Design Methods for Plastics using Deformation Data

5 Design Methods for Plastics using Deformation Data Isochronous and Isometric Graphs 

The most common method of displaying the interdependence of stress, strain and time is by means of creep curves. However, it should be realised that these 

The creep modulus will vary with time, i.e. decrease as time increases, in a manner similar to that shown for the relaxation modulus. The classical variation of these moduli is illustrated in Fig. 2.9. On log-log scales it is observed that there is a high value of creep or relaxation modulus at short times. This is referred to as the Unrelaxed Modulus and is independent of time. Similarly at long times there is a low value Relaxed Modulus which is also independent of time. 

Occasionally in creep analysis it is convenient to use a Creep Compliance instead of the creep modulus. This is simply given by 

These latter curves are particularly important when they are obtained experimentally because they are less time consuming and require less specimen preparation than creep curves. Isochronous graphs at several time intervals can also be used to build up creep curves and indicate areas where the main experimental creep programme could be most profitably concentrated. They are also popular as evaluations of deformational behaviour because the data presentation is similar to the conventional tensile test data referred to in Section 2.3. It is interesting to note that the isochronous test method only differs from that of a conventional incremental loading tensile test in that (a) the presence of creep is recognised, and (b) the memory which the material has for its stress history is accounted for by the recovery periods. 

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