused by the approach of the chain sequences between entanglements to their limits of extensibility. First we need the knowledge of the molar mass of these sequences. This molar mass, denoted Me, can be obtained with the aid of Eq. (9.86), 

The degree of polymerization Ng and the contour length /ct,e of chains with molecular weight Me are 

With the knowledge of /ct,e and Re the draw ratio at the onset of strain hardening can be estimated. Calling it max, we write 

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Plastic Sheet. Poly(methyl methacrylate) plastic sheet is manufactured in a wide variety of types, including cleat and colored transparent, cleat and colored translucent, and colored semiopaque. Various surface textures ate also produced. Additionally, grades with improved weatherabiUty (added uv absorbers), mat resistance, crazing resistance, impact resistance, and flame resistance ate available. Selected physical properties of poly(methyl methacrylate) sheet ate Hsted in Table 12 (102). 

In general, polycarbonate resins have fair chemical resistance to aqueous solutions of acids or bases, as well as to fats and oils. Chemical attack by amines or ammonium hydroxide occurs, however, and aUphatic and aromatic hydrocarbons promote crazing of stressed molded samples. Eor these reasons, care must be exercised in the choice of solvents for painting and coating operations. Eor sheet appHcations, polycarbonate is commonly coated with a sihcone—sihcate hardcoat which provides abrasion resistance as well as increased solvent resistance. Coated films are also available. 

Polystyrene. Polystyrene [9003-53-6] is a thermoplastic prepared by the polymerization of styrene, primarily the suspension or bulk processes. Polystyrene is a linear polymer that is atactic, amorphous, inert to acids and alkahes, but attacked by aromatic solvents and chlorinated hydrocarbons such as dry cleaning fluids. It is clear but yellows and crazes on outdoor exposure when attacked by uv light. It is britde and does not accept plasticizers, though mbber can be compounded with it to raise the impact strength, ie, high impact polystyrene (HIPS). Its principal use in building products is as a foamed plastic (see Eoamed plastics). The foams are used for interior trim, door and window frames, cabinetry, and, in the low density expanded form, for insulation (see Styrene plastics). 

When crazing limits the ductility in tension, large plastic strains may still be possible in compression shear banding (Fig. 23.12). Within each band a finite shear has taken place. As the number of bands increases, the total overall strain accumulates. 

Fig 23 11 Crazing in a linear polymer molecules are drawn out as in Fig. 23.10, but on a much smaller scale, giving strong strands which bridge the microcracks. 

At the present time it is generally accepted that the toughening effect is associated with the crazing behaviour.Because of the presence of the low-modulus rubber particles most of the loading caused when a polyblend is subject to mechanical stress is taken up by the rigid phase (at least up to the moment of... 

When a craze occurs around a rubber droplet the droplet is stressed not only in a direction parallel to the applied stress but also in the plane of the craze perpendicular to the applied stress (see Figure 3.9). Such a triaxial stress leading to dilation of the particle would be resisted by the high bulk modulus of the rubber, which would thus become load bearing. The fracture initiation stress of a polyblend should not therefore be substantially different from that of a glass. 

Figure 3.9. Rubber particle straddling craze perpendicular to stress is subjected to triaxial stresses and because of its high bulk modulus becomes load bearing. (After Bucknall )...

The rubber particles should not be so small that they are completely embedded in a craze. It is interesting to note that in high-impact polystyrene crazes tend to be about 2 p.m thick and the optimum particle sizes observed as a result of experience are quoted in the range 1-10 p.m. For ABS the figures are about 0.5 p.m and 0.1-l.Op.m respectively. 

The presence of many crazes is considered to distribute stresses which would otherwise be concentrated at the tip of a few growing cracks. Additionally there is some evidence that when a propagating craze reaches a particle it often divides so that if there is a large number of particles per unit volume there is a high dissipation of energy. 

As may be expected of an amorphous polymer in the middle range of the solubility parameter table, poly(methyl methacrylate) is soluble in a number of solvents with similar solubility parameters. Some examples were given in the previous section. The polymer is attacked by mineral acids but is resistant to alkalis, water and most aqueous inorganic salt solutions. A number of organic materials although not solvents may cause crazing and cracking, e.g. aliphatic alcohols. 

Internal stresses occur because when the melt is sheared as it enters the mould cavity the molecules tend to be distorted from the favoured coiled state. If such molecules are allowed to freeze before they can re-coil ( relax ) then they will set up a stress in the mass of the polymer as they attempt to regain the coiled form. Stressed mouldings will be more brittle than unstressed mouldings and are liable to crack and craze, particularly in media such as white spirit. They also show a characteristic pattern when viewed through crossed Polaroids. It is because compression mouldings exhibit less frozen-in stresses that they are preferred for comparative testing. 

Notch sensitivity and susceptibility to crazing under strain. 

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