Finite Element Analysis: light touch or deep dive?

29 Apr 2024 6min read

Finite Element Analysis (FEA) is a digital twinning tool which allows engineers to simulate device behaviour whilst shortcutting the costly process of physical prototyping. It is widely accepted as a powerful tool for medical device development, however it can be difficult to decide both when to use it and how much time and resource to allocate to it. As with so many other aspects of medical device development, the answer lies in taking a risk-based approach.

In practice, this means it is useful to have some understanding of what the failure modes might be of your device – the various ways in which your components or system might fail. This will help inform what it is you want to simulate and what kind of simulation you want to run.

What is Finite Element Analysis?

Finite element analysis is a great tool for quickly checking whether your components are adequately robust to survive the forces imparted on them.  As shown in the figure below, FEA sits within a range of engineering analysis tools. Depending on the complexity of the subject, the time involved in running a simulation can vary from a relatively light touch review of linear materials, to a more detailed analysis of dynamic content. It’s important to note that care must be taken when considering these ‘load scenarios’ and whether your FEA result is a true representation of what the part will experience in use. This might involve a design review or design failure mode and effect analysis (DFMEA), or simply trialling your design with some 3D printed components.

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Typical applications of FEA in medical device development

There are many applications of FEA in medical device development. Common uses include checking the feasibility of an idea, such as whether your device design will likely fail under its intended loads, or to quickly compare several design options.

It can also be useful for testing key materials within your device. Plastics are almost the universal material of choice for medical devices, however many contain components which are highly loaded, sometimes for extended periods of time. These require careful analysis to ensure that the polymers can withstand the rigours of these loads.

For more information on typical FEA applications, read our top five uses for finite element analysis.

Light touch vs deep dive

While the benefits of Finite Element Analysis are clear, the challenge is often determining how deep to go. There are many applications of FEA throughout the development process. In the earlier stages, a more “light touch” approach could be used to help rapidly determine the feasibility of a concept, to help determine if you are roughly on the right track. This might involve setting up a simple model to give the green light required for continuing your design, for example.

As you progress through your development and device design, it is likely you will start to need a more detailed and robust analysis. This is where a “deeper dive” FEA can help. For example, you may need to determine which dimensions and parameters impact the intended performance of your device, or how susceptible it is to external factors such as temperature or environment. Analysis tools such as FEA, tolerance analysis and mathematical modelling can be used to do just that.

To help illustrate the difference between light touch and deep dive approaches, the following is an example based on the kind of analysis Team frequently carries out on activation systems for auto-injector devices.

FEA for an auto-injector

In the example shown below, an FEA has been conducted on an auto-injector which uses a trigger pin to hold a pre-compressed spring. The device has two overhanging teeth which are flexed inwards to release the spring force.

A simple static FEA suggests that the part is adequately strong to hold the spring force, with the peak stresses predicted by the analysis well below the material’s yield stress. The stress contours in the image are scaled from zero to the material yield stress. The fact that no red is visible in this analysis shows that the stresses are generally low.

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If you were taking a light touch approach to FEA, you might decide to stop there. However, in this case, the spring is compressed at the assembly stage and then stored in this configuration. This means that the effects of creep need to be considered, therefore a more in-depth analysis might be required to investigate this. Creep is the phenomenon of loaded parts stretching or relaxing over time such that they either change shape (deformation creep) or lose any tension or compression built up in them (stress relaxation creep). It can affect any part which experiences a load over a long period of time.

Once creep is taken into account, the same trigger pin experiences significant deflection over time, with the trigger teeth flexing inwards, risking accidental activation of the device. Here, we start to see the benefits of a deep dive FEA approach, as this design risk might not otherwise have been highlighted until much later in the development process when physical performance testing is conducted on prototypes, which requires significant capital investment. One of the challenges of this type of analysis is acquiring the appropriate material data to build an accurate model, but the benefits of doing so are clear.

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Using FEA to make device improvements

Using the information gained from the initial analysis, we are then able to optimise the original design, adding a security pin to prevent the tabs from deflecting inwards as a result of the creep. Some additional teeth can also be added to help spread the load from the spring force. This type of trigger pin would often already have a security pin to protect against dropping, so here we are simply adding extra functionality to an existing part.

As shown below, when re-running the simulation with the new optimised design, the creep deflection reduced by over 50%.

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Why you should use FEA

This simple example shows how a first pass analysis can provide useful, but incomplete, information. By taking Finite Element Analysis a step further, significant insights can be gained into the workings of your medical device. This will require some investment of time and effort, but doing so can help to identify major design risks and mitigate them early in the development process, for example before committing to tooling, helping you save both money and time in the long run.

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