Description: The following diagnostic enhancements are included in Abaqus/Standard:

• Diagnostic output to the printed output (.dat) file has been added to encourage use of hybrid element types in Abaqus/Standard for cases of nearly incompressible elastic and hyperelastic materials. Hybrid element type names end in “H” (for example, C3D8H). Hybrid elements help avoid numerical problems when a material is incompressible or nearly incompressible. As in past releases, hybrid elements must be used for fully incompressible materials. Use of hybrid elements will often greatly improve convergence behavior when a material is nearly incompressible. In Abaqus 6.13 a warning message is issued to the printed output file for Abaqus/Standard if the effective initial Poisson's ratio exceeds 0.48 (which corresponds to the initial bulk modulus exceeding 24.667 times the initial shear modulus) within non-hybrid elements. By default, an error message is issued instead of a warning message if the effective initial Poisson's ratio exceeds 0.495 (which corresponds to the initial bulk modulus exceeding 100 times the initial shear modulus) due to the high likelihood of numerical problems without hybrid elements in such cases. An environment variable setting (nonhybrid_incompress100_diag = WARNING) is provided to convert this error message to a warning message (allowing the simulation to proceed past the batch preprocessing phase), although this is not generally recommended.

• Warning messages appearing in the message (.msg) file stating that the system matrix has negative eigenvalues, which are issued if Abaqus/Standard detects that the system of equations is not positive definite, have been enhanced such that potential causes listed within the warning message are more specialized to characteristics of the model. Previously, the list of potential causes would include comments about procedures that did not exist in a model. The list of potential causes for negative eigenvalue warning messages now includes an entry related to the use of non-hybrid solid elements with a material having an initial Poisson's ratio exceeding 0.48 if this is a characteristic of the model. For general nonlinear steps, negative eigenvalue warnings are often an indication of numerical or physical instabilities in a model. Convergence behavior is sometimes poor when negative eigenvalue warning messages are issued. Negative eigenvalue warnings do not always indicate a serious problem.