Question

Why is a 3D CAD model more than just a "digital drawing" when used for engineering simulation, and what key information must be added to this geometric model to perform a meaningful analysis like a stress test?

Answer 1

This question highlights the fundamental difference between defining an object's form (CAD) and predicting its function (Simulation). While a 3D CAD model is the essential starting point, it is merely a geometric shell—a perfect, lifeless description of shape. To perform a meaningful simulation, this shell must be imbued with the properties of physics.

Here is the breakdown of what a CAD model provides and the critical information that must be added for simulation.

Part 1: The 3D CAD Model – The Geometric Foundation

A modern 3D CAD model (from software like SolidWorks, Fusion 360, or CATIA) is far more than a simple drawing. It is a precise mathematical database that defines:

• Exact Geometry: The surfaces, curves, and solid volumes of the object with perfect accuracy.
• Topology: The relationship between geometric entities (which edges connect to which faces, etc.). This is crucial for the simulation software to understand the object as a continuous whole.
• Assembly Relationships: In a multi-part model, it defines how components are mated, constrained, and positioned relative to one another.
• Basic Metadata: It may contain preliminary information like the assigned material name (e.g., "AISI 1020 Steel") and the resulting mass properties.

However, this model knows nothing about how it will behave under real-world conditions. It cannot bend, break, heat up, or experience force. It is simply a perfect, rigid blueprint.

Part 2: Transforming Geometry into a Physics Model for Simulation

To perform a meaningful analysis, such as a Finite Element Analysis (FEA) stress test, an engineer must add several layers of physical and environmental information. This process transforms the geometric model into a "physics model."

The key information that must be added is:

1. The Mesh (Discretization)
The first and most critical step is to break down the model's complex, continuous geometry into thousands or millions of small, simple, interconnected shapes (elements) like triangles or tetrahedrons. This network of elements is called a mesh.
* Why? It is mathematically impossible for a computer to solve physics equations (like stress/strain) on an infinitely complex shape. By discretizing the model, the computer can solve these equations for each simple element and then combine the results to approximate the behavior of the entire object. The quality of the mesh directly impacts the accuracy of the simulation.

2. Material Properties
The basic material name from the CAD model is not enough. The simulation needs specific, quantitative physical properties that define how that material behaves. For a mechanical stress test, this includes:
Young's Modulus (Modulus of Elasticity): How stiff is the material? How much does it resist being deformed elastically?
Poisson's Ratio: When the material is stretched in one direction, how much does it thin out in the other directions?
Yield Strength: The amount of stress at which the material will begin to deform permanently (plastic deformation).
Ultimate Tensile Strength: The maximum stress the material can withstand before it begins to fracture.
* Density: The mass per unit volume, which is essential for calculating the effects of gravity or acceleration.

3. Loads and Boundary Conditions (The Environment)
This step defines how the object interacts with the real world. Without this, the simulation is meaningless.
Boundary Conditions (Constraints): This defines how the object is held in place. Are certain faces fixed, as if welded to a wall? Is it sitting on a flat surface? Is it attached with virtual bolts? Without constraints, applying a force would simply cause the object to accelerate into infinity according to Newton's laws.
Loads: This defines the external forces or conditions being applied to the model. This can be a force (e.g., 500 Newtons applied to a specific face), pressure (e.g., 150 PSI inside a tank), gravity, or a thermal load (e.g., one side is heated to 300°C).

In summary:

A CAD model answers the question, "What is it?" by defining its shape, size, and assembly.

A Simulation answers the question, "What will it do?" by taking that shape and applying a mesh, real-world material properties, and physical interactions (loads and constraints). This allows engineers to see a visual prediction of high-stress areas, deformations, or temperature gradients, enabling them to identify and fix design flaws before a single piece of metal is cut.