Young time dependence

It is important to differentiate between brittie and plastic deformations within materials. With brittie materials, the behavior is predominantiy elastic until the yield point is reached, at which breakage occurs. When fracture occurs as a result of a time-dependent strain, the material behaves in an inelastic manner. Most materials tend to be inelastic. Figure 1 shows a typical stress—strain diagram. The section A—B is the elastic region where the material obeys Hooke s law, and the slope of the line is Young s modulus. C is the yield point, where plastic deformation begins. The difference in strain between the yield point C and the ultimate yield point D gives a measure of the brittieness of the material, ie, the less difference in strain, the more brittie the material. 

Another manifestation of a time dependence to particle adhesion involves the phenomenon of total engulfment of the particle by the substrate. It is recognized that both the JKR and MP theories of adhesion assume that the contact radius a is small compared to the particle radius R. Realistically, however, that may not be the case. Rather, the contact radius depends on the work of adhesion between the two materials, as well as their mechanical properties such as the Young s modulus E or yield strength Y. Accordingly, there is no fundamental reason why the contact radius cannot be the same size as the particle radius. For the sake of the present discussion, let us ignore some mathematical complexities and simply assume that both the JKR and MP theories can be simply expanded to include large contact radii. Let us further assume that, under conditions of no externally applied load, the contact and particle radii are equal, that is a(0) = R. Under these conditions, Eq. 29 reduces to... 

Rothenfluh A, Abodeely M, Young MW 2000 Short-period mutations ofper affect a douhle-time-dependent step in the Drosophila circadian clock. Curr Biol 10 1399-1402 Rutila JE, Zeng H, Le M, Curtin KD, Hall JC, Rosbash M 1996 The tim mutant of the Drosophila rhythm gene timeless manifests aUele-specific interactions with period gene mutants. Neuron 17 921-929... 

Fig. 2. The time dependency of volume, Young s modulus, long period and weight of the swollen PVA film during soaking in the iodine solution...

Youngs, D. L., Time-dependent multi-material flow with laige fluid distortion. In Numerical Methods for Fluid Dynamics (K. W. Morton and M. J. Baines, eds.). Academic Press, New York, 1982, p. 273. 

A variant is the micro-pipette method, which is also similar to the maximum bubble pressure technique. A drop of the liquid to be studied is drawn by suction into the tip of a micropipette. The inner diameter of the pipette must be smaller than the radius of the drop the minimum suction pressure needed to force the droplet into the capillary can be related to the surface tension of the liquid, using the Young-Laplace equation [1.1.212). This technique can also be used to obtain interfacial tensions, say of individual emulsion droplets. Experimental problems include accounting for the extent of wetting of the inner lumen of the capillary, rate problems because of the time-dependence of surfactant (if any) adsorption on the capillary and, for narrow capillaries accounting for the work needed to bend the interface. Indeed, this method has also been used to measure bending moduli (sec. 1.15). 

Bell IR, Schwartz GE, Peterson JM, et al Possible time-dependent sensitization to xe-nobiotics self-reported illness from chemical odors, foods, and opiate drugs in an older adult population. Arch Environ Health 48 315-327,1993a Bell IR, Schwartz GE, Peterson JM, et al Self-reported illness from chemical odors in young adults without clinical syndromes or occupational exposures. Arch Environ Health 48 6-13,1993b... 

Relaxation Time - Maxwell proposed a model in the 19 century to describe the time-dependent behavior of viscous materials such as pitch or tar. This model has also been applied to plastics and polymers. A parameter has been defined in this model called relaxation time that is a characteristic of the plastic material. Relaxation time is the ratio of viscosity to the Young s modulus of elasticity. 

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