Example: An exercise bike that can support 250 lb. Does the engineer find a large enough weight that deforms the bike, then take a fraction of that as the amount it can support?
In: Engineering
An engineer will use math to predict how a structure will behave.
So, so much math.
These days, much of that math is done on a computer.
What is even more amazing is how much of that math was done historically using a slide rule.
>Does the engineer find a large enough weight that deforms the bike, then take a fraction of that as the amount it can support?
Basically, yes.
The materials used by the engineer has known mechanical properties. Those properties are used in design calculations to determine the stress in the structure. Sometimes, this is as easy as F/A, but most of the time it requires several calculations to find all the stress states.
After determining that, a factor of safety may be applied. This just describes how much “extra” load bearing capability the design has. For example, if the rated load for an exercise bike is 250 lb but it has a factor of safety of 2, it can actually hold up to 500 lb before it starts to deform.
This process can start with either determining what the required load capability needs to be, or determining that after the design has been completed. Although usually there are multiple iterations of the design, so you bounce back and forth between these two approaches.
Old way (school way or double checking way) would be treating each part of the bike as a perfectly rigid, one-dimensional member. Creating a free body diagram (method of slices) at every joint, and solving for every load. You can then compare that to the deformation and stresses of every member/joint. The member/joint with the lowest factor of safety is your limiting load, if the FOS is lower than desired, you back calculate your load based on the desired FoS.
New way is just let <insert CAD program here> do finite element analysis for you.
Lot and lots of history on materials will allow for mathematical predictions of how materials will perform. Also engineers typically build in a minimum of 10% safety factor.
Engineers know at what point a material will fail. They know this by looking up the values in a table. The people who made the table know this because somewhere a long time ago, the material was meticulously tested in a lab environment and the results were recorded for the world to know.
Once you know the material properties, it’s a matter of taking the size and shape of your material, and plugging it all into a formula (or modeling it in software) and verifying that the load does not reach the limit.
What is this “limit” you ask? Typically it’s the point at which the material will bend without returning to its original shape, modified by a safety factor. In your example, let’s say the safety factor is 2. That means the product would break at 500 lb under perfect conditions, but is limited to 250 lb to account for things like user error, defects in the material, etc.
These days the easiest way is FEA (finite element analysis). You take a CAD model of the structure, add constraints and loads and run a simulation to see how it responds. Typically you add a factor of safety of 3-5 maybe. So if I run an analysis and determine a structure will deform at 900 lbs load I would rate it for 300 pounds, that’s a factor of safety of 3. You can also do vibration analysis, corrosion, etc.
As others have noted, an engineer will use math. The strength of materials given their thicknesses and orientation are known.
QA will pull a couple of the bikes off the production line and subject them to the test you describe.
We have a pretty good idea where the weakest designed section is.
Maybe it’s a welded joint or a bolt. You can calculate the maximum force to fail the connection and divide it by 2 or 3 as a factor of safety for your advertised maximum load allowed.
For simple structures there are tables that give a lot of information for different materials, like their weight or tensile strength for a given material, thickness, shape etc. they also use design software that simulates the given shape/structure being under load which serves as the baseline around which you begin designing it. Then come the actual real life tests with weights and some times special jigs that apply different forces to it. Lastly comes refinement which is to see if you can reduce the amount of material or complexity of the structure without compromising its overall load capabilities.
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