Yield Stress and Modulus

In the ordered state, the suspension can have a substantial modulus (see Fig. 6-31). Buscall et al. (1982b, 1991) have derived an expression for the high-frequency modulus from the interparticle potential by calculating the work required for particles to move affinely with the flow. The result of Buscall et al. follows from the Zwanzig-Mountain expression, Eq. (6-20), where g r ) can be replaced by a delta function since the suspension is crystalline. Neglecting the osmotic term, the result is (Evans and Lips 1990 Buscall 1991) 

From Eqs. (6-69) and (6-70), the ratio of the yield stress to the modulus works out to be (Buscall 1991) 

Theoretical expressions similar to those presented in this section have recently been successfully used to predict the modulus and viscosity of suspensions of charged rod-like particles (Solomon and Boger 1998). 

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Figure 5.1 Relation between chain length and select polymeric properties such as Tg, yield stress, and modulus.

In general, substitution of polar atoms and polar groups for nonpolar or less polar moieties results in an increase in the Tg and such mechanical properties as yield stress and modulus. Thus condensation polymers such as nylons, polycarbonate (PC), and polyesters are typically higher-melting and exhibit higher Tg s, tensile strength and associated properties, but typically lower impact strengths and associated properties which require some flexibility (Table 5.3). 

Figure 12.7 Plot of yield stress and modulus data according to Kitagawa s equation (T0 = 22°C) for different epoxy formulations (see Table 12.1 for abbreviations). (Reprinted from Cook et al., 1998, Copyright 2001, with permission from Elsevier Science.)... 

Gupta et al. (39) has studied the effect of dynamic cross-linking on tensile yield behavior of PP/EPDM rubber blends. They prepared blends of PP/EPDM in internal mixer by simultaneous blending and dynamic vulcanization. Dimethyl phenolic resin vulcanized PP/EPDM blends showed higher yield stress and modulus than unvulcanized PP/EPDM blend (Fig. 14.17 and Table 14.1). They found the increase in... 

Decomposition temperatures, whether sharp or vague, are seldom actually reached in service life. Reinforced composites are preeminently load-bearing materials, and it is their temperature-dependent mechanical properties, such as T, or the closely related heat distortion temperature, that usually determine the maximum use temperature, at least for short or intermediate term use. Strength, yield stress and modulus all decline with increasing temperature, reflecting the increasing mobility of the molecular structure, and unacceptable levels of physical property loss will often occur well before the onset of thermal or thermo-oxidative degradation. 

The mechanical properties, fiacture stress, fiacture strain, yield stress, and modulus, were measured (67) for doses up to 100 Mrads. Fracture strain was found to increase with dosage but shear yield strength decreased. Decreases averaging roughly 50% were noted for the other properties. Therefore, it is evident that the -radiation produces a predominance of chain scission over crosslinking in the epoxy. It was concluded that irradiation does not have much effect on the graphite fibers but that it degrades mechanical properties of the epoxy matrix. 

The modification of the interfacial tension influences the particle size obtained in the blend, but does not influence yield stress and modulus, which both depend on rubber concentration rather than on particle size (Borggreve et al. 1987). 

The yield stress and modulus of all adhesives decrease with increasing temperature and humidity. 

PP PA6 PP-g-MA SEBS-g- MA PP-g-MA improved yield stress and modulus and SEBS-g-MA improved elongation and impact strength 125... 

Figure 8.6 shows a representative set of stress-strain curves for the 12 1 melt-drawn samples at each nanotube concentration. The addition of nanotubes significantly alters the stress-strain behavior of the fibers. The ultimate stress, yield stress and modulus increase with the addition of nanotubes. In contrast, the ultimate elongation slightly decreases with the addition of nanotubes. The significant increases in ultimate and yield stress combined with a small decrease in ultimate elongation lead to the observed increases in toughness. 

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