Understanding Finite Element Analysis and Its Benefits for Your Medical Device
There are products from our grandparents’ and even great-grandparents’ eras that are still in use today. In their time, technology didn’t advance nearly as quickly, so products were expected to last a lifetime — and they were built accordingly. Everything was “built to last”.
Today’s development standards are different, even for medical devices. Products evolve faster because of improved technology as well as new, more readily available use and safety information. We’ve also come to expect products to be faster to manufacture (to keep up with the speed of innovation), lighter, easier to use and more aesthetically pleasing.
Enter FEA, or finite element analysis. FEA helps the product designer find the balance between a medical device’s durability for its expected life, and the above expectations.
There are two primary modes of finite element analysis: linear and non-linear. It’s important to understand the limits of each mode and determine which is appropriate for a part or assembly’s function.
What is Finite Element Analysis?
At its core, FEA is a complex mathematical simulation that models the stresses within a material.
These days, engineers use advanced computer software to conduct FEA. The software analyzes the millions of small elements that make up a single shape, i.e., component. It simulates the physical react of a part under the expected load(s).
When the part is stressed, FEA allows the designer to see weak points of the geometry. Adjustments to the design can now be made based on this, by taking away material where it’s nonessential and putting it where it is needed.
The Top 3 Benefits of FEA for Your Medical Device
Some may feel pressured to skip the FEA step because even the less expensive linear option (as opposed to non-linear; more on this below) has a price and takes time, in an already time-constrained development process.
But the benefits are many.
1. Design Parts Exactly for Their Function
As mentioned, an FEA reveals a part’s strong and weak points by simulating its reaction to forces. With the results of an FEA in hand, you can create, essentially, a minimum viable part. A part that uses precisely the right amount of material and functions exactly as it should — with no excess.
2. Save Time and Money on Your Overall Development Process
Because you’re able to design your parts more precisely to its purpose, you’re using less material, which saves time and money on manufacturing.
In today’s environment where many medical devices are made of plastic, injection mold modifications are costly and require additional time. One of the largest time saver is to avoid modifications to mold tooling.
3. Support Your FDA Submission
Medical devices must undergo testing and prove their safety and efficacy. It’s a rigorous process.
As discussed, an FEA can reveal a part weakness which can be addressed prior to any verification testing. This certainly relieves some of the anxiety a design team feels. And it follows that your product will more likely pass any applicable tests and make it through the FDA’s stringent design controls processes.
In sum, planning and proactivity are key in medical device development, and conducting an FEA is another early-stage safeguard you can put in place to ensure the process goes smoothly later on.
Understanding Linear Finite Element Analysis
As mentioned, there are two types of FEA: linear and non-linear. Let’s discuss linear first.
During FEA, a load is applied to the part, and the material properties and part geometry react to the loading. Linear FEA assumes all the relationships are linear. That is when a load is applied, the material properties, part constraints, and part stiffness all behave in a linear manner (think of straight lines on a graph).
The Limitations of Linear FEA — and Why It’s Still the Popular Choice
We noted that some would rather forgo FEA entirely. But if they have to choose one option, they almost always choose linear FEA.
Most engineers can conduct linear FEA internally with their existing CAD (computer-aided design) systems. There’s no need to spend more to hire an outside partner, making linear FEA an attractive choice. Linear FEA is also considerably quick to run, from just a couple of hours up to one day. Finally, linear is cost effective, roughly costing $1,500.
Linear FEA certainly has its place. But there are definite limits to linear version of FEA. Foremost depending on the deformation, physical structures exhibit non-linear behavior. As a result, linear FEA can provide misleading predictions.
The Right Conditions for Linear FEA
Because most parts don’t behave linearly, linear FEA requires very particular circumstances to produce accurate results. Linear FEA is only valid if:
- The material properties remain linear over the entire range of the deformation. This means:
- Hooke’s Law applies,
- Young’s Modulus is assumed constant, and
- All stresses are below the material’s yield strength.
- The deflection/rotations are small enough such that:
- Neither the force direction nor force distribution changes during deflection,
- The deflection does not strengthen the design (such as inflating a pressure vessel),
- The geometry of the part does not significantly change shape,
- The size and shape of the contact area remains constant while the load is applied, and
- The application is viewed as static, no impact forces.
If the linear analysis model is not posed well, there can be several convergence problems. One of the pop-up warnings in the software may request you to click the “large displacement” button, activating the non-linear large displacement mode.
When the large displacement mode is on, the loading is applied incrementally; this requires additional time to resolve. But all the stiffness variables and constraints are still considered to be linear.
In summary, linear FEA should only be used under very particular conditions: small deflections and all constraints are linear.
Understanding Non-Linear Finite Element Analysis
As in life, things don’t always behave linearly. Non-linear FEA must be used for deflections that go beyond the linear portion of the material properties and resultant part deformations.
Here is when the actual material stiffness must be tested. This will require test samples to be molded or cut out of the actual material, ideally in the actual color. (Some plastic’s pigment do affect the mechanical properties.) For plastic parts, the test specimen is typically molded in the “dog-bone” shape as described in ASTM D638.
There are several labs both nationally and worldwide that have these kinds of tensile test machines that can pull three samples for you, then send you the tabulated data. This will cost about $500. The tabulated stress-strain data will be inputted into the FEA software.
Non-linear FEA is more expensive and time consuming than its linear counterpart – it takes a week or more, and roughly costs $10k.
The Right Circumstances for Non-Linear FEA
Non-linear FEA does not have the strict parameters of linear FEA. In fact, you must conduct a non-linear FEA if the following conditions exist:
- The displacement is large, causing the loading to significantly change direction with deflection, or the deflection is greater than 5% of the deformed part’s length.
- The loading is dynamic, such as shock loading.
- Deformation changes the restraint or contacts. Imagine a spherical surface deforming as it presses into a flat surface.
- The part geometry deforms significantly due to the deformation. For example, pressure vessels inflating. –
- The material stiffness is not linear as the part deforms. This occurs when the part stresses are beyond the material yield strength, or the part is made of a rubbery material.
Pay Now or Pay Later
That was a lot of technical information, but it comes down to this:
You could opt to perform a quick linear FEA and then fine-tune your parts later. or pay for non-linear FEA and get results that are closer to reality.
Keep in mind that, even though non-linear FEA is initially more expensive and time consuming, you’ll save time and money in the long run because you won’t be tweaking your medical device’s parts in preproduction runs.
Either way, you’ll realize benefits from conducting an FEA for your medical device’s parts.