Basic Concepts and Definitions

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This section describes the different types of material models that Patran supports and the methods by which you can input material property data.

Patran provides five homogeneous material types that you may select as the Object on the Materials application form: Isotropic (same properties in all directions), 2D and 3D Orthotropic (properties vary in primary directions), and 2D and 3D Anisotropic (properties vary in arbitrary directions). In general the 2D material types should only be used by planar elements and the 3D formulations should only be used by solid elements.

Type (Object)	Structural Characteristics
Isotropic	Same properties in all directions (two elastic constants).
2D Orthotropic	Properties vary in primary directions (six elastic constants).
3D Orthotropic	Properties vary in primary directions (nine elastic constants).
2D Anisotropic	Properties vary in arbitrary directions (six elastic constants).
3D Anisotropic	Properties vary in arbitrary directions (21 elastic constants).

In addition to the homogeneous materials, you may also define composite materials that are based on layering homogeneous materials using one of several methods. Composites are the most complex materials; Patran provides several subforms and material properties just for composites. The theory and equations that support the composite materials implementation in Patran are described in detail in the Patran Reference Manual, Volume 3, Part 5: Functional Assignments.

In order to define a composite material, you must define the homogeneous materials that form the layers, the thickness of each layer, and the orientation angle of the layers relative to the standard coordinate axis being used for the model. The orientation is particularly important for orthotropic and anisotropic materials, whose properties vary in different directions.

The following table describes the four supported construction methods for composites. Each method is supported with different construction forms within Patran. Some composite forms display a spreadsheet for inputting thickness and orientation values, while other forms hold mathematical formulas. Two of the composite construction methods can be implemented in more than one way; there are five Halpin-Tsai submethods and two Short Fiber submethods.

In a structural analysis, a constitutive model describes the stress-versus-strain behavior of the material properties used in a model. Here are a couple of examples of constitutive models:

Method	Variations	Algorithm	Intended Application
Laminate	Includes a choice of stacking sequence convention for structural analyses.	Classical Lamination Theory	Laminated shells and solids.
Rule of Mixtures	n/a	Volume-weighted averaging	3D composites with multiple phases, arbitrary orientations, and arbitrary volume fractions.
Halpin-Tsai	1. Continuous fiber 2. Discontinuous fiber 3. Continuous ribbon 4. Discont. ribbon 5. Particulate	Halpin-Tsai equations	2-phase composites.
Short Fiber	1. 1D composite 2. 2D composite	Monte-Carlo integration combined with volume-weighted averaging	Short fiber composites whose orientation distribution can be described by a Gaussian curve or surface.

• Linear elastic: the material deforms proportionally to how much force is applied (linear) and returns to its original shape when you remove the load (elastic). In the simplest case, this kind of material can be defined by two constants:

• Poisson's Ratio, which relates strain in different orthogonal directions (analogous to how much peanut butter squirts out the sides of a sandwich when you press the bread slices together).

• Elastoplastic: the stress-versus-strain curve is elastic up to a certain level of stress, and then plastic (that is produces permanent deformation) above this. Since you now need more than a constant to describe this, it is often defined as points on a stress-strain curve.

You can define multiple constitutive models for a single material (such as elastic, plastic, and creep models). For example, a material can have an elastic representation and an inelastic one under the
same name.

You can configure the models as Active or Inactive prior to starting the analysis job. You can also leave more than one model enabled at the same time if needed. Patran attempts to include all Active constitutive models when you submit an analysis. For example, to use a simple elastic model for checkruns, set all other constitutive models to Inactive.

After selecting the type of material model that best represents the behavior of a material, you build the material model by specifying the appropriate material properties. To manually input material property values, you use Patran’s Material Property application forms.

Fields are an Patran feature that allows you to describe how one quantity varies in relation to another quantity. You can use the Fields feature for many applications, including defining a variable
material property.

Materials can be defined to vary as a function of temperature, strain, strain rate, time and/or frequency, using a material property field. You can define a material property field by entering tabular data in the Fields top menu selection. An example would be the dependence of an elastic modulus on temperature.

You can create Material Property Fields that define distributions of any property with respect to any combination of temperature, strain, or strain rate. Materials remain in the database unless specifically deleted and thus provide an archival record. Use the Show action on the Materials form to display properties versus temperature, strain, and strain rate in either tabular form or as XY plots. You can also show the resultant stiffness and compliance matrices.