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- This indicates when a part can fail
- FOS of 1 means the current simulation/reaction will begin to fail at 1 times the max equivalent stress, 2 means the structure begins to fail at 2 times the max equivalent stress, etc.
- FOS of 1 is the absolute bare minimum a structure should have before it is deemed manufacturable, but some parts of the frame must have a 1.1 minimum, due to high safety goals/concerns (although you should always shoot for something >1.6 FOS)
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Solids
Start Here → Solids Lecture.pdf
There are 4 main types of stresses each tube will encounter. Those include:
- Normal Stress: Stress that acts perpendicular to the cross-section of interest (compression, tension, or rather axial force)]
- Shear Stress: Stress that acts tangential (parallel) to the cross-section of interest (like a cutting force)
- A common application of this is bolt shearing. This is where pre-tensioning, torqueing, and bolt calculations come in to play (tbd, I am learning about these currently)
- Torsional Stress: A shear stress that is caused by a moment about the longitudinal axis of the tube (think of a twisting force)
- Bending Stress: (THIS IS THE BAD ONE): Stress caused by a moment that has tension and compressive stress at each end (think of the cantilever beam problem below
- Also, bending stress is related to moment of inertia
- Simple Bending or Pure Bending A beam or a part of it is said to be in a state of pure bending when it bends under the action of uniform/constant bending moment, without any shear force. Alternatively, a portion of a beam is said to be in a state of simple bending or pure bending when the shear force over that portion is zero. In that case, there is no chance of shear stress in the beam. However, the stress that will propagate in the beam as a result will be known as normal stress. Non-uniform bending occurs when shear forces are introduced.
- Above is the formula for bending stress on this cantilever beam. Here, we have bending stress on the left side, with the calculated bending moment times the vertical distance from the neutral axis, all over the moment of inertia from the neutral axis
- What you really need to take from this is not necessarily all the algebra behind bending stress, but rather the things that affect the value itself, which in this case is bending moment, moment of inertia, and vertical distance.
Fluids/Thermodynamics
Specific to Ergonomics, you'll also utilize some Fluid Mechanics and Thermodynamics knowledge as it pertains to the braking system.
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- Simply put, if we assume the flow rate is the same across the entire tube/line, when the tube has a larger cross-sectional area/diameter, there's more volume in the tube, meaning the fluid travels slower and with higher pressure to achieve the same flow rate
- On the opposite side of the spectrum, when the tube or line has a smaller cross-sectional area/diameter, there's less volume, meaning the fluid must work harder and travel with more velocity to achieve the same flow rate, albeit with less pressure
- This is applicable when thinking about the sizing of your lines, especially as your braking ability is determined by the ability to move pressure from the brake pedal, through the fluid and to the actual brakes.
Additionally, we have the different types of flow, laminar and turbulent flow, at play as well.
- Laminar Flow refers to a flow with little turbulence, creating a smooth, consistent stream
- Turbulent Flow is the opposite, referring to a more chaotic, inconsistent flow, that occurs due to turbulence
- Turbulence is just a term for for irregular motion, like swirling and unpredictable changes in motion
- Point here being, you want to achieve laminar or close to laminar flow when possible as it's predictable and consistent. You want predictable and consistent braking ability right?
Lastly, piggybacking off of the types of flow, consider the pathing of your tubing. Consistent and smooth fluid streams are naturally better suited to straight, consistent paths, but inevitably you might need to change the directions and pathing of your lines.
- When creating direction changes in these lines consider the resultant drag caused by the abrupt change in the fluid's path as well as the turbulence it can create, which further affects the pressure and velocity values of the flowing fluid
- The red pockets on the corners symbolize points of flow separation, which just means that the smooth stream is broken up into sections, causing turbulent flow and pressure losses.
- All in all this just means that you should for as straight and consistent as a path as possible, and if you have to, utilize more gradual, smoother turns, as more abrupt and constant turning hurts fluid flow, leads to excess drag, as well as lots of pressure drops and turbulence
Topology Optimization
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Useful Tidbits
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