Diving into the world of Practical FEA (Finite element analysis)

"Have you ever wondered how modern-day aircraft are able to handle the rough physical conditions to which they are exposed while flying? How cars withstand what Indian roads throw at them? Aren’t they magnificent? Actually, these are outcomes of rigorous engineering and testing. FEA plays a very important role in the design and development of these complex mechanical systems and makes them fit for the various tough conditions they will experience during their lifecycle.”

There is an abundant amount of literature out there that provides a very deep and clear understanding of practical FEA. But the main motive of this particular article is to educate anyone, even without an engineering background, to gain a brief idea about FEA and its importance in the industry.

First of all, let us understand what Finite Element Analysis really is. A very crisp definition would be as follows: 'Finite Element Analysis can be defined as the process used to simulate simple or complex mechanical systems to understand their behaviour under various load conditions.' After conducting the analysis, we receive various results (as requested by the analyst). We can then interpret the results and take necessary actions with regard to the design of the product (e.g., addition of a component like a stiffener, increasing the thickness of a plate, removal of a part, etc.).

Let me explain with a very simple example, which I call "The Four-Legged Example". Imagine someone asks you to sit on a beautifully designed chair. Now, you take a look at it, and you are not sure whether it will be able to withstand your weight (not judging your weight, but I surely will have second thoughts before sitting on it though). There are three ways we can test whether you can sit on it comfortably (without breaking it, obviously):

• Use an analytical method, basically using formulas to calculate the forces the chair can handle because of your body, and make a judgment. But the drawback of this method is that it uses various approximations and assumptions that can result in very conservative results. Guess what? You sat on it, and the chair broke in half. Or the chair manufacturer just ended up using extra material and increased his overall costs.
• An experimental method, i.e., you go ahead and sit on it right away. But there is a 50/50 chance that the chair breaks, and the designer asks you to pay for it. In today's engineering world, a 50/50 probability ratio is not considered viable at all. Imagine relying upon this ratio for an aircraft wing that is supposed to fly over 100 people. That would be disastrous.
• Now, a numerical method. Here comes the role of FEA. We build a virtual mathematical model of the chair and load it with your weight (including the factor of safety). We make use of the powerful computational power modern-day computers possess and let them conduct a series of calculations. Now, we receive a set of crucial results, such as maximum stresses induced within the chair, and decide whether you can sit on it or not. Voila, you can go ahead and sit on it (but please don’t jump on it).

Let me explain the word 'stress' in a very simple way. Sometimes, we undergo a lot of stresses in our lives. Every person has a limit they can handle. When we cross that limit, we often tend to give up. On the positive side, we can also learn from it and redesign/equip ourselves better to withstand the stresses, or we can also decide that we aren’t suitable for such an application. Similarly, stresses are internal forces that are induced within a material, giving you an idea of its strength. Yes, we can correlate a lot of engineering topics with what we all face every day. Surprisingly, FEA as a topic holds a lot of lessons that teach us to understand our lives better.

Now, we can observe that the process of FEA doesn’t cost much because it is also a non-destructive method of testing. Another added advantage of FEA is that it is less time-consuming when compared to experimental methods. (If you break the chair, you’ll have to redesign and sit on it again; guess what, it broke again). There are also various products which cannot be tested experimentally. For example, what if I wanted to test how well a big Genset enclosure is able to withstand the high wind pressure loads? I would have to build a wind tunnel capable of handling such a structure, which would cost a lot for a company and may not be feasible sometimes. Therefore, in today's industry, a dedicated FEA department is incorporated into major companies (automobile, aerospace, prosthetics, etc.). They all conduct their analysis in-house to safeguard their intellectual property.

Practical FEA involves three main steps:

1. Pre-processing
2. Analysis
3. Post-processing

Now, let us discuss all three of the above steps in brief. Please note these three steps are very crucial, and a separate book can be written on all of them, which is beyond the scope of this article. I would encourage the readers to refer to a book called “Practical Finite element analysis for mechanical engineers by Dominique Madier”. Dominique and his team have done a very beautiful work in making the reader to gain an in-depth knowledge in this particular field.

