Simulation to reality: Finite Element Analysis (FEA) for device safety and performance
Published: March 2025
Gavin McDougall, Design Consultant
At the core of many medical devices are dynamic mechanical design systems and features critical to affecting overall device performance, feel and, crucially, ease of use. These features must be designed in such a way that any expected user or system forces fall within acceptable ranges often specified in the Product Design Requirements. At the same time, operational stresses within the components must be minimised to remain at a safe level throughout the life of the product, to reduce the risk of failure. Finding a basis for these mechanical design features can be challenging.
Finite Element Analysis (FEA) is a powerful tool to enable the prototyping and iteration of design features in a virtual environment prior to physical prototyping. Described in basic terms, it’s a computerised method of breaking down a geometry into smaller ‘elements’. It analyses solving complex numerical equations to arrive at a prediction of how a product or feature will react to real-world stresses or forces.
Simulation software and setup
At Shore, we extensively use the Creo Advanced Simulation Extension, seamlessly integrating the results within our PTC Creo environment. This extension allows us to study how advanced, nonlinear effects should influence product design.
The key to setting up an efficient analysis is a specific refinement of the CAD model and tailoring it to the FEA process. FEA relies on the computational power of the workstation, and the larger or more complex the model, the more computational power is required. Unnecessary complexity in the model can create long run times in the analysis, which can add up over multiple design iterations. A little planning and rationalisation early in the process can often pay dividends later in terms of time saved during computation. It is common for half or quarter-symmetrical models, to remove features irrelevant to the area of focus, and remove radii from non-critical areas. While retaining a balance in terms of introducing other potential issues such as unrealistic stress concentrations at sharp edges).
Material considerations
We operate a centralised material database that is available to all our engineers at Shore. This provides a detailed and controlled record of all materials used or considered for previous projects and enables quick and easy searching for suitable materials. Material definitions include information relating to structural performance that can be used as a basis for FEA (for example stress-strain response, values for Young’s Modulus, Tensile Yield Stress etc.).
Linearity considerations
Broadly speaking, analyses can be considered as linear or non-linear, however, linearity can be applied to discrete aspects of the study. For example:
- Geometric nonlinearity (large displacements): Changes in the geometry of the part from the initial to the final state can affect the response of the structure. This is especially true when solving large displacements. For example, where a geometric linear solution applies the entire displacement or loading in one step based on the initial geometry; this can lead to inaccuracies for large displacements. A non-linear large displacement analysis applies the loading over many steps, re-evaluating the stiffness matrix before each loading step. Related to this is the case of contact between components.
- Material nonlinearity (stress-strain response): Many materials used in engineering applications can be considered non-linear (i.e. a perfect linear relationship between stress and strain does not exist), these could commonly take the form, for example, of following an elastoplastic or hyperelastic stress-strain curve. Depending on the application consideration is given as to which nonlinearity cases should be used on a case-by-case basis for each specific analysis.
Case study: needle retraction mechanism for improved patient safety
A medical device recently developed at Shore includes a spring driven release mechanism for mimicking the automated retraction of a needle, which allows for improved patient safety by reducing the risk of needlestick injuries.
The first stage of the mechanism operation involves the use of a spring to drive together and latch two components by means of a flexible clip.
The performance of the clip feature benefits from being optimised according to the balance of two main considerations:
- Low force requirement to open the clip (since the available spring force is limited).
- Once latched, high resistance to unlatching from opposing forces.
An initial concept for the clip design was developed based on a traditional cantilever snap fit geometry in a low friction Acetal material. After modelling the basic design, it was decided to use FEA to first gain confidence in the performance of the clip, and then to further optimise the design to reduce latching force and minimise expected stresses, before committing to validation using physical components.
In this case, a simplified assembly of the two relevant components was studied in an analysis which included the consideration of large displacement and geometry nonlinearity. A prescribed displacement was applied to the clip part, which has the effect of moving the component a sufficient distance in the axial direction to fully deflect the clip. The study also included consideration of contact surfaces with friction (coefficient estimated at μ=0.2) between the clip feature of one component and the latch of the other.
The resulting fringe strain plot animation of the key components is shown below. The maximum expected strain is well within the Tensile Strain of the material, with a large factor of safety.
Creo Simulate allows for the reaction force at the prescribed displacement constraint to be plotted for every time step of the analysis. Therefore one of the useful outputs from the study is a force vs. displacement plot; in this case the plot shows the estimated assembly force. Through careful iteration of the geometry, we were able to lower the assembly force to around 1N while keeping the maximum stress / strain values well within safe limits.
Real world validation
The initial FEA results gave us good confidence that the refined geometry would give us the desired performance when translated to real production injection moulded components. Based on informative FEA studies, along with other Design for Manufacture and New Product Introduction (NPI) process techniques in our toolkit; we decided to proceed with production.
At this stage, another of Shore’s key capabilities was used to verify and validate the production product. The two components were force-tested in isolation on our Universal Test Machine (UTM). A compression test was set up to mimic the original virtual FEA setup.
The force vs. displacement plot from the real-world UTM testing is shown below, overlaid on the original FEA plot. The forces were shown to track very closely with the initial virtual results, effectively validating the FEA study, and verifying that the components meet our requirements.
Mapping simulation to real world results
FEA is an extremely useful tool to have at our disposal throughout the design and development phase. Using FEA early in the NPI process can provide a clearer picture of how parts and products will react to forces, displacements and stresses, before committing to prototyping and tooling. Saving time and expense and increasing the likelihood of getting the design right the ‘first time’.
Simulation adds significant value to medical device development. Partner with Shore for expert FEA-driven design solutions that take you from concept to reality. Contact our Business Development Manager, Holly Milston, to discuss this.
