Materials Mechanical Properties Terminology

kbd512 · 2024-03-27 12:13:49

After seeing some of the responses on this forum about Carbon Fiber being similar in strength to stainless steel, I felt the need to start a topic about basic materials mechanical properties definitions and meanings.

We have people repeating and likely believing, apparently following some erroneous off-hand remarks from Elon Musk, that stainless steel, one of the weakest structural materials in common use, is similar in strength to Carbon Fiber, merely because Carbon Fiber is frequently used with a resin (plastic) matrix to bind the fibers together, commonly known as "CFRP" or "Carbon Fiber Reinforced Plastic".

That is false, period. I don't care what Elon Musk thinks about material mechanical properties. Testing has proven what different materials can or cannot withstand in the way of loads applied to them. Belief in actual test data is not required. The data show that CFRP is stronger than stainless steel, except when exposed to high temperatures.

From any temperature between absolute zero and about 200 degrees Fahrenheit, high modulus (high tensile strength) Carbon Fiber, combined with an appropriate epoxy resin (not all epoxies are suitable for use at cryogenic temperatures), CFRP is dramatically stronger than stainless steel over the same temperature range. Above 200 degrees Fahrenheit, but especially above 300 degrees Fahrenheit, all steels, even the weakest ones, rapidly become "stronger than CFRP", because the resin itself begins to soften / weaken and structurally fail (fail to bind the Carbon Fibers together).

TorayCA (Toray Composite Materials America, Inc) Carbon Fiber is the high modulus material most engineers are referring to, when they speak of "high modulus Carbon Fiber". Specifically, they're talking about the T700 or T800 fibers produced by that company, or any other company that also produces engineering Carbon Fiber materials to the same mechanical property specifications as TorayCA. These Carbon Fiber materials are certified aerospace materials accepted by the FAA / EASA / NASA / ESA, for fabricating aerospace structures, similar to how 6061-T6 Aluminum alloy and 304L austenitic stainless steel are also certified aerospace materials. If you use a material that is not certified and of known provenance / origin, then you will have to prove to the governing body that your materials meet performance specifications suitable for your particular application. In actual practice, if you want to build a jet airliner, for example, then you're going to use certified materials or your airliner, or get new materials certified ahead of time, or your new jet airliner design is not passing the FAA or EASA certification process.

This is also why soda can Aluminum is not accepted as an aerospace material. Sure, it's Aluminum, and it might work for some uses in aircraft and spacecraft, but it is not Aluminum that meets the standards set forth for certified aerospace materials. Both the manufacturer of the material and aircraft fabricator will take test samples of the materials they intend to use, send them to certified labs to test, and if the specimen sent fails to meet specifications, then the entire lot of material is rejected. There's a paperwork trail that leads directly back to the materials point of origin. If there's any question about the suitability of the material for the intended use case, then it will be tested. If it's found to not meet specifications, then there will be a lot of lawyers and product liability involved. A very lengthy and detailed set of chain-of-custody documentation is kept by the manufacturer and the aircraft fabrication firm that uses the material. The same applies to Carbon Fiber and all other materials used in type certificated aerospace vehicles. The military uses their own system, with even more stringent testing and acceptance requirements. These certification systems exist specifically because of the price of engineering failures, that was ultimately paid for in blood.

When Airbus, Boeing, Lockheed-Martin, Northrop-Grumman, Orbital ATK, RocketLabs, ArianeSpace, SpaceX, ULA, etc are producing airliners or tactical fighters or rocketry components made from Carbon Fiber, those two materials are called out by name when stiff / strong / light composites must be used to meet strength / stiffness / weight performance targets. When Boeing