Creep compliance structural-relaxation times

Table 14.1 Characteristics M, M /Mn and Tg (based on DSC and defined at ts = 1,000sec) and parameters A, / and Z extracted by analyzing the creep-compliance J(t) curves or viscoelastic spectra G (ta) of the polystyrene samples, whose structural-relaxation times TS, structural-growth parameters s and frictional factors K are displayed, respectively, in Figs. 14.13, 14.14 and 14.15. Also shown are the K values at 127.5°C of samples A, B, C and F2 along with the average value of K shown in Table 10.1 and the Mw, MwjMn, and Tg (DCS) of F2. The reference theory used in each analysis is indicated. 

The use of solid polymers for long-term structural applications that are stiffiiess limited in design requires the determination of the modulus of elasticity or the compliance for loading in tension (or compression). For metals, this is a simple matter since the value determined from a tension test, a compression test or a flexural test will be the same. However, for solid polymers, the tensile and compressive compliance are not the same, and thus, the flexural compliance will be different from both[l], combining as it does tensile and compression in a single test. To furdier complicate matters, all three compliance values are time-dependent, for polymeric solids[2]. The creep compliance is the time-dependent creep str divided by the constant applied stress that caused the creep, but the inverse is not the same as the relaxation... 

If the Boltzmann superposition principle holds, the creep strain is directly proportional to the stress at any given time, f Similarly, the stress at any given lime is directly proportional to the strain in stress relaxation. That is. the creep compliance and the stress relaxation modulus arc independent of the stress and slrai . respectively. This is generally true for small stresses or strains, but the principle is not exact. If large loads are applied in creep experiments or large strains in stress relaxation, as can occur in practical structural applications, nonlinear effects come into play. One result is that the response (0 l,r relaxation times can also change, and so can ar... 

---