Finite element modelling, in-depth orthosis case-study discussion, what I did wrong at first interpretation (LBYMF4C)

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About Thumbnail: Different materials for ankle-foot orthosis subjected under same static loads

OVERVIEW: I want to share a story about a case study I presented in class. I explained why the material chosen, ABS, is the most practical material to be utilized for the ankle-foot orthosis considering it produces the least max stress felt and has a low safety factor (SF). At the time, the instructor asked me about safety factors and how it is relevant to my analysis or why a lower SF should be preferred. My line of thinking was that a lower SF accounts to less modifications needed to compensate the deformation that is being made upon force felt in the bottom of the orthosis.

It felt natural at the time but looking at it now, I may have been hindsighted with how I presented my case for this design and would like to recap on this further in this blog!

I will try to explore more on the consideration being made when doing finite element analysis and methods. You may see first the progression that I did on knowing FEA or you may skip to that section on what really matters on materials selection for orthosis case study~!

What is FEA?:

Finite element analysis (FEA) is part of the engineering design process. It has many applications mainly to validate the design and material selection for the workpiece of interest. This is done by simulating the material elements that comprise the design to calculate approximate solutions to differential equations (yes, think of calculus that considers element body interaction and getting its summation or area under curve). It’s considered to be a black box model as the numerical solution does not have an immediate explanation of how it made the solution but we do know of the user inputs. The more defined the inputs (physical properties and assumptions), the better the result of the computation.

Of course, this can vary in other applications, computation fluid dynamics (CFDs) also tackle the same idea to model but consider fluid element. On the other hand, vibration inputs of the system is called modal analysis.

The general modeling procedure is as follows:

• Decomposition of the components
• Develop element models
• Assembly
• Apply boundary conditions and loads
• Solve for primary unknowns
• Calculate dependent variables set

We define the mesh density, support/ground, applied force/load and lastly the type of response that should be approximated.

Further explanation and discussion of this is provided below on the lecture slides and videos I have saved :>

Mechanical Basics for a Cap Fillet Design

Now we know the application for FEA, the most commonly used software for FEA is Ansys that covers many multiphysics foundation and simulation for its friendly student package. This is what I used in exploration of finite elements and how to do various analysis.

To gain proficiency for the software, I had to understand the workbench of the softare. Using the Stress Wizard, I was able to set up and solve a structural model for stress, deflection, and safety factor for the example workpiece to be a cap fillet.

Problem statement: − The model geometry consists of a STEP file representing a control box cover. The design requirements for the cover include that it must withstand an external pressure of 1.1 MPa. − The cover is to be made from aluminum alloy. − Our goal is to verify that the maximum stress in the cover will not exceed the material yield strength under the design pressure load.

I particularly like the answers that I got where you may check via the report I made below! TLDR, I was able to refine the mesh for a global element size of 1mm that reflected the use of aluminum alloy to deform under 1.1 MPa, despite its yield strenght of 276 MPa. Just increase thickness is the main recommendation.

2D Gear and Rack-Pinion system:

On a difference scenario, we are now tasked with a moment displacement on the system (meaning force felt on a distance). We are designing a press that should be capable of delivering 2500 N of force to the rack. I was able to simulate a pivot from the remote force point for a axial force felt directly in the pinion gear.

Goal:

• In order to design the mechanism for applying the load, we need to know the required torque in the gear to produce the necessary force.
• We’ll apply the desired force in the rack and extract the moment reaction at the gear.
• We will use a “Remote Displacement” to constrain the gear (instead of a fixed support) because this type of support provides rotational as well as translational constraints.

Aside from this, I feature in the report below specific cases and uniqueness of the ANSYS workbench. Partciularly the use of named selections to easily simulate pressure felt in a model's fasteners; I also was able to do basic CAD and additional components of beam connections on a platform via the object generator within ANSYS. Check the report below!

Beam Connections

The geometry for this activity now considers a 2-part flange assembly. The fasteners holding the flange together are not modeled explicitly therfore there is a need to use Mechanical’s beam connection feature on workbench to simulate them.

I was able to simulate a remote force to represent a structural load whose line of action is located some distance away from the flange. Copying my findings here, I was able to conclude that:

-the beam connections specified with defined behaviors were able to handle the structural load well with relatively low spikes of concentrated stresses or deformations made upon the application of the remote force in the geometry model. The structural loading offset was critical in the observation of how the system may resist the given axial and momental reactionsmade by these forces.

-Based on the resulting information provided through the beam probes, it was clear that there were low attributed stresses at the fasteners and further exploration dictates that the finite element connectors are directly placed within these fasteners and may be deformed in prolonged usage. Better distribution of the forces impacted within the flange through design modification is recommended with added safety factors for the geometry model.

Mesh Evaluation

In this activity, an arm from a mechanism will be solved using several different meshes and the results will be compared. Our primary goal is to explore how mesh changes can have dramatic effects on the quality of the results obtained.

TLDR: circular geometry, use a sweep method; complex/irregular shape should be a tetrahedron or hex dominant method; body sizing components that defines the mesh size should be tweaked depending on the computation allocation of the system :>

Modal Analysis

Lastly, the hardest type of analysis is the vibration analysis considers the frequency of the load felt. The goal is to investigate the natural vibration characteristics of the machine frame shown here. We will solve two modal analyses using two different mounting arrangement and compare the results.

This is important if the model can withstand natural frequencies felt especially if its a machine frame as shown in my report below. Particularly, adding more fasteners decreased the deformation (duh), but the deformation difference was huge! 9 millimeters to be exact when covering a 4- vs. 8-mounting hole.

The interesting question is which configuration and number of mounting holes can save money while achieving the same performance?

Realization on Ankle-Foot Orthosis Case study!

Going back to my question at the start of this blog, what is the determining variable for a suitable material for my case study? For more detail of the case study, I present here below my slides and also the accompanying baseline and study we followed for this application.

Safety factor (SF) is determined by the allowable strength / maximum working stress which just means that a higher SF is preferred, not lower! as the material is able to tolerate the stress being applied as simulation. This means that lower SF is more prone to higher risk actually to deform under load. 😓😓😓

The more correct way of thinking should be the trade-off between strength, stiffness and fatigue among other variables like user experience. The right balance for the SF directly impacts the design against unexpected loads but having a high SF makes it stiff for practically use.

So looking at its again in a differnt view the material Nylon should be recommended since it has a higher SF of 4.14 than the 2.56 for ABS.

Let me know down the comments if you agree with this new findings for the study.~~

REFLECTIONS: It is very confusing to see the values first and I do admit that I made mistakes in my thinking that I believe made even the instructor hallucinate on our values 😅. What is important is to really double check with a co-designer and understand the crux of the problem and why we are doing the FEA at the first place which is to validate the design and selection.

Having a solid background on doing the calculations by-hand definitely grounded me with this type of analysis and surely will be useful in the field if I go into designing products. Again, its important that we know the values we are dealing with as this may make or break the interpretation of the results that we provide to stakeholders and those interested on our findings. Stay vigilant on these pitfalls when doing FEA studies!