Pre-processing is the very initial step of any FEA project. Here the analyst understands the requirements of the customer and plans out the entire FEA project ( I call it: “laying the basement” because the efficacy of the next steps are basically the outcome of this process). During this step the input received from the client side (such as CAD models of the product, project details and requirements) are carefully analysed and discussions are conducted with the client to make sure the analyst and the client are both on the same page with respect to the outcomes of the project. Sometimes the analysts are also requested to submit a DOU (Document of understanding). CAD (computer-aided designs) are basically the virtual representation of a product. Here, the CAD geometry is also prepared (Cleaning up the geometry) to make it suitable to meshing.

Meshing can be defined as a process in which the model in broken down into finite number of tiny pieces so that we can derive finite number of points and calculations can be conducted on them. Finally connections are made (like fasteners between various components, or representation of welds) and a detailed model is prepared. Last but not the least, a final check is conducted to make sure out Finite element model represents the actual physical nature of the mechanical system in practical and feasible manner.

Let us move to the next step. The Analysis, as the name suggests, during this step the analyst conducts the various analysis that are requested buy the client. Going back to the chair (yes, I love chairs), we may need to conduct a linear static analysis to get a basic idea about stresses induced at the legs of the chair. Again, there are a variety of analysis which are available according to the requirements which are to be fulfilled. I as an analyst have conducted various analysis tests both in a linear and a non-linear system. During this step too, the analyst is responsible to explain his observations in a very brief format and any assumptions he/she has made to make the model perform better for analysis to the client. This makes sure the client is satisfied with the explanations, and trusts the model with its representation of the practicality. Again, to reiterate the efficacy of the analysis completely depends upon how well the analyst has understood the requirements of the client and how efficiently the model is meshed. If its messed, yes you will receive messed up results which sometimes mayn’t make any sense (I am an engineer who loves puns). It requires a lot of experience and inherent engineering judgement to troubleshoot the results and find out what caused the issue.

Finally ! We are at the last step. We receive a lot of result data as an outcome of an efficient FEA analysis. Not every result is of use. We post-process the results, ie. Filter out what is required and what is not. For example you get a big bag of assorted chocolates, yummy. But we need to filter out the dark chocolates as well as the ones with crunchy almond coating (beware if your recipient has a nut allergy). Yes, you may ask me? why not just but two separate bags of each flavour ? Wait, what if your client suddenly asks for the mint flavoured chocolate (I am not a big fan of this flavour though) ? You’ll have to run all the way back to the store and buy it. Sometimes your car may run out of gas. Or, the shop just got closed. Directly correlating this, sometimes a analysis may run for several hours or even days in big models. You will have to re-run the entire analysis asking for that particular output again. Or, we may face a technical issue (I fell sick that day, or genuinely your Pc’s mother board got fried up). Now, we document the results in a way which is easily comprehendible and gives the reader a complete idea of what the mechanical system is behaving like upon loading. The analyst should always try to keep his report simple yet add adequate amount of technical details so that a person even from a non-engineering background is able to understand what is really happening. This maybe a situation where the group of engineers have to forward the report to the stakeholders and they may not be all well-versed the technical jargons. Its easy to include all numbers and complete the report, but it is an separate art to develop a efficient report. After this, I have a big cup of cappuccino and wait for the client to revert back with any updates. These updates may consist of design changes (as we discussed in the chair example) and we may have to re-run the updated model and observe how this new guy behaves.

I believe, this article was successful in achieving the goal of providing the reader with a brief dip if not a dive into the world of FEA. We analyst work hard, staring hours and hours together at our monitors looking at beautiful Von-mises stress contour plots and deformation patterns, to make sure the end consumer can rely upon the products right from a simple plastic chair for sitting, to the big rockets that carry expensive telecommunication devices into the outer space. I take pride when I look at a person sitting on chair (the chair to be specific) and it didn’t break. I take pride, when the aircraft faces heavy unprecedented turbulence and yet made it through with ease because I know there was an analyst who read the numbers and passed the design. To all analysts out there, keep burning the nights lamp so that things can fly, things can withstand the sands of time.

By,

Aniruth Paramasivan

An FEA engineer striving to make the community better.