| HN Mirror

Ah engineers who do not understand the fundamentals of the values they are using. I expect this will piss you off, but I expect it is important you know.

The easiest way to visualize Young’s modulus is typically using a stress/strain curve from a tensile force test, but can also be visualized by a compressive force test.

Young’s modulus is the elastic region that is ‘under’ both curves, since it’s a value for how much something can safely bend* and return to it’s original shape without permanent damage, and bending causes both compressive and tensile forces in a material.

The lowest value of the two upper limits - of course - will set the upper elastic limit!

Specifically, to a very simple approximation, the ‘inside’ of a bent beam will be limited by compressive strength, the ‘outside’ by tensile. The overall beams strength will be whatever the lowest of the two values is, as that is when the beam will fail somewhere, hence, that what you want to use in civil engineering, eh?

*at a first approximation. It is of course much more complex than that.

For a column, the limit is the lowest of the ability of the column to prevent deflection (tensile) and bear the load without ‘pancaking’ (compressive strength). Pancaking is resisted by the full cross sectional area of the column, while deflection is generally only resisted by (to a rough approximation - greatly depending on the actual geometry, as I noted in my prior comment!) half the thickness of the column, so tensile strength of the material is usually the limiting factor for most simple columns of non-trivial length, where unsupported column deflection is the dominant failure mode.

Notably, this is why concrete often uses rebar in civil engineering, as concrete has trash tensile strength on it’s own and requires massive volumes to have sufficient tensile strength to form acceptable beams or unsupported columns, or use only very limiting forms to ensure only compressive forces actually occur. Unreinforced Concrete’s Young’s modulus is trash for this reason.

Which for civil engineering is the best bet!

Also, add a good safety factor on top due to all the other materials fundamentals that they apparently don’t teach in civil engineering school?

Which hey, I get, because they’d be too overwhelming eh? And they’re generally drowned in the noise at that scale anyway.

Depending on the sub discipline, mech-e, aero-e, etc. will of course have to know these things at a finer level of detail.

Someone computing fuselage thickness, designing an engine connecting rod or turbine blade, etc. needs to know what is going on at a much finer level eh?

Those folks will also have to quantify the type of failure modes they expect (fatigue limit, tensile failure, compressive failure, wear, etc.) and design around it.

Young’s modulus isn’t fully useless in those usages, but more specific values tend to be far more useful, as Young’s modulus is fundamentally the lowest value of a mix of material properties.