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Getting Confidence in FEA Design Optimisation

July 6, 2021
Posted in Posts
July 6, 2021 Nikita Gurov

FEA Design Optimisation

Every engineer wants to make confident designs based on Finite Element Analysis. We want to develop a product knowing that it’s strong and perfectly optimised for real world usage, without having to apply time consuming prototypes testing or using only traditional methods of hand calculation.

This is where design optimisation using FEA can deliver incredible benefits. It’s a fast, accurate and inexpensive way to create and test product designs. It provides an opportunity to look inside the design using a powerful magnifying FEA lens and also a magic wand to instantly change the design.

Why the traditional design-analyse approach add design complications  

In the beginning of my stress engineering career, I found that the traditional Design-Analysis cycle was time consuming and expensive. There were two main reasons for inefficiency and high cost of the design optimisation:

CAD & CAE packages integration

A lack of a proper connection between CAD and FEM software. As a result, even minor modification of the design led to the recreation of the FEA model, including geometry preparation and manual mesh updates. This was fine for simple designs but could turn into a nightmare for assemblies.

Designer & Stress Analyst roles

The design process was divided into two different and very separate functions – the design itself (creating shapes etc.) and its stress analysis. In most cases these functions were completed by different people – designers and stress analysts. Thus, every iteration of the design and its further analysis required extra communication between designers and stress analysts. The result was often an overengineering as designers were making conservative guesses due to the lack of FEA knowledge.

Combined, these reasons added extra layers of complication while attempting to optimise design using FEA.

In today’s CAD and CAE software market the first issue has been largely resolved – FEM is either integrated in CAD packages as an add-in or it easily consumes CAD files.

Autodesk Inventor Nastran is a great example of a FEA tool seamlessly integrated into a CAD package, so that the designers can use FEA as they produce their designs and complete optimisation faster.

Do you want to improve your FEA skills or get advice on your FEA model?

FEA is the perfect tool to deliver engineers confidence with their inventions

When dealing with design optimisation, there are two ways.

Typology Optimisation

Topology Optimisation is a finite element tool that helps to generate a new optimum shape by maximising efficient use of material.

Quick and efficient way to find optimum shape.

Often the optimum shape is too expensive from a manufacturing perspective as it doesn’t take into consideration available manufacturing capabilities. So, to produce shape after Topology Optimisation may require advanced machining or 3D printing.

Good tool for prototypes. It gives the designers good insights at the conceptual design stage.

Shape Optimisation

Shape Optimisation helps to modify an existing design to eliminate its deficiencies.

Manual process of trial and error but it could be automated through parametric design study.

Helps to modify an existing design to eliminate its deficiencies taking into consideration available manufacturing capabilities. So, it can be used for any manufacturing methods.

Good tool for any design stages.

Shape Optimisation Scenario

In the following test scenario I’ll focus more on effective workflows of Shape Optimisation using the tow bar model.

In Static Linear, Static Nonlinear & Explicit: Basic Principles to Test Your Own Design I showed how to identify design ultimate loads for the tow bar model using Autodesk Inventor Nastran and discovered that the reasonable design load is 5,000N.

Following on this example I would like to optimise tow bar design and lower the maximum stress via Shape Optimisation.

Model Settings

Let’s imagine that as a result of careful considerations we set the design target to keep the maximum stress under 200MPa.

Weldments are included in the model using solid elements. This allows us to create a relatively realistic model of welded connections. It is acknowledged that there are several methods for weldments analysis that are out of scope of this study.

Model symmetry is used to reduce the number of elements and simplify the model.

Model is hybrid, i.e. it contains shell and solid elements. This allows for both model simplification and more detailed analysis at the same time. Please refer to Shells vs Solids article to learn the difference between different element types.

Shell vs Solid Elements: are they similar?

Tensile Strength, Yield – 285MPa (41.3ksi)

Disclaimer: the model is built to demonstrate a Shape Optimisation approach only, with no intent to confirm that the design meets any industry standards. Also, a detailed weld analysis is not in the scope for this article.

Step 1

Original design

We use original tow bar design and FEA as a starting point for the Shape Optimisation process.

There is a spot (coloured in grey) at the corner of the safety chain ring where stress increases the desirable limit of 200MPa.

Do you want to improve your FEA skills or get advice on your FEA model?

Iteration 1

Remove the safety chain ring

It looks obvious that the root of this issue is in the stiff safety chain ring attached at a 90-degree angle. So, let’s find out what’s going to happen if we remove the ring.

It certainly looks better, so one option to improve design is to attach the safety chain ring at the bottom of the receiver tube. However, the stress value of 247MPa is still higher than our target maximum of 200MPa.

Now I will continue searching for the design configuration that hits the design target.

Click on image to enlarge…

Iteration 2

Modify safety chain ring

From the original design we learned that the perpendicular connection between the tube and plate is the main source of high stress values.

So, is it possible to use the safety chain ring attachment plate as the main tube reinforcement and what if we can make gradual increases in bending stiffness?

Through several pretty simple modifications of the safety chain ring design I found the optimal shape that doesn’t “cut” material off the main tube but actually reinforces it.

It seems that this idea works as the maximum stress value is now under 200MPa.

Click on image to enlarge…

Do you want to improve your FEA skills or get advice on your FEA model?

When is Shape Optimisation successful?

The beauty of modern CAD/CAE software like Autodesk Inventor Nastran is that it allows for quick modifications of the design and its subsequent FEA. It took me less than an hour to prepare and analyse all tow bar configurations. How good is that?

OK, let’s now talk about using FEA optimisation to produce stable and close to reality results.

Shape Optimisation is one of the benefits that FEA can bring to the design process. It opens up the opportunity to create better and more reliable designs without spending extra time and money on prototype testing.

Although modern software technology allows for quick Shape Optimisation there are still many engineers struggling to get reliable and stable results when using FEA. This significantly decreases the level of confidence they are receiving when making decisions based on their FEA models.

As many engineers know there’s nothing more frustrating than producing poor quality design.

That’s why it does not surprise me when I often get asked by engineers

How do I know that my finite element model is reliable?

How do I get FEA results that are close to the practical testing?

Why does my FEA model give unstable and unrealistic results?

This is where a good understanding of FEA processes comes into play.

Introducing rocket-science technologies as FEA to a wider engineering community has incredible benefits as it allows for greater efficiency and quality of engineering designs and products.

But there still remains a danger of developing FEA models that are not giving correct results and lead to poor engineering decisions.

10 Key FEA Steps

Every engineer can benefit from

Over the years of practicing and teaching FEA I learned that every FEA model can be prepared and solved while delivering realistic results if an engineer is following specific steps.

That’s why I’ve taken my knowledge and experience and condensed them into 10 Key FEA Steps that I use in my own work. These steps can be easily followed to optimise my product design, regardless of what stage I’m at or goals I’m trying to achieve.

Depending on what I’m trying to achieve, I use different levels of FEA sophistication while applying the same 10 Key FEA Steps. Using the appropriate FEA approach at various design stages gives required results without spending unnecessary extra time or wasting materials.

Refer to the table below to get a general understanding of the FEA approach for different design stages.

Do you want to improve your FEA skills or get advice on your FEA model?

The next steps in your FEA journey

  • Would you like to learn practical FEA and start confidently using it in your everyday work?
  • Are you looking for a highly accessible and practical FEA course where you can build finite element models from scratch?
  • Do you want to learn a range of FEA tricks and time-saving tools for your own designs?

Then I would love to hear from you! Please get in touch – fea@wordpressmu-726877-2424848.cloudwaysapps.com

If you would like to learn more about my FEA journey and my approach, please click here.

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