Aramids creep

The proposed model for creep rupture based on the condition of maximum shear strain and the Eyring reduced time model explain the observed relations concerning the lifetime of aramid, polyamide 66 and polyacrylonitrile fibres. However, with increasing temperatures, in particular above 300 °C, chemical degradation of PpPTA also determines the lifetime. Furthermore, the model... 

The tensile strength of aramids are comparable to cast metals. In additions, they exhibit low creep and low water absorption. Therefore, these materials... 

The weak van der Waals bonds between the molecules in UHMWPE give it very poor heat resistance. The fibres melt at 150°C and their properties deteriorate as the temperature inaeases above room temperature. Under high stress, the fibres tend to creep extensively and can break after a short time under load. A secondary slow heating, under tension, when approaching the melting point, increases modulus and reduces CTeep. It is extremely resistant to chemical and biological attack and has better abrasion and fatigue resistance than aramid fibres. ... 

Although aramid fibres have high inherent tensile strength, particularly in uni directional construction, in composites they tend to have creep rates very much higher than similar glass or carbon composites (reference 1.9). 

Short fibres of glass, rayon, aramid, asbestos and cellulose as reinforcing fillers, have been broadly used in rubber industries due to their high modulus, high strength and low creep. In recent years especially, natural fibres such as jute fibre, cellulose fibre, " coir fibre," " sisal fibre," " etc. have been also widely used in NR composites because they are enviromnental friendly, cheap, abundant and renewable. However, natural fibres also have some disadvantages such as moisture absorption, quality variations, low thermal stability and poor compatibility with the hydrophobic polymer matrix. 

The time-dependent behaviour is different in the two types of highly oriented, chain-extended polymer fibres. Table 1 gives the results of studies in FIBRE TETHERS 2000 (1995), which were made because creep rupture is a concern in deep-water mooring of oil-rigs. The low-load creep in aramid fibres is due to a straightening of the initial. structure. It reduces in rate, even on a logarithmic scale, with time and is not a source of creep rupture. In Vectran, the creep is less and is absent after 10 days under load. 

In this report we present a model for the primary creep of well-oriented aramid fibres. It has been shown for well-oriented fibres of PET, cellulose and poly-(p-phenylene terephthalamide), abbreviated here as PpPTA, that the dynamic compliance, S, is a linear function of the second moment of the orientation distribution of the chains / / /2/ /3/. By measuring S during creep and relaxation of a fibre, the changes in the orientation distribution can be followed. As shown here, such an experiment offers a valuable tool for the investigation of the viscoelasticity in polymer fibres. 

Creep and stress relaxation of aramid fibres are a linear function of log(time). The creep rate of unconditioned fibres increases linearly... 

Raman and IR spectroscopy have been used in a number of studies of molecular load distributions and deformation mechanisms in PP, usually in combination with a mechanical loading device (tension or compression). The topics studied include true loads on atomic bonds, chain scission under stress, stress relaxation and creep, residual stresses, and stresses along aramid fibers in a PP matrix during pull-out testing. 

Giannopoulos, I. P. (2009), Creep and creep-rupture behaviour of aramid fibres , PhD thesis. University of Cambridge, UK. 

COMPARISON WITH COMPETITIVE MATERIALS Frictional properties of thermoplastics differ from those of metals. Wear of metals depends on adhesive, abrasive, fatigue, and pressure wear. In the case of plastics wear depends on adhesion and deformation of softer plastic material. Frictional forces are proportional to velocity rather than load as is the case of metals. It is always advantageous to combine two different plastics to work together (as opposite to one plastics material for two working together parts). Reinforcement by glass, carbon, and aramid fibers enhances wear resistance by increasing thermal conductivity and creep resistance. 

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