Pseudoelastic design method

These assumptions are not always justifiable when applied to plastics unless modification has occurred. The classical equations cannot be used indiscriminately. Each case must be considered on its own merits, with account being taken of such factors as the mode of deformation, the service temperature and environment, the fabrication method, and so on. In particular, it should be noted that the past traditional equations that have been developed for other materials, principally steel, use the relationship that stress equals the modulus times strain, where the modulus is constant. Except for thermoset reinforced plastics and certain engineering plastics, many plastics do not generally have a constant modulus. Different approaches have been used for the nonconstant situation some are quite accurate. The drawback is that most of these methods are quite complex, involving numerical techniques that are not attractive to designers. One method that has been widely accepted is this so-called pseudoelastic design method. In this method appropriate values of such time-dependent properties as the modulus are selected and substituted into the standard equations. 

The problems of exact design for a viscoelastic plastic with non-linear properties are severe. For example, in Fig. 8.14(a) the stress-strain curve is linear only at the smallest strains (below 0.2 %). Most plastic parts are designed to operate at strains well above 0.2 %, and in this case exact stress analysis is impossible. In practice, a safe approximate procedure known as the pseudoelastic design method is used. The salient features of the method, which is very straightforward to apply, are as follows. 

This is an example of strain-limited design. We apply the pseudoelastic design method, specifying the duration of loading as 5 hours (which is 18 (XX) seconds). In order to determine the modulus, we need the isochronous stress-strain curve for / = 18(XX)s. Substituting in the equation... 

The pseudoelastic design method may be used for components submitted to intermittent loading, provided that the intervals during which the material is unloaded are sufficient to allow virtually complete recovery. Some manufacturers provide recovery data that enable the validity of this assumption to be tested. Alternatively, the Boltzmann superposition principle may be used to determine whether the assumption gives a satisfactory approximation (see... 

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