Use xStiff to Insert Extraneous Stiffness Parameters

Add extraneous stiffness parameters to your Helius PFA user-defined materials.

For Abaqus/Standard to define the stiffness matrix for a particular element, it must be able to answer the following four questions:

1. Which materials reside within the element?
2. How is each material distributed within the element?
3. How is each material oriented within the element?
4. Beyond the basic stiffness that the element obtains directly from the constitutive relations of the materials present within the element, are there any additional, extraneous stiffness parameters that are needed to improve, or stabilize the behavior of the element?

An understanding of these extraneous stiffness parameters becomes important when using Helius PFA materials. When using standard Abaqus materials, the extraneous stiffness parameters are calculated automatically. Helius PFA materials are not standard Abaqus materials, but are instead user-defined materials. Abaqus is unable to calculate the extraneous stiffness parameters for user-defined materials. You must compute the required extraneous stiffness parameters and include them in the appropriate section definition. This process is complex because the extraneous stiffness parameters are dependent upon the specific combination of element type and section definition type. This complex process of manually calculating and entering extraneous stiffness parameters led to the creation of an automatic extraneous stiffness parameter calculator (xStiff).

xStiff allows you to take a standard input file containing user-defined materials and automatically create a new input file containing all the required extraneous stiffness parameters. The calculator is a command-line tool that is called from the Helius PFA 2015 Command Shell.

Note: Helius PFA cohesive materials and Abaqus/Explicit analyses do not require any extraneous stiffness parameters.

xStiff supports the following Abaqus section definitions:

• Solid sections (homogeneous and composite)
• Shell sections (homogeneous and composite)
• Membrane sections

The following are limitations in xStiff:

• xStiff does not support *INCLUDE keywords. These may exist in the input file, but if important information pertinent to the xStiff calculations exists in the exterior file, behavior of xSTIFF is undefined.
• xStiff will not calculate extraneous stiffness parameters if they already exist for a section.
• If a section does not need extraneous stiffness parameters, e.g. a section only contains fully integrated solid elements, xStiff will still compute the parameters and insert them into the new input file. Abaqus will simply ignore any extraneous stiffness parameters it does not need.