Preparing CAD for FEA starts with a clean solid model of the parts that carry load, in STEP or native format, with units stated and materials identified. The geometry is then simplified: features that do not affect the result, such as logos, cosmetic threads, small chamfers and holes far from the load path, are removed or suppressed, while fillets, holes, ribs and joints in the load path are kept. Fasteners and contacts are given a deliberate representation, thin parts may be reduced to midsurfaces, and for CFD a closed fluid volume is created from the space the fluid occupies.
The aim of CAD defeaturing for simulation is a model that answers the question with as little unnecessary detail as possible. Every small feature adds elements, and very small features force very small elements around them, which increases solution time without improving the answer. Removing the wrong feature does the opposite: take out the fillet where the peak stress sits and the result is wrong at the one place that matters. The principle is easy to state. Keep what lies in the path of load, heat or flow, and simplify everything else.
Which files to send
- STEP. The most dependable neutral format for solid geometry, carrying units and assembly structure. AP214 and AP242 are both widely supported.
- Native files. Autodesk Inventor files and most other native formats can be accepted. They keep features and parameters, which makes it quicker to suppress details or try design changes.
- IGES. Older and surface-based. Files often arrive as surfaces with small gaps rather than closed solids, and need repair before they can be used.
- STL and other mesh formats. Faceted and without units. Sometimes usable for external CFD around a scanned shape, but poor for structural analysis, where curved surfaces and accurate fillets matter.
Send the assembly with meaningful part names, and say which parts matter to the question. A note of the units is worth adding even when the format carries them, since a part modelled in inches inside a metric assembly is a common source of confusion.
Defeaturing: what to remove and what to keep
| Feature | Usually remove or simplify | Keep when |
|---|---|---|
| Logos, text and embossing | Almost always | Practically never, unless they shape a surface the flow depends on |
| Small fillets and chamfers | Away from areas of high stress | At internal corners in the load path, where they set the peak stress |
| Small holes | Far from the load path | Near high stress, or where they carry fasteners or coolant |
| Modelled threads | Replace with plain cylinders | Only when the thread itself is being assessed |
| Draft angles | Where they complicate the mesh and barely change stiffness | On thin moulded walls where thickness varies along the draft |
| Ribs and gussets | Rarely | Almost always: they carry load and stiffen walls |
| Cables, labels, seals and small internal parts | Remove, or replace with point masses | When they carry load, block flow or conduct heat |
| Small gaps and overlaps | Close or remove | When the gap is real and controls flow, heat or contact |
Two features need particular care. Fillets at internal corners control the local stress, as explained in our guide to stress concentration factors, so they stay wherever load passes. Unintended overlaps between parts, sometimes only a few tenths of a millimetre in an assembly built for appearance, become interference contact in a structural model and blocked passages in a flow model, so they need resolving one way or the other.
Fasteners and threads
Bolts and screws can be represented in several ways, and the right one depends on the question:
- Bonded joints. The parts are tied together at the contact faces or around the bolt holes. Suitable when the question is overall stiffness, or stress well away from the joint.
- Simplified bolts. Each bolt is represented by a beam or connector between the holes, with a preload. Suitable for many bolted frames and flanges where the joint loads matter.
- Solid bolts with contact and preload. Bolt, nut, washer and clamped parts are modelled as solids with contact between them. Needed when the question is whether a joint separates or slips, or how much fluctuating load the bolt carries in fatigue.
In each case, threads are usually removed and the threaded length tied to the hole, since detailed thread stresses need a dedicated local model. Supply the bolt size, property class (such as 8.8), number of bolts and tightening torque or preload, and say which joints might separate or slip in service. Welded joints are usually represented by merging the parts, with the weld geometry included where the weld itself is under assessment.
Contact between parts
Wherever two parts touch, the model needs to know how they interact. Bonded contact ties them together and keeps the analysis linear. Frictionless or frictional contact lets them slide, open and close, which is closer to reality in many assemblies but makes the analysis non-linear, because the load path changes as surfaces separate. For contact to be represented cleanly, faces that touch should be coincident in the CAD or have a deliberate, known gap. Press fits and interference fits should be flagged as intentional, with the interference stated, so that they are modelled as fits rather than corrected as CAD errors.
Thin parts and midsurfaces
Sheet metal, thin castings, plastic housings and panels are thin compared with their other dimensions. Meshing them as solids needs several elements through the thickness for bending to be represented well, which multiplies the size of the model. Shell elements avoid this: the part is represented by its midsurface, a surface halfway through the wall, and the thickness is assigned as a property. A wall thinner than about a tenth of the part’s other dimensions is a common guide to when shells are appropriate.
Midsurfaces extract cleanly from constant-thickness sheet metal, and less cleanly from castings and mouldings whose walls vary, where thickness has to be mapped region by region. Slender frames built from tube, box section or angle can be reduced further, to beam elements with section properties. For the same reason, sheet metal sent as native sheet metal features, with its thickness defined, is easier to work with than the same part as a generic solid.
Fluid volumes for CFD
CFD solves the flow in the space the fluid occupies, but CAD normally models the solid parts around that space. The first task in preparing a CFD model is therefore to create the fluid volume:
- Internal flow through pipes, valves, manifolds or enclosures needs every inlet and outlet capped so the internal space can be extracted as a closed volume. Any gap at a joint or seal becomes a leak path, so the model must be watertight.
- External flow around a vehicle, structure or product needs an enclosing domain large enough that its boundaries do not disturb the flow near the object, commonly several body lengths upstream and further downstream.
- Conjugate heat transfer needs both: the solids that conduct heat and the fluid around them, sharing their interfaces.
Small gaps need a deliberate decision. A clearance of a fraction of a millimetre between a heat sink and its duct can carry a real bypass flow, while a gap between two panels that is sealed in the real product should be closed. Filters, grilles and heat exchanger cores are usually represented as porous regions with a known pressure drop, and fans as boundary conditions using their fan curves, rather than modelling every passage and blade.
Units, materials and properties
Name a material and grade for every significant part, such as 6082-T6 aluminium, S355 steel or PA66 with 30 % glass fibre, along with any temper, heat treatment or surface treatment. For plastics, state whether the grade is filled or unfilled and, where moisture matters, whether it is dry or conditioned: polyamides lose a considerable part of their stiffness as they absorb water. Glass-filled mouldings are also stiffer along the flow direction than across it. If parts operate well above or below room temperature, give the temperature range, since stiffness and strength change with it.
Load and boundary information
Geometry is only half the model. The other half is how the part is loaded and held:
- Loads: magnitude, direction, where and how they are applied, whether they are static, cyclic or sudden, and how many times they occur.
- Supports: how the part is actually held, such as bolted to a stiff frame, resting on rubber mounts or clamped in a fixture. Supports that are stiffer in the model than in reality change the stresses.
- Thermal conditions: power dissipation, ambient temperature, coolant flow and temperature, and any heating or cooling cycles.
- Flow conditions: flow rates or pressures at inlets and outlets, fluid properties, temperatures and fan or pump data.
- Criteria: what counts as acceptable, such as allowable stress, deflection, temperature or pressure drop, and the code or standard you specify.
When to bring in a specialist
None of this has to be perfect before an enquiry. Send the model you have and the questions you need answered; preparing the geometry is part of the study, and knowing which features matter is easier when the design intent is explained. SixGate™ accepts STEP and most native formats. The team designs in Autodesk Inventor, so the analysis model and the production model can be kept in line as the design changes, and the same people can take the result through detail design to the manufacturing drawings and production. Find out more about our FEA and CFD work, or tell us about your project.