Nylon creep behaviour

For some plastics, particularly nylon, the moisture content can have a significant effect on the creep behaviour. For such plastics, creep curves are normally available in the wet and dry states and a knowledge of the service conditions enables the appropriate data to be used. 

Temperature and humidity levels will be critical to test results and sample conditioning prior to testing should be carefully carried out (e.g. moisture pick-up and crystallisation in nylon on storage). Any small differences in test pieces (e.g. crystallinity, degree of crosslinking, moulded-in stress, orientation, etc.) can alter the creep behaviour of the material significantly [5]. 

There appears to be no information on the uniaxial creep of polymers used as structural adhesives such as is available referring to the creep of adhesive joints in lap-shear or torsion. The latter is reserved for Chapter 7 where the few data that are available are given. An apparent exception is the careful study of a nylon-epoxy adhesive (FM 1000) by Shen and Rutherford (1972). These authors achieved something approaching uniaxial stress using a cylindrical butt joint in direct tension. However, what they called creep was simply the delayed elastic response. They likened the adhesive to a metal in its behaviour and used the classical but inappropriate concept of separating the creep behaviour into three stages as was done many years ago for metals. 

When thermoplastics are reinforced with short fibres, both stiffness and strength may be increased, but these improvements are accompanied by a reduction of the ultimate strain for high concentrations of fibres [1-5]. This brittle behaviour is incompatible with growing engineering applications in which tlie parts are subjected to cyclic loadings or impacts, as in automotive under-the-hood applications. Thus, very tough nylon thermoplastics with enhanced creep and impact resistances have been produced by the introduction of a rubbery phase to the... 

In the course of extensive studies of the creep and recovery behaviour of textile fibres already referred to, Leaderman [13] became one of the first to appreciate that the simple assumptions of linear viscoelasticity might not hold even at small strains. For nylon and cellulosic fibres he discovered that although the creep and recovery curves may be coincident at a given level of stress - a phenomenon associated with linear viscoelasticity (Section 4.2.1) the creep compliance plots indicated a softening of the material as stress increased, except at the shortest times (Figure 10.4). Thus, the creep compliance function is non-linear and of the... 

The electrical conductivity of hydrophilic polyamides " and the photoconductivity of nylon-6,6" are reported. The time dependence of two transient photocurrents suggest the possible formation of a space charge in the polymer. Time-dependent effects in the form of creep measurements have also been used to examine the influence of moisture on the behaviour of nylon-6,6. Other low molecular weight molecules, whose effects on the properties of this polymer have been reported are surfactant and both acid and disperse dyes. Also with a textile connotation was a paper with more general application describing the determination of amino-acid N groups in nylon-6 and -6,6. ... 

Measurements made in the vicinity of a major relaxation region, for instance creep tests on isotactic polypropylene close to room temperature, are sensitive to small changes in temperature, so that a controlled temperature environment is essential. For some polymers, such as nylon, it is also essential to control the humidity, because the presence of moisture in the polymer has a dramatic effect on the mechanical behaviour [2] (in nylon by reducing the effect of interchain hydrogen bonding). Whenever possible, several nominally identical... 

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