Nylons fatigue properties

Acetal resins are strong, stiff thermoplastics with good fatigue properties and dimensional stability. They also have a low coefficient of friction and good heat resistance. Acetal resins are considered similar to nylons but are better in fatigue, creep, stiffness, and water resistance. Acetal resins do not, however, have the creep resistance of polycarbon-... 

Aromatic polyamides (aramids) split longitudinally due to fibrillation, whereas nylon shows plastic deformation under the same conditions [33], Gupta [34] showed skin-core effects in polyester fibers. Hearle and Wong [35] studied the fatigue properties of nylon 6,6 and PET. 

Property Summary Nylons are recommended for general-purpose gears and other mechanical components. Acetals for maximum fatigue life, for highly accurate parts, or exposure to extremely humid conditions. Phenolic-fabric laminates for low-cost, thin stamped gears or parts. Polycarbonates for intermittent, very high impacts (not recommended for applications involving repeated cyclical stress). TFE-filled acetals for heavy-duty applications. 

Stiffness, resistance to deformation under constant applied load (creep resistance), resistance to damage by cyclical loading (fatigue resistance), and excellent lubricity are mechanical properties for which acetal resins are perhaps best known and which have contributed significantly to their excellent commercial success. General-purpose acetal resins are substantially stiffer than general-purpose polyamides (nylon-6 or -6,6 types) when the latter have reached equilibrium water content. 

Acetal polymers and copolymers are engineering materials and are competitive with a number of plastics materials, nylon in particular, and with metals. Acetal resins are being used to replace metals because of such desirable properties as low weight (sp. gr. 1.41-1.42), corrosion resistance, resistance to fatigue, low coefficient of friction, and ease of fabrication. 

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