Nonlinear Analysis
The ever increasing use of laminated composite structures in
the aerospace, automotive, chemical and sporting goods
industries, demands better understanding of their material and
structural behavior not only in the linear range but also in the
nonlinear range. To meet the increased application of
laminated composite structures in these industries, NISA II,
known for its outstanding capabilities for modeling composite
structures is now enhanced with nonlinear formulations.
Currently, the four and eight noded degenerated composite
shell elements have nonlinear capability whereas for the solid
element development is in process.
Analysis highlights for nonlinear composites:
Geometrical nonlinearity with total and updated Lagrangian
formulation which includes large displacements and large
rotations.
Material nonlinearity with elastoplastic material model and the
following yield criteria:
- Hill's anisotropic yield criterion with yield parameters that
are updated during deformation history.
- Modified Hill's anisotropic yield criterion (C.F. Shih and D.
Lee) which takes into account the differential strength in
tension and compression and Bauschinger effect.
- Program accepts individual effective stress-strain curves in
the principal material directions. Bilinear, piecewise linear
or Ramberg-Osgood options are available for curved
definitions.
Nonlinear Solution Procedures
- Simple incremental
- Newton-Raphson and Modified Newton-Raphson Iteration
- BFGS, Aitken, and Modified Conjugate Gradient
Techniques
OUTPUT - The linear analysis output and post-processing are
also applicable to nonlinear analysis. In addition, both the
effective plastic strain and the effective stress are output at
Gauss as well as nodal points. History plots are also available
through the post-processor.
Loading and Boundary Conditions
NISA II/Composites allows the user a large variety of loading
conditions as shown below:
- Point loads
- Follower point loads (for non-linear analysis)
- Specified non-zero displacements
- Pressure loads
- Follower pressure loads (for non-linear analysis)
- Body forces (centrifugal and linear accelerations)
- Thermal loads
- Thermal gradient through thickness for shell and beam elements
Kinematic Constraints
Kinematic constraints are relations among the unknowns that must
be satisfied during the solution. The following forms of kinematic
constraints are available:
- Specified displacements
- Rigid Links (elements)
- Multi-point-constraints
- Coupled displacements
Failure Theories
NISA-composites includes the following failure theories:
- Maximumstress
- Modified Hill-Von Mises
- Tsai-Wu
- LDelamination
For material nonlinearity, the following yield criteria are available:
- Hill's anisotropic yield criterion
- Modified Hill's anisotropic yield criterion
All theories are applied on a ply-by-ply basis, in the principal
material coordinate system of each layer. Delamination failures are
predicted on the basis of interlaminar stress exceeding a specified
allowable value.
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