Strain point definition

The data obtained from the resistance measurements are shown in Fig. 4.10. The assigned values of conductivity are limited in accuracy because the measured resistance was found to be somewhat time dependent. The [100] datum at the lowest strain was particularly so and a definite resistance value cannot be assigned to that point. 

It is shown that model, end-linked networks cannot be perfect networks. Simply from the mechanism of formation, post-gel intramolecular reaction must occur and some of this leads to the formation of inelastic loops. Data on the small-strain, shear moduli of trifunctional and tetrafunctional polyurethane networks from polyols of various molar masses, and the extents of reaction at gelation occurring during their formation are considered in more detail than hitherto. The networks, prepared in bulk and at various dilutions in solvent, show extents of reaction at gelation which indicate pre-gel intramolecular reaction and small-strain moduli which are lower than those expected for perfect network structures. From the systematic variations of moduli and gel points with dilution of preparation, it is deduced that the networks follow affine behaviour at small strains and that even in the limit of no pre-gel intramolecular reaction, the occurrence of post-gel intramolecular reaction means that network defects still occur. In addition, from the variation of defects with polyol molar mass it is demonstrated that defects will still persist in the limit of infinite molar mass. In this limit, theoretical arguments are used to define the minimal significant structures which must be considered for the definition of the properties and structures of real networks. 

SMART MATERIALS. From a technical and simple point of view, a smart material is a material that responds to its environment in a timely manner. To expand on this definition, a smart material is one that receives, transmits, or processes a stimulus and responds by producing a useful effect, which may include a signal that the material is acting upon it. Stimuli may include strain, stress, temperature, chemicals, an electric field, a magnetic field, hydrostatic pressures, different types of radiation, and other forms of stimuli. Transmission or processing of the stimulus may be in the form of an absorption of a photon, of a chemical reaction, of an... 

The sample must have reached steady state before cessation of the test or the application of a second step. Steady state in a creep test is seen as a constant slope in the strain curve. A constant slope in the stress curve may also be seen in a stress relaxation test, but often the signal is lost in the noise. A material that is liquid-like in real time will need a test period of 5 to 10 min. A stress relaxation test is likely to be somewhat shorter than a creep test since the signal inevitably decays into the noise at some point. A creep test will last indefinitely but will probably reach steady state within an hour. For a material that is a solid in real time, all experiments should be longer as molecular motion is, by definition, slower. Viscoelastic materials will lie in between these extremes. Polymer melts can take 1 hr or more to respond in a creep test, but somewhat less time in a stress relaxation test. 

As can be seen in Table 4, the "activation energy" derived from the series of exposures that includes silk fabric heated at 150°C predicts a half-life for the strain-to-break of 85 years at 20°C. The half-life at 20°C calculated from the series of exposures that covers the 70°C to 110°C range is markedly different, only 32 years. Such a large effect produced by inclusion of only one data point indicates that results obtained at this temperature may be suspect. By definition, the activation energy must be independent of temperature over the range of extrapolation. 

Fracture properties arguably are not rheological properties, but mechanical properties, given the standard definition of rheology (see above). However, failure of structure in foods, especially high-fat foods, usually begins at strains below the actual fracture point, the point at which the... 

In ductile materials the maximum in the stress-strain curve is the yield point. Two definitions of yield points are in use (see Fig. 13.68) ... 

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