Department of Mechanical Engineering

University of Nevada, Reno

Reno, NV 89557

The objective of this work is to predict the biaxial behavior of the soft tissues (such as cardiac muscle, skin, lung, blood vessel, etc.) using modified fabric flexible composite theory whose input parameters are determined from uniaxial tests through an analytical model. A soft tissue is considered as an orthogonal composite element containing wavy fibers in a soft incompressible matrix. Results show that the fiber stiffness and the degree of fiber waviness play the most important roles for the material deformation. Major accomplishment is here to predict biaxial behavior of the soft tissues using only uniaxial tensile test results. Reasonably good agreement has been found between experimental measurements and theoretical predictions.

The experimental results have shown that the stress-strain curves of fabric flexible composites and the soft tissues, under various biaxial loads, are very similar. The biaxial mechanical behaviors of the soft tissues have been extensively studied. Most researchers believe that the uniaxial data cannot be generalized to characterize multiaxial behavior. A common approach is to propose a set of constitutive equations, then use data regression method to obtain input values from both simple and biaxial test results. This usually gives a good description of the stress-strain behavior for the particular material but gives no physical meaning of these input values obtained from the mathematical curve fittings.

Microstructural approach is, generally, more advantageous compare to the phenomenological approach (Humphrey and Yin, 1987), it is however rigorous to apply.

Pao et al. (1980) suggested that the mechanical properties of the fiber bundles must be predicted first to compute the stresses.

The biaxial stress-strain relation of a soft tissue can not be obtained by superimposing two individual uniaxial test results due to the large deformation (Mitra, 1995). On the other hand, classical solid mechanics will not be sufficient to study these types of materials due to the fact that there is a significant amount of geometrical rearrangement within the fibers because of the deformation and this results in a highly nonlinear problem (Luo and Chou, 1990).

Many biological materials are incompressible, viscoelastic and anisotropic; they often demonstrate nonlinear behavior with a large deformation range.

During the initial stage of the material deformation, the fibers get straightened out with little stretching. Thus, the composite is usually "soft" and the stress-strain relation is dominated by the matrix properties. Intermediate stage shows very complicated and interesting phenomenon regarding matrix-fiber interaction. At the final stage of the deformation, the straightened fibers are much stiffer than the matrix, hence carry most of the load and the composite becomes "stiff". The overall deformation of the composite mainly depends on the fiber stiffness and the length to which the fibers can be straightened (degree of fiber waviness), as well as the matrix property. First part of this paper discusses how to characterize these parameters from a uniaxial tension test. Second part deals with predicting biaxial behavior of the soft tissues by applying the modified fabric flexible composite theory with known input parameters. Present work is the first attempt, our knowledge, to predict the biaxial behavior of the soft tissues using the uniaxial experimental data.

A passive cardiac tissue is selected to test the model due to extensive experimental data available and being a subject of great interest for many years.