Getting Started with Topology Optimisation in MSC Nastran SOL 200: A Practical Guide

Topology optimisation answers a fundamental engineering question: given a block of material, a set of loads, and a set of constraints, what is the most efficient shape? The result is a structure that places material only where it contributes to load transfer, removing everything that does not carry stress. The output is not a finished design, but a concept geometry that reveals the optimal load paths, which the designer then interprets into a manufacturable shape.

MSC Nastran SOL 200 provides topology optimisation integrated with the structural solver, the same SOL 200 that also handles sizing optimisation (topometry) and shape optimisation (topography). This integration means the optimisation has direct access to all of Nastran’s structural analysis capabilities: static loads, multiple load cases, modal frequencies, buckling eigenvalues, and frequency response targets can all drive the optimisation simultaneously.

The Topology Optimisation Workflow

Step 1: Define the Design Space

The design space is the volume within which the optimiser is free to add or remove material. Everything outside this volume, bolt holes, mating surfaces, packaging boundaries, is designated as non-design space and preserved throughout the optimisation.

Defining the design space correctly is the most important step. Too small a design space constrains the optimiser artificially. Too large a design space includes regions that cannot physically contain material (interfering with adjacent components).

Practical tip: start with the maximum envelope that packaging and assembly allow, then subtract non-design regions for mating interfaces, bolt patterns, and clearance zones.

Step 2: Define Loads and Boundary Conditions

The optimiser needs the same load and boundary condition definitions as a standard Nastran analysis. For a structural bracket, this typically includes:

• Applied forces at attachment points (from Adams load extraction or hand calculations)
• Boundary conditions at mounting locations (fixed, pinned, or spring-grounded)
• Multiple load cases representing different operating conditions

SOL 200 optimises against all load cases simultaneously, the resulting topology satisfies every load case, not just the worst one. This multi-load-case capability is essential for real engineering problems where a bracket must handle loads from multiple directions.

Step 3: Set the Objective and Constraints

The most common topology optimisation formulation is:

• Objective: Minimise compliance (maximise stiffness), or equivalently, minimise strain energy
• Constraint: Volume fraction, the optimiser must use no more than a specified fraction of the design space (e.g., 30% of the original volume)

Alternative formulations include minimising mass subject to stress or displacement constraints, maximising the first natural frequency (for vibration-sensitive components), or maximising the buckling eigenvalue.

Step 4: Apply Manufacturing Constraints

Raw topology optimisation produces mathematically optimal but often unmanufacturable shapes, organic, lattice-like structures with internal voids and undercuts. Manufacturing constraints guide the optimiser toward shapes compatible with the intended manufacturing process:

• Minimum member size: prevents thin features below the manufacturing resolution
• Draw direction: ensures the result can be extracted from a casting mould or formed in a stamping die
• Symmetry: enforces geometric symmetry planes
• Extrusion constraint: forces uniform cross-section along an axis (for extruded profiles)

For Indian manufacturing, the draw direction constraint is particularly important when the optimised part will be produced by investment casting or die casting, the geometry must be mouldable without undercuts.

Step 5: Run the Optimisation

SOL 200 iterates between structural analysis and design variable update. Each iteration:

1. Performs structural analysis for all load cases
2. Computes sensitivity of the objective and constraints to design variable changes
3. Updates the element density distribution (removing material where it is not needed)
4. Checks convergence

Typically 20-40 iterations are needed for convergence. Compute time scales with the number of elements in the design space and the number of load cases, but is generally manageable, a model with 500,000 elements and 5 load cases runs overnight on a modern workstation.

Step 6: Interpret and Redesign

The topology optimisation result is not a finished design. It is a density field where each element has a value between 0 (void) and 1 (solid). The engineer interprets this result:

1. Identify the primary load paths (regions of high density)
2. Clean up the topology, remove disconnected islands, smooth boundaries
3. Create a CAD geometry that follows the optimised load paths while respecting manufacturing, assembly, and aesthetic requirements
4. Verify the redesigned part with a standard Nastran analysis (SOL 101/103) to confirm it meets all structural requirements

Beyond Topology: Topometry and Topography

SOL 200 also provides:

Topometry optimisation: optimises the thickness distribution of shell structures (e.g., body panels, stiffened shells). Each element can have a different optimal thickness. Useful for aerospace skin panels and automotive floor panels where adding thickness in high-stress regions and thinning low-stress regions saves mass.

Topography optimisation: determines the optimal bead pattern for sheet metal structures. Beads (raised features stamped into sheet metal) increase local stiffness. Topography optimisation places beads where they provide the greatest structural benefit.

Why Buy from GSAS

GSAS provides MSC Nastran licensing in India, including SOL 200 optimisation capability, with INR invoicing and training workshops. Our application engineers support teams in Bengaluru, Hyderabad, Chennai, Pune, Mumbai, Delhi NCR, and Visakhapatnam with topology optimisation workflow setup, manufacturing constraint definition, and result interpretation.

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