Finite Element Analysis – Predicting the Real World

Overview

When designing products and components, it is vital to ensure that their integrity and reliability within the bounds of ‘normal usage’ will not be compromised.

Alongside product and technology design processes, Finite Element Analysis (FEA) can be used to model how product structure and will respond to stress, and from this to predict any possibility of structural failure.

At Cambridge Design Technology we use FEA alongside or instead of prototyping (where all eventualities must be modelled or tested).

Why is FEA used?

When mechanical designing a product or mechanical engineering component, FEA is used to predict how a solid body (3D CAD model) will respond to real world physical effects.

Examples of these physical effects can include repeated impact or stress, the strain from sudden or prolonged loading, vibration and temperature changes – as well as a variety of other usage and condition scenarios.

In each of these conditions, FEA will predict whether, and if so how the part would fracture, bend, buckle, overheat or wear out when subjected to these effects, or indeed may demonstrate that the part will work according to its design intent.

Benefits of FEA

For engineering designers, project managers and product development managers, FEA modelling is useful for its ability to highlight possible design and material flaws in parts whilst still at the CAD stage.

It is particularly useful for modelling behaviour of parts which will be subject to certain stress conditions during the course of everyday usage, and for which testing is always required.

In some cases, designers move straight from a CAD model to early concept prototyping – and from there onto functional testing. If a design is found to be structurally or materially flawed at the prototyping stage, re-prototyping can result in costly time delays and iterations in design cycles – with negative time-to-market impact.

This is where FEA comes into its own. FEA testing can help design managers to:

• Reduce design project times and the overall ‘design-to-manufacture’ cycle
• Optimise product designs, thus keeping prototyping and production costs low
• Discover and eliminate possible sources of component failure before production and manufacture.

The benefits of this include:

• Testing for failure in ways that may not be possible with prototyping alone
• Assessment of safety of the product when very close to failure, allowing for minimum factors of safety
• Increased confidence that the design will work as intended
• Time and costs savings associated with re-prototyping
• Quantifiable improvements to the part design such as strength, weight reduction, improved durability and optimised production costs.

FEA applications

The vital function of FEA is to ensure that ultimately, parts, products and component assemblies can perform and function correctly, as intended for the duration of their life – and beyond.

This is particularly key with components where exceptionally high stress loads may be encountered. As such FEA is very widely used for structural design and analysis in the aerospace, automotive, marine, sporting goods and petrochemical industries – as well as in a whole range of other industrial sectors.

And of course proper and safe functioning is not restricted to these high stress environments. It applies equally to consumer and business goods product manufacturing where everything from an office chair to a garden seat must be subjected to rigorous testing for safety and compliance purposes.

The list of applications for FEA is therefore long, but common examples where it used by product designers include the following, in the context of design optimisation and lifetime durability prediction:

• Structural integrity testing
• Fracture testing
• Fatigue testing
• Pressure testing
• Loading and stress testing
• Vibration or shock impact
• Thermal loads and temperature change
• Airflow and fluid dynamics
• Drop testing

How Does FEA work?

FEA splits a solid body into hundreds and thousands of tiny elements, cubes or pyramids

Each of these elements is subjected to scientific and mathematical calculations and formulae.

The FEA software then assembles each elemental data set and forms a set of results showing how a part reacts to thermal or structural loads.

Linear and non-linear FEA explained

All manufacturing materials have tolerance limits on the stresses and pressures they can endure.

For example plastic materials (such as rulers) have an ‘elastic deformation limit’, whereby when bent and released, they will return to the original shape.

Certain rulers when bent too far and released will remain bowed and not return to their original shape, and this is called ‘plastic deformation’.

If bent further again, beyond what might be expected to be the ‘normal’ limits for bending a plastic ruler, the product will reach its tolerance limit and break.

In this case the limits and ‘yield stress’ of the plastic material have been exceeded and so the material has fractured.

Using the example above, we can define linear analysis and non-linear analysis:

• Linear analysis models the material in its elastic deformation phase
• Non-linear analysis is used to model plastic deformation.

In either case, it is important at the design stage to ensure that the ruler when bent within a tolerable range will not remain permanently deformed. Thus linear FEA analysis is used to ensure that the design results in an end product that will not permanently buckle or deform.

Steady state analysis further tests for changes in the material behaviour when a thermal or structural load is applied. Steady state analysis predicts the end state of the part without reference to time, so does not take into account whether the reaction to thermal or structural load takes place in a few milliseconds or several hours, days or weeks.

Cambridge Design Technology and FEA

At CDT we use PTC Creo Simulate and SolidWorks to perform our FEA analyses, using in particular linear static structural and thermal analysis forms of FEA, though we also offer non-linear analysis and computational fluid dynamics for the modelling of airflow and fluids.

We use FEA to increase confidence in our designs form two perspectives:

• Confidence in design optimisation: we ensure that real world loads to which our designs may be subject at prototyping stage are modelled using FEA. This means we can be confident that the design is robust enough to withstand testing before moving to the costly and time-consuming prototyping process. It may also result in reduced costs and shorter time periods over the prototyping cycles;
• Factor of safety: we ensure that we have enough margin or “headroom” in our designs that factors of safety are considered (we aim to work to a 10x factor of safety). By using FEA we can determine peak stresses in our designs and ensure that these peaks are optimised to be at least 10x less than the facture or yield stresses of the materials that will be used in the final design.

With our extensive experience in engineering and product design consultancy, we completely understand how critical the requirement for optimised design is.

We are also very familiar with the multiple and often conflicting priorities faced by project managers and product development management managers including cost, speed to market, efficiency, weight and size – as well as structural durability, reliability and safety.

For regulatory compliance and quality assurance purposes, we are always happy to provide comprehensive analysis documentation and records to be included in your product development file.