Free Body Diagram (FBD) – Aerospace Applications
We touched on the concept of a free body diagram (FBD) in some other blog posts, such as those listed below:
- https://www.stressebook.com/stress-engineering-interview-questions-part-3/
- https://www.stressebook.com/classical-hand-calculations-in-structural-analysis/
- https://www.stressebook.com/shear-clip-freebody-diagram-fbd/
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Free Body Diagram – Definition:
If you asked me what a free body diagram is, then I’d define it this way:
A free body diagram is a graphical representation (or a schematic) of a segment or component or assembly under load. It must show all the ‘applied’ loads/moments on the segment or component or assembly. It must also show all the ‘reaction’ loads/moments required to balance the ‘applied’ loads/moments. The free body diagram thus demonstrates ‘static equilibrium’ of the free body by itself.
Let us assume for the remainder of this blogpost that the free body is a component in a sub assembly.
Free Body Diagram – Significance:
So what makes a free body diagram of a component so special and important?
First of all, it shows that you understand what the loading is on this component. But this assumes you drew the free body diagram correctly as defined above. Now THAT, my friend, is half the battle at ‘Normandy’. Why? Because in stress analysis, the most important aspect you need to be work out first is loading. Some of you more experienced folk may know this very well.
Secondly, with a good free body diagram, you also understand how component reacts the load. The load passes through this load path called ‘component’. The component then transfers the load to other components along the load path down the line.
It also demonstrates that you are a good stress or design engineer. It is one of the essential qualities you need to master in stress engineering.
If you would like to learn more about these qualities, then visit these posts:
- https://www.stressebook.com/good-stress-engineer-qualities-part-1/
- https://www.stressebook.com/good-stress-engineer-qualities-part-2/
- https://www.stressebook.com/good-stress-engineer-qualities-part-3/
- https://www.stressebook.com/good-stress-engineer-qualities-part-4/
Without a clear understanding of the “free body diagram” you are kind of shooting in the dark in stress analysis. Your analysis is basically relegated to ‘garbage in garbage out’. So make sure you have a good grasp on the basics of a free body diagram.
Free Body Diagram – Static Equilibrium:
The basic concept behind a free body diagram is known as “Static Equilibrium”. But what in the world is that?
Simply put: Static equilibrium of a component is defined as a condition or state of the component in which the sum of all forces and moments acting on the component in each direction is zero. This includes any reactive forces and moments.
Free Body Diagram – Example:
Let us look at an example to truly understand the importance of a free body diagram.
Shear Clip:
Light weight, thin sheet metal built up beam systems often use shear clips in the airframe.
If you would like to learn more about the mechanics of the diagrams below, then click this link: https://www.stressebook.com/shear-clip-freebody-diagram-fbd/
A different example is also analyzed in this blog post: https://www.stressebook.com/stress-engineering-interview-questions-part-3/
The figure above shows a shear clip loaded in shear. Each leg of this shear clip is ‘free bodied’ in the figures below.
As we can see, the free body diagram must ‘balance’ the ‘free body’ in static equilibrium. At the end, all the forces balance each other for a net zero load in any direction. In the example above, you don’t need a detailed FEA to determine these loads. With an accurate FBD, you know the shear loads in each fastener. You can then use the resultant shear load to determine fastener shear and hole bearing margins of safety. Shear clips are not meant for tension applications. Those are called ‘tension clips’. The critical shear clip section is at the transition from one leg to the other in ‘shear’. You can then calculate the critical section shear margin of safety for this section.
Conclusions:
In a component like the shear clip above, ensure that the joint is more critical in bearing rather than fastener shear. In other words, your fastener should not be the first to fail under load. It is better to have a lower bearing margin than a fastener shear margin. In case failure occurs, the fastener is still there to transfer load even if it is redistributed due to bearing yield. Here is a link to an article for more reading on bearing vs shear critical joint analysis.
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