[A_D_E_P_T]

> Tests showed the new alloy achieved a compressive yield strength of more than two gigapascals while retaining its ductility, meaning it bends without breaking.

This is sleight-of-hand.

For metals, the operative properties are usually ultimate tensile strength and tensile yield strength. Compressive strength is typically a non-factor for most engineering alloys; only concrete is judged by its compressive strength, and sometimes various engineering ceramics. (e.g. SiC, compressive strength = 3.9 GPa.)

About ten years ago, there were a lot of papers on amorphous metal alloys that had "extreme strength" -- compressive strengths in the 5-6 GPa range -- but tensile strength was not reported and very low. Some measure of ductility was also present, but it too was very low. Those amorphous alloys were classical brittle materials; more ceramic-like than metal-like. I fear the same is probably the case here, with the alloy potentially fracturing along crystal type grain boundaries.

Until they report actual tensile strength and elongation, don't believe the hype. High compressive strengths are not very useful.

[jordanb]

Steel is used for compressive applications all the time, not only slender column.

[lnenad]

> High compressive strengths are not very useful.

I mean, it depends on the use case right? Modern tall buildings/skyscrapers with metal framing use metal pillars where high compressive strength is very useful.

[A_D_E_P_T]

Absolutely nobody is using an expensive novel alloy for 2GPa compressive strength. A 2GPa tensile strength, depending on various other factors such as corrosion resistance and thermal properties, could however be very interesting. (The strongest bulk steel alloys generally peak at ~3.6GPa tensile, which is approximately their theoretical maximum, but there are a lot of applications where steel simply can't be used. Nickel superalloys are typically 0.9 to 1.1GPa UTS.)

[lnenad]

That's true, but it's still factor and if price comes down having that much compressive strength at your disposal could be an interesting choice for supertall skyscrapers.

[A_D_E_P_T]

2GPa isn't a super-high value for compressive strength. Regular tool steels can exceed it. Technical ceramics routinely exceed it. If you normalize to weight, various cement/concrete formulations can also surpass it.

Also, the price of a Ta-Hf-Zr-Nb-Ti alloy isn't especially elastic. Tantalum in particular is an unavoidably expensive element.

I'll grant that what would be exceptional is a metal with a >2GPa compressive strength, a >2GPa tensile strength, and decent damage tolerance and ductility. I don't think that this paper describes a material with that outstanding combination of properties, though -- it's much more likely to describe a simple brittle material.

[lazide]

Interestingly, the design of beams and the like (used for columns) still means tensile strength is the limiting factor, not compressive strength.

For compressive strength alone to matter, you’d essentially need a solid steel column - which is so prohibitively heavy, it would often end up collapsing under it’s own weight (in tensile failure, most likely) before it got up to a useful height.

[max51]

I'm a structural engineer, and this is incorrect. For structural steel (e.g., A992, 350W, etc.), the tensile strength is the same as the compressive strength. That's why we only need one value for both.

And yes, when we design beams, we check the stresses in both tension and compression. For a symmetrical shape like an I-shaped beam, both verification will be the exact same formula so we don't need to calculate both explicitly.

For columns, we do check the compressive stress, and it is the controlling failure mode for short columns. Long, slender columns will buckle before the compressive stress exceeds the allowable limit.

[lazide]

It depends on the geometric stability of the column shape, which is why I said what I said. The reality is that compressive stress limits are much lower in steel than tensile, as you’re noting.

They are only the same in an abstract theoretical sense, not an actual one.

And bucking within the column (and/or the resistance too it) is primarily resisted by tensile strength.

[max51]

>It depends on the geometric stability of the column shape, which is why I said what I said.

What you said was like 99% completely false.

>that compressive stress limits are much lower in steel than tensile

No they are not.

> as you’re noting

I never acknowledge that. Your lack of understanding in what column buckling is doesn't change anything. Unironically, you should ask chatgpt, it can probably give you a high level explanation that is more compatible with your understanding of physics than what I can give you.

Buckling is a stability problem, not a resistance problem. The compressive stress resistance is literally not part of the formulas that we use to verify it. The column could have 100 MPa, 350 MPA, or 359918 MPa compressive resistance and it would change nothing when it comes to buckling. Only the elastic modulus and the physical shape of the column (length, inertia, etc.) is relevant for buckling verification.

>And bucking within the column (and/or the resistance too it) is primarily resisted by tensile strength.

Than please explain to me why we don't need the tensile (or compressive) resistance of the material to know the buckling resistance of the column. I really want to hear that one.

[lazide]

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.

[lnenad]

Yeah, nothing exists in a vacuum :), even for concrete columns rebar is added.