Schapery model creep recovery

The creep and recovery behaviour of an UHMWPE was studied in the region of small xmiaxial deformations by Zapas and Crissman [152]. These results are used to illustrate the capability of the Schapery model to represent the viscoelastic/viscoplastic behaviour of UHMWPE. Creep and recovery experiments were carried out on specimens under creep stresses in the range 1-8 MPa. In Figure 7.9 are plotted the creep compliances obtained, showing to be stress dependent above 1 MPa. Using the appropriate values for the model parameters, the strain under creep and creep-recovery loading conditions were very well captured as shown in Figure 7.10. 

Fig. 60. Comparison of experimental (circles) creep and recovery behavior of a glass-reinforced phenolic resin with the predictions from the Schapery (147-149) nonlinear viscoelastic model. o Experimental Data A Predicted Recovery Data (Nonlinear Theory). After Schapery (147), with permission.

Consider a nonlinear viscoelastic material which is well modeled by the Schapery approach. Would it be possible to determine all seven (7) material parameters only using creep tests That is, not using recovery (unloading) data or a multiple steps in stress Give a detailed explanation for your answer. 

Figure 60 illustrates two-step creep and recovery data at different stresses for a reinforced polymer along with the predictions finm the Schapery creep formulation and those obtained from simply applying a modified form of the Boltzmann superposition principle. Without going into the details of the procedures of obtaining all the parameters, it is clear that the model captures much of the observed nonlinear response, while the modified Boltzmann rule does not. (Note that the modified Boltzmann rule simply assumes additivity of responses, but without the linearity assumptions.) Figure 61 shows the creep and recovery data... 

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