Creep structural parameters

Figure 16.2 The creep curve and structural parameters as a function of time for nickel. Tests at temperature T = 673 K (0.39 T ) stress o = 130 MPa (1.7 x 10 V)-...

Let us stress once more that, irrespective of the specific solid-state deformation model used [271-275,292], the values of creep activation parameters in polymers are interconnected and depend on the degree of deformation and temperature. Their changes are related in a regular manner to structural alterations in polymers and to their spectra of molecular motions. Due to the changeability of deformation kinetic... 

The four-parameter model is very simple and often a reasonable first-order model for polymer crystalline solids and polymeric fluids near the transition temperature. The model requires two spring constants, a viscosity for the fluid component and a viscosity for the solid structured component. The time-dependent creep strain is the summation of the three time-dependent elements (the Voigt element acts as a single time-dependent element) ... 

The creep stress was assumed to be shared between the polymer structure yield stress and the cell gas pressure. A finite difference model was used to model the gas loss rate, and thereby predict the creep curves. In this model the gas diffusion direction was assumed to be perpendicular to the line of action of the compressive stress, as the strain is uniform through the thickness, but the gas pressure varies from the side to the centre of the foam block. In a later variant of the model, the diffusion direction was taken to be parallel to the compressive stress axis. Figure 10 compares experimental creep curves with those predicted for an EVA foam of density 270 kg m used in nmning shoes (90), using the parameters ... 

Structural failure may occur when the overall structural cross-section cannot support the applied load or, when the critical flaw size ac is exceeded by preexisting discontinuity or by reaching the critical crack size through fatigue, stress corrosion cracking or creep mechanisms. Using fracture mechanics the stress at a crack tip can be calculated by a stress-intensity parameter K as,... 

Mechanical Characterization of Sulfur-Asphalt. The serviceable life of a pavement comes to an end when the distress it suffers from traffic and climatic stresses reduces significantly either the structural capacity or riding quality of the pavement below an acceptable minimum. Consequently, the material properties of most interest to pavement designers are those which permit the prediction of the various forms of distress—resilient modulus, fatigue, creep, time-temperature shift, rutting parameters, and thermal coefficient of expansion. These material properties are determined from resilient modulus tests, flexure fatigue tests, creep tests, permanent deformation tests, and thermal expansion tests. 

The creep test is a simple and inexpensive test for viscoelastic foods which provides valuable information on the rheological parameters. Davis (1973) pointed out that indeed too much information can be obtained from the creep test. For example, an eight-parameter rheological model defined by eleven parameters was required for shortening and lard. One drawback with creep studies using concentric cylinder systems is that the materials structure is disturbed when the sample is being loaded. 

The creep-compliance technique has been used extensively by Sherman and co-workers for the study of ice cream, model emulsions, margarine, and butter (Sherman, 1966 Shama and Sherman, 1969 Vernon Carter and Sherman, 1980 Sherman and Benton, 1980). In these studies, the methodology employed was similar to that for ice cream, that is, the creep-compliance data on a sample were described in terms of mechanical models, usually containing four or six elements. Attempts were made to relate the parameters of the models to the structure of the samples studied. However, with increased emphasis on dynamic rheological tests and interpretation of results in terms of composition and structure, the use of mechanical models to interpret results of rheological tests has declined steadily. 

Most of the correlations discussed in sections ll.B and ll.C estimate the mechanical properties of polymers in terms of several material parameters of a more fundamental nature. Creep, stress relaxation and fatigue, which are all very important in determining durability, are discussed in Section 1 l.D. As discussed in Section 1 l.E, the key contribution of our work is the development of the general correlations presented in this book to calculate the fundamental material parameters, significantly extending the range and structural diversity of polymers for which existing structure-property relationships for the mechanical properties can be applied. 

If all or some of the particles to be agglomerated by pressure are elastic, under certain conditions, the temporary elastic deformation can be converted into permanent plastic change of shape. For this to occur, the most important parameters are time and temperature. If elastically deformed solids are kept under pressure for some time, certain structural features, such as lattices, dislocations, etc., move into stable new positions by creep. At elevated temperatures, but still well below the softening point, most solids, even those that are brittle or tough at ambient temperatures, become more malleable and deform readily under pressure. 

Selection criteria with respect to structural applications at high temperatures have already been discussed by Sauthoff (1989). First, such phases must have sufficient strength at the service temperature which also means sufficient creep resistance. The creep resistance scales with the diffusion coefficient and with the shear modulus (e.g. Jung et al., 1987), and both parameters scale with the melting temperature (Frost and Ashby, 1982). Thus the... 

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. 

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