This week I was introduced to FEA, Finite element analysis. FEA is a powerful numerical modelling technique. It allows us to simulate the behaviour of complex geometry by breaking that geometry down into many little elements and analysing how these elements interact together. Often we use it to visually display an important quantity on a structure, like this stress distribution on a clips.
Red = Higher stress
Bule = Lower stress
Good pointers I had noted was that although there are high stresses, it does not mean its too high. FEA can be used to simulate a range of physical behaviour including analysis of:
- static structures (stress analysis)
- buckling modes (when thin structures collapse under compressive loads)
- heat transfer
- dynamic/vibration analysis
Stress analysis of structures.
FEA will always need testing, no matter what’s in the program. I’ll need to physically test this to verify what The computer has calculated. I started this process by Building my frames in fusion, there were 4 different type of structures in this to show the strength differences. These were the 4 shapes.
I first started by making my shape using the measurements, assuring all measurements are the same for the common faces.
I then split the the top bar down the middle, giving me a middle point reference where the force will be applied. I also split the bottom bar to lock that in place so the force would cause it to compress.
I then continued using the simulation section of Fusion 360. I hadn’t used before so has a play around. I input the force onto the slit line and the direction it should go in. I then input my material – MDF.
The system, with that input information, and along with your designed structure, can simulate and calculate the Load cases. You’ll end up with the minimum and maximum stress levels.
I Proceeded to do a physical test of this in the Lab. Id be using this machine, that has a clamp on the bottom to hold it in place and a bar on the top that would compress down. This device allows you to switch different clamps and tool in and out.
I then had all four Pieces ready to test. I marked on each one the split points, like I did on Fusion 360. This would also guide me when placing the structures into the machine in the right place.
During the process I could examine the results on the screen. Here they were…
| Structure | 1 | 2 | 3 | 4 |
| Weight (g) | 10.7 | 14.4 | 15.2 | 18.2 |
| Max Force (n) | 66 | 144 | 185 | 286 |
Following previous tests, I then explored more stress and strain. They are concepts that are used to describe how a body responds to external loads. The testing I was doing was ‘uniaxial loads’ meaning all the loads are applied on one axis. Like with the pervious structure tests.
Here we will be working with these forms below and stretching them to breaking points.
The machine we previous used will calculate everything we need, but I covered the laws and equations involved:
- linear behaviour (proportional: σ=E.ε or stress is proportional to strain in the linear region).
- elastic behaviour (recoverable: <σYield or it bounces back!)
- Elastic modulus = Young’s modulus, E (GPa) = slope; E = σ/ε
- Poisson’s ratio, v = -εLateral / εLongitudinal
- σYield = Elastic limit (end of straight bit!) = 0.2 % offset proof stress
- σUTS (ultimate tensile strength) = maximum stress
- εFailure = maximum strain
- strainEnergy = area under F.s curve
- resilience = area under σ.ε curve
As the structure gets stretched, internal forces are reacting within the piece until it breaks. This will give you a ‘strain curve’
I did some testing with wooden and plastic models, here are the results:
| Max Force (N) | Max Force (kn) | ||
| Steel | 13,596 | 13.6 | Ductile |
| Acrylic | 2720 | 2.7 | Brittle |
| MDF | 1243 | 1.2 | Brittle |
