Cracking and crazing

In the region of the impact, the plastic wiU experience a shock wave. In brittle fracture at low temperatures, the high frequency displacement will cause the energy to be constrained in a narrow zone where the impact occurs. Since the polymer is rigid, no molecular motions to accommodate the stress are possible. Then aU the energy breaks bonds or overcomes polymer-q)olymer interactions and so separates the polymer chains, causing the material to break and fail. 

Ductile failure is observed when it is possible for the polymer to exhibit rubber-Hke characteristics above the glass transition. So, the crazing by formation of 

As was shown in Chapter 5, the relevant motions of the polymer chains are controlled by the timescale of the stress application, by the available energy, and by the free volume in the system. We reiterate in order to move, molecules need time, energy and space. So the response of the polymers, and their ability to absorb energy and move, determines whether the fracture is ductile or brittle, glassy smooth or accompanied by crazing and material pull out . 

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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. 

Bituminous This term is used for products obtained from both petroleum and coal tar sources but the petroleum products are the more widely used. These materials are very resistant to moisture and tolerant to poor surface preparation. They are only available as black, dark brown or aluminum pigmented. The last has reasonable outdoor durability but, without the aluminum, the film will crack and craze under the influence of sunlight. Normally they cannot be over-coated with any other type of paint, because not only will harder materials used for over-coating tend to crack or craze but there is also a possibility that the bitumen will bleed through subsequent coats. The best use is as a cheap waterproofing for items buried or out of direct sunlight. 

Coal tar enamel This is derived from the coking of coal and is further distilled to produce coal tar pitches. It is used for hot application on-site. It will crack and craze if exposed to sunlight but has been employed successfully for over 50 years for the protection of underground or immersed structures. The main use is now for the exteriors of buried or immersed pipelines. Different types of enamel are available to give various degrees of heat resistance. It is now generally used for pipelines below 155 mm diameter. 

Good stress-crack and craze resistance, brittle Good processibility, no stress whitening Higher-heat styrenic, brittle... 

VI. Stress Cracking and Crazing in Fluids Summary Problems References... 

Fine Earthenware.—The English manufacture may be selected as the type of this ware it is composed essentially of a mixture of plastic clay and quartz, and is, therefore, more refractory, harder, and denser than the delft ware, the chief ingredients of which are potters clay and marl, and which is in consequence soft and calcareous. Fine earthenware again is always coated with a transparent glaze the commoner sorts with enamel, producing the favorite Majolica ware. Neither kinds, however, are fitted for oulinary use, as they either split when exposed to heat, or the glaze becomes cracked and crazed. 

The method of caustics has also been used to study the formation of cracks and crazes formed by exposure of PMMA to solvents (259). ISO 4599 (260) has been developed to better control the application of stress using a jig having the curve of the arc of a circle for shaping the specimen and maintaining a set curvature during exposure to the agent. After a predetermined time the specimen is tested for tensile or flexural properties and compared to preexposure test values. ISO 4600 (261) uses the technique of impressing an oversized ball or pin into a hole drilled in the specimen to apply a strain. 

If Vt 1240 meters/sec in the matrix and branching will occur in the rubber at 29 meters/sec, we calculate A/Co = 0.047. Thus, branching can occur after a matrix crack acceleration distance of only 2 to 5/x (assuming a Griffith crack length of 50-100fi) hence, ample room for the development of fast cracks or fast crazes exists in the ABS structure. Note that the expressions for craze instability, acceleration, and speed (Equations 1, 6, 7) show that the macro strain rate of the specimen is irrelevant— fast cracks and crazes propagate in specimens strained even at slow creep rates. 

Williams JG, Marshall GP (1975) Environmental crack and craze growth phenomena in polymers. Proc R Soc Lond Ser A 342(1628) 55-57... 

The aim of this review is to concentrate mainly on these fundamental aspects of the fracture behavior of glassy thermoplastics. In the first Section, following an outline of the relevant fracture mechanics theory, the optical interference method is described and the nature of the results obtainable from it is discussed. The next Section then considers the behavior of cracks and crazes in specimens subjected to quasistatic loading, whilst the final Section examines the role of crazing associated with fatigue crack growth. 

Fig. 5. Interference fringe patterns (X = 546 nm) and microdensiiometer traces of an unloaded and loaded (K, = 20,8 N/mm ) crack and craze in PMMA...

Using a modified Dugdale model with a variable craze stress along the craze zone this effect has quahtatively been interpreted At positions where the constant stress Dugdale model gives displacements higher than the measured ones the actual craze stress must be higher. In the case of PC a closer inspection reveals a stress peak at the crack and craze tip. Kambour predicted just such a stress distribution in a craze from the analysis of the stress distribution around a craze (without a crack)... 

Fig. 1.1. Crack and craze openings in PMMA as measured by interference optics (points) (Arrangement of molecules and deformation behavior of fibrils are schematically indicated)...

Fig. 2. Penny shape crack and craze grown from a defect. Note the large crack extension with the crack-tip craze ...

In addition to the separate or combined effects of heat, oxygen, and radiation, polymers may deteriorate due to exposure to water (hydrolysis) or different types of chemical agents. Condensation polymers like nylons, polyesters, and polycarbonates are susceptible to hydrolysis. Structural alteration of some polymers may occur as a result of exposure to different chemical environments. Most thermoplastics in contact with organic liquids and vapors, which ordinarily may not be considered solvents for the polymers, can undergo environmental stress cracking and crazing. This may result in a loss of lifetime performance or mechanical stability and ultimately contribute to premature mechanical failure of the polymer under stress. 

Gap-filling resins are produced by incorporating into UF resins plasticizers, such as furfuryl alcohol, and fillers to minimize shrinkage and consequent cracking and crazing. 

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