Materials are considered to be isotropic if the properties are not dependent on the direction. The isotropic material properties are listed below. Depending on the element type, analysis type and loads, not all of the material properties may be required.
Mass Density:
The mass density of a material is its mass per unit volume. Mass density is applicable to all structural elements. This property is required in all MES/ nonlinear structural analyses involving gravity or acceleration loads and dynamic effects.
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For quasistatic analyses, increasing the mass density can help to stabilize the solution. Naturally, such analyses cannot include dynamic effects nor gravity loads. 
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See the page "Converting Mass Units" in the section "General Options: Unit Systems" for tips on converting the mass density to the appropriate units. Alternatively, define a Display Unit system that uses the provided units for the mass. 
Modulus of Elasticity:
The modulus of elasticity is the slope of the stress versus strain curve of a material until the proportionality limit, or yield stress. It is also referred to as the Young's modulus of a material. The modulus of elasticity is applicable to all MES/ nonlinear structural elements except for 2D and 3D kinematic elements and is required for all MES/ nonlinear structural analyses.
Thermal Coefficient of Expansion:
The thermal coefficient of expansion is based on the contraction and expansion of the material due to a temperature difference. This is applicable for all MES/ nonlinear structural element types except for 2D and 3D kinematic elements. This is required for any MES/ nonlinear structural model containing thermal loads.
Poisson's Ratio:
Poisson's ratio is found by taking the negative lateral strain and dividing it by the axial strain for an axially loaded member. Typical values for Poisson's ratio range from 0.0 to 0.5. This is applicable for all MES/ nonlinear structural element types except for trusses, 2D kinematic and 3D kinematic elements. This is required for all MES/ nonlinear structural analysis types.
Shear Modulus of Elasticity:
The shear modulus of elasticity is the slope of the shear stress versus shear strain of a material until the proportionality limit. This is also referred to as the modulus of rigidity. This is applicable to all MES/ nonlinear structural element types except trusses, beams, 2D kinematic and 3D kinematic elements. If zero is entered, the software will use the equation to calculate shear modulus of elasticity, where E is the modulus of elasticity and is the Poisson's ratio.
Damping:
This property is only applicable for truss, brick and tetrahedral elements. This damping is not material (or hysteresis) damping as discussed in vibration textbooks. The basic equation for this material damping is:
F = C[k]v
where: C = the damping coefficient (time)
[k] = the stiffness matrix (force/length)
v = relative velocity between nodes in a part (length/time)
Since the material damping is assigned to a part, it affects only that part. This is useful to provided isolated damping to the model.
This material damping should only be used in cases where the user has done sufficient testing and is trying to match the model results to the test data.
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Yield Strength:
When using a "Linear" or "Isotropic" material model with beam elements, the Yield Strength is used for code checking as a parameter for the allowable stress. Otherwise, the yield stress has no effect on the results. That is, plasticity effects are not included. (Use a "von Mises" material model to include the effects of plasticity in the analysis.)