Viscoelastic Type Research Articles

Comparison of mathematical models for cat lung and viscoelastic balloon derived by Laplace transform methods from pressure-volume data.

The mechanical properties of some hollow organs are most conveniently described by a pressure-volume relationship. If the material exhibits hysteresis, thep-v relation must include provision for time-dependent or path-dependent properties. Provided the amplitude of deformation is fairly small and the hysteresis is primarily of the viscoelastic type, a linear description is possible. That this may take the form of a simple transfer function in which material properties are implicit is illustrated for the case of a rubber balloon. The transfer function was derived from the pressure transients which follow step changes in volume produced in a fluid-filled plethysmograph. The applicability of the transfer function in predicting responses to other forcing functions was tested by varying the balloon volume sinusoidally over a frequency range of 1000, at 4 different amplitudes. The good agreement between the linear model and all types of data justifies the use of Laplace transform methods and the assumption that superposition holds. When isolated cat lung is tested in the same manner, the transfer function quantitatively predicts the magnitude ratio of sinusoidal responses but only about two-thirds of the phase angle. The additional energy loss per cycle is interpreted as arising from static hysteresis. The analysis thus provides a simple means of estimating the relative contributions of viscoelastic (dynamic) and static hysteretic processes to the total damping in a material.

On the disturbances in an infinite viscoelastic medium due to transient normal forces and transient twists on the surface of a spherical cavity

In this note the disturbances in an infinite viscoelastic medium ofViogt type with a spherical cavity have been considered. The problem has been solved separately for (1) transient radial forces and (2) transient twist acting on the inner spherical surface of the cavity. Exact solutions have been obtained in both cases.

Rheology of Surface Films. IV. Viscoelastic Properties of 6-Nylon Films at Air/Water Interface

Abstract (1) Using a surface-rheometer, the straintime curves at constant shear stress, the stress-relaxation curves at constant strain, and the stress-strain curves at constant rate of strain were determined with the spread films of 6-nylon at air/water interface, and the rheological data were measured in relation to their dependence upon stress, temperature, and molecular weight of the sample under varying compressions, and were arrayed in the light of the mechanical models. (2) It was found that the rheological characteristics of the film were classified into two types according to the degree of compression, i.e., a simple elastic type at large areas which involves a Hookean elasticity alone, and a viscoelastic type at considerably small areas, which is characterized by an instantaneous elasticity, the delayed. elasticity, and stationary flow under constant stress. (3) The stress-strain curves at constant rate of strain revealed that the film, which was simply brittle at large areas transformed to tough as the compression proceeded, the mechanical properties resembling to that in bulk. (4) It was also observed that the film strength, which was readily set up after compression, was considerably improved by the rhythmic strains applied to it, indicating the two-dimensional rheopexy. (5) Instantaneous elastic modulus versus-area curve showed a conspicuous plateau at the areas from about 33 to 24 Å2/residue, which suggests that at least two different phases are experienced during the process of compression. (6) Temperature dependence of the instantaneous elasticity suggests that the elasticity is mainly due to the energy effect rather than entropy effect. (7) Molecular weight variations of the film elasticity and of the gelation area were examined together with their pH dependence of the substratum, and discussions were made in molecular terms. (8) The stationary flow under constant stress behaved as a plastic flow, which is characterized by a yield stress above which flow practically begins to occur. Apparent activation energy of flow was estimated as about 6 kcal./mol. (9) Overall forms of the strain-time curves and of the stress-relaxation ones were found to be well fitted by the mechanical models of six parameters. (10) From the temperature dependence of the relaxation times, it was suggested that at least two different modes of relaxation mechanism do exist within the films of high compression.