Elongational stress

Mechanical and Chemical Properties. Colorants, especially pigments, can affect the tensile, compressive, elongation, stress, and impact properties of a polymer (5). The colorants can act as an interstitial medium and cause microcracks to form in the polymer colorant matrix. This then leads to degradation of the physical properties of the system. Certain chemicals can attack colorants and there can be a loss of physical properties as well as a loss of the chromatic attributes of the colorant. Colorants should always be evaluated in the resin in which they will be used to check for loss of properties that ate needed for the particular appHcations. 

For freely suspended bioparticles the most likely flow stresses are perceived to be either shear or normal (elongation) stresses caused by the local turbulent flow. In each case, there are a number of ways of describing mathematically the interactions between turbulent eddies and the suspended particles. Most methods however predict the same functional relationship between the prevailing turbulent flow stresses, material properties and equipment parameters, the only difference between them being the constant of proportionality in the equations. Typically, in the viscous dissipation subrange, theory suggests the following relationship for the mean stress [85] ... 

The reduction in stress which takes place in a test strip of rubber held at constant elongation. Stress relaxation measurements are used in the study of the ageing of rubber vulcanisates, the degradation of the network structure resulting in a reduction of the tension. 

Physical characterization of polymers is a common activity that research and development technologists at the Dow Chemical Company perform. A material property evaluation that is critical for most polymer systems is a tensile test. Many instruments such as an Instron test frame can perform a tensile test and, by using specialized software, can acquire and process data. Use of an extensometer eliminates calibration errors and allows the console to display strain and deformation in engineering units. Some common results from a tensile test are modulus, percent elongation, stress at break, and strain at yield. These data are then used to better understand the capabilities of the polymer system and in what end-use applications it may be used. 

Figure 9 shows the result by Fukada, Date, and Hirai (1966) for uniaxially drawn poly(y-methyl L-glutamate) film (PMLG). The conformation of the chains is predominantly the a-helix. The sample was drawn twice as the original length and the frequency of stress was 20 Hz. The observed anisotropy is well expressed by the tensor (59) which leads to the d-constant of the film for elongational stress in the film plane (yz-plane),... 

If the duration and intensity of the elongational stress are sufficiently large, the macromolecule can be stretched to a greater extent. Keller was able to prove this using a cross-slot device producing a planar elongational flow field with the aid of flow diffraction. 

Lodge rubberlike liquid (3.3-15) Constant = constant l/2 = 0 No predicts elongational stress growth rj1 (l, k) Yes... 

Figure 7.14. Schematic of fat crystal network under extension when the weak-link theory is applicable ( is the diameter of crystal clusters, La is the size of the microscopic system, AL is the extension due to elongational stress, a is the size of a primary particle within a cluster and da is the interfloc distance).

This latter method can also be used in uniaxial elongation using the transient elongational stress (OE(t,e) = Xii(t,e) - T (t,e)) ... 

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