Superpositions and shift factors

In many applications, plastic parts carry reasonably constant mechanical loads over periods up to few years. The polymer will creep during the lifetime of the part. At moderate load levels, long-term prediction of creep from short-term tests is possible, because the viscoelastic response of polymers (creep, stress relaxation) measured at different temperatures superimpose when shifted along the time axis [24]. 

The first step is to carry out a series of creep tests at different temperatures, usually above the glass transition temperature of the polymer. A fixed weight is suspended on a sample at a fixed temperature, and the resulting sample elongation Al is measured as a function of time, usually for less than a couple of hours. 

Below the glass transition temperature a linear extrapolation of the shift is used. The creep behaviour can then be predicted at any other temperature and time, by combining equations (2.1) and (2.2), provided that no other effects (environmental interactions, chemical or physical aging) occur over the complete time scale. 

The prediction of the long-term viscoelastic response of a polymer part subjected to any given temperature history is possible through the integration of 

It is evident that the various degradation and ageing phenomena discussed in the above sections occur simultaneously. The interactions between these phenomena and the resulting acceleration of property change are subject to widespread investigations, particularly in the field of composites [27.  

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