Stress strain ratio

Eor reinforcement, room temperature tensile strength and Young s modulus (stress—strain ratio) are both important. Typical values for refractory fibers are shown in Table 2. 

Isotropic construction Identifies RPs having uniform properties in all directions. The measured properties of an isotropic material are independent on the axis of testing. The material will react consistently even if stress is applied in different directions stress-strain ratio is uniform throughout the flat plane of the material. 

Condensation polymers tend to exist below their Tg at room temperature. They typically form fairly ordered structures with lots of strong interactions between the various chains giving strong materials with some, but not much, elongation when stretched. They are normally used as fibers and plastics. They have high stress/strain ratios. 

Elasticity is nonlinear the secant modulus (the stress/strain ratio) decreases rapidly as strain increases. It can be reversible at very high strains, but this property can be rarely checked for in thermosets owing to their brittleness at T > Tg. 

The similarity of TMDSC and dynamic-mechanical analysis (DMA) raised the question vdiether a complex heat ccqxicity would be of use in analogy to the stress/strain ratio (7 4). One writes for the reversing heat C2q>acity of Equation (4) the complex expression for Cp... 

Dynamic mechanical analyzers can be divided into resonant and defined frequency instruments. The torsion pendulum just described is, for example, a resonant instrument. The schematic of a defined-frequency instrument is shown in Fig. 4.155. The basic elements are the force generator and the strain meter. Signals of both are collected by the module CPU, the central processing unit, and transmitted to the computer for data evaluation. The diagram is drawn after a commercial DMA which was produced by Seiko. At the bottom of Fig. 4.155, a typical sample behavior for a DMA experiment is sketched. An applied sinusoidal stress, o, is followed with a phase lag, 6, by the strain, e. The analysis of such data in terms of the dynamic moduli (stress-strain ratios, see Fig. 4.143) at different frequencies and temperature is the subject of DMA. 

The analyses of several polymers by dynamic mechanical analysis, DMA, are described in Sect. 4.5 in connection with the brief description of the DMA equipment. It was observed in such experiments that neither the viscosity nor the modulus are constant, as is assumed for the discussion of energy and entropy elasticity, outlined in Sects. 5.6.4 and 5.6.5, respectively. One finds a stress anomaly when the elastic limit of a material has been exceeded and plastic deformation occurs. Other deviations have the stress depend both on strain and rate of strain. Finally, a time anomaly exists whenever the stress/strain ratio depends only on time and not on the stress magnitude. 

To describe the data at the bottom of Fig. 6.19, one defines a complex modulus (stress/strain ratio), G, as given in Eq. (1). Analogous expressions can be written for Young s modulus and the bulk modulus. The real component G represents the in-phase component of the modulus, and G , the out-of-phase component i stands for the square root of -1, as usual. 

In addition to the torque-rise behaviour the steepness of the curve may be expressed with the torque/time ratio, which is the stress/strain ratio, i.e., modulus. Between the two gum-rubbers in question, there is a significant difference in modulus. The gum rubber giving a steeper curve is stiffer than the other one. The peak may be interpreted as the failure point [4]. The stiffer rubber has a smaller strain to break and the softer rubber is more deformable. The former tends to fall in Region I of mill processability and the latter in Region II. The former may contain macrogel in a significant amount, if it is an emulsion-polymerised diene rubber. See Chapters 4 and 6 for additional information. 

---