Mechanical property measurement stress-time curves

The isothermal curves of mechanical properties in Chap. 3 are actually master curves constructed on the basis of the principles described here. Note that the manipulations are formally similar to the superpositioning of isotherms for crystallization in Fig. 4.8b, except that the objective here is to connect rather than superimpose the segments. Figure 4.17 shows a set of stress relaxation moduli measured on polystyrene of molecular weight 1.83 X 10 . These moduli were measured over a relatively narrow range of readily accessible times and over the range of temperatures shown in Fig. 4.17. We shall leave as an assignment the construction of a master curve from these data (Problem 10). 

The mechanical properties were evaluated by two sets of tensile measurements. Typical stress-strain curves are shown in Figure 4. The modulus and stress decrease with increasing aging time. Similar results are observed for all aging samples at all three temperatures. Both testing methods provided essentially the same tensile data at 400% extension. The scatter of the tensile data is due to the experimental error associated with the measurement. 

The most common type of stress-strain tests is that in which the response (strain) of a sample subjected to a force that increases with time, at constant rate, is measured. The shape of the stress-strain curves is used to define ductile and brittle behavior. Since the mechanical properties of polymers depend on both temperature and observation time, the shape of the stress-strain curves changes with the strain rate and temperature. Figure 14.1 illustrates different types of stress-strain curves. The curves for hard and brittle polymers (Fig. 14.1a) show that the stress increases more or less linearly with the strain. This behavior is characteristic of amorphous poly-... 

Figure 4.9 Mechanical stability of 3D biomimetic scaffolds during in vitro culture. Compressive mechanical properties of biomimetic scaffolds with and without cells measured under wet conditions, a-d) Representative stress-strain curves of 3D biomimetic scaffolds during 28 days of cell culture a) cell-seeded, b) cell-free scaffolds after 14 days of culture, c) cell-seeded, d) cell-free scaffolds after 28 days of culture, and e) cell-seeded scaffolds were able to maintain their mechanical properties, whereas the cell-free scaffolds showed significant decrease in compressive modulus after 28 days of culture due to the polymer hydrolytic degradation. ( ) indicates significant decrease for the same type of scaffold at different time points, p <0.05. Cell-seeded scaffolds showed higher compressive moduli at all the time points compared with cell-free scaffolds due to the reinforced extracellular matrix secreted by cells during the cell culture. Reproduced with permission from M. Deng, S.G. Kumbar, L.S. Nair, A.L. Weikel, H.R. Allcock and C.T. Laurencin, Advanced Functional Materials, 2011,21, 2641. 2011, Wiley-VCH [3]...

Viscoelastic characteristics of polymers may be measured by either static or dynamic mechanical tests. The most common static methods are by measurement of creep, the time-dependent deformation of a polymer sample under constant load, or stress relaxation, the time-dependent load required to maintain a polymer sample at a constant extent of deformation. The results of such tests are expressed as the time-dependent parameters, creep compliance J t) (instantaneous strain/stress) and stress relaxation modulus Git) (instantaneous stress/strain) respectively. The more important of these, from the point of view of adhesive joints, is creep compliance (see also Pressure-sensitive adhesives - adhesion properties). Typical curves of creep and creep recovery for an uncross-Unked rubber (approximated by a three-parameter model) and a cross-linked rubber (approximated by a Voigt element) are shown in Fig. 2. 

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