Crazing conditions

Thermal expansion The thermal expansion of the film is only about one-fifth that of aluminium, and cracking or crazing is observed when anodised aluminium is heated above 80°C. The fine hair-cracks produced do not seem to impair the protective properties of the coating if anodising conditions have been correct. 

There are other conditions that result from the frozen-in stresses. In materials such as crystal polystyrene, which have low elongation to fracture and are in the glassy state at room temperature, a frequent result is crazing it is the appearance of many fine microcracks across the material in a direction perpendicular to the stress direction. This result may not appear immediately and may occur by exposure to either a mildly solvent liquid or vapor. Styrene products dipped in kerosene will craze quickly in stressed areas. 

Applied stress There are TPs that will craze or crack under certain environmental condition. Products that are highly stressed mechanically must be checked very carefully. Polypropylene, ionomer, chlorinated polyether, phenoxy, EVA, and linear polyethylene are examples that offer greater freedom from stress crazing than some other TPs. Solvents may crack products held under stress. TSs is generally preferable for products under continuous loads. 

Nomnetallics As stated, corrosion of metals apphes specifically to chemical or electrochemical attack. The deterioration of plastics and other nonmetallic materials, which are susceptible to sweUing crazing, cracking, softening, and so on, is essentially physiochemical rather than electrochemical in nature. Nonmetallic materials can either be rapidly deteriorated when exposed to a particular enviromnent or, at the other extreme, be practicidly unaffected. Under some conditions, a nonmetallic may show evidence of gradual deterioration. However, it is seldom possible to evaluate its chemical resistance by measurements of weight loss alone, as is most generally done for metals. 

The formation of voids in the rubbery phase in HIPS influences its mechanical properties. The formation of voids is believed to facilitate the energy dissipating deformation processes, i.e., crazing and shearing. Crazing and shearing are facilitated under conditions in that the rubber particles can easily cavitate. 

Depending on the polymer chemical structure and MW and on the deformation conditions (temperature and strain rate), two types of deformation heterogeneities are observed crazes and shear deformation zones. 

Under conditions where chain mobility is very low, such as at low temperatures, the loss of entanglements occurs by chain scission this is what happens for polystyrene air crazes. Of course, chain scission crazes (CSC) are MW-independent as soon as chains are long enough to be entangled (at too-low MWs, no craze can be formed, only cracks happen). 

Under conditions where chain mobility is high enough, typically at high temperature and low strain rate, the loss of entanglement in the active layer at the craze-bulk interface can occur by chain disentanglement, resulting in chain disentanglement craze (CDC). 

A convenient technique for studying the crack tip craze propagation in amorphous polymers deals with optical interferometry. It has been applied to the examination of PMMA behaviour at various temperatures and crack speeds under conditions of stable propagation [44,45]. 

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