Material Property Check

In order to prevent analyses from being carried out on models with poor material definitions, a material property check is incorporated into the pre-processing phase. The material property check is controlled by the CHECKMAT parameter (see the PARAM input format).

There are several levels of requirements that material data needs to satisfy:

1.	Symmetry requirements - These are assured by the input formats.

Mathematical requirements - These have to be satisfied so that the stiffness matrix can be formulated at all, and are usually not very strict. For example, to avoid division by zero, MAT1 in 3D must have n¹-1 and n¹0.5. Composite homogenization adds additional mathematical requirements. Failure to meet these criteria will always result in an error termination.

3.	Stability requirements - By default, "semi-stability" combined with "not-all-zeros" stability based on the following definitions is required:

•	Full stability - This means that the material is stable; when pulled it stretches rather than shrinks, etc. For example, E>0 assures stability for a rod. In a mathematical sense, stability assures that the stiffness matrix (with proper support) will be Semi-Positive Definite (SPD).

•	Semi-stability - This is a slight extension of stability to include borderline cases such as allowing E=0. It guarantees that the stiffness matrix (with proper support) will be Semi-Positive Semi-Definite (SPSD). However, this can lead to infinite or very large compliances.

•	Not all zeros - This is an additional requirement that at least one deformation mode of the material is active (non-zero stiffness). This avoids element stiffness matrices that are identically zero. However, it does not prevent infinite or very large compliances.

An error termination will occur if a material has negative stiffness.

A warning is produced if any individual mode of deformation has zero stiffness.

An error termination will occur if all modes of deformation have zero stiffness.

4.	Consistency requirements - This is a requirement that user provided data be internally consistent. For example, specifying E, G and Nu for isotropic material may lead to inconsistent data.

5.	Practical material requirements - These are requirements that correspond to typical practical materials. For example, while stability requirements allow for negative Poisson's ratio, the natural materials usually have positive n (although some composites and nano-materials with negative n exist).

You may choose to perform only those checks that are necessary to avoid crashes in the element routines (that is the Mathematical Requirement checks). This is facilitated through the specification of PARAM,CHECKMAT,NO in the bulk data section of the input deck.

The checks performed for MAT1, MAT2, MAT3, MAT8, and MAT9 are outlined in the following topics:

Material Property Checks for MAT1

Material Property Checks for MAT2

Material Property Checks for MAT3

Material Property Checks for MAT8

Material Property Checks for MAT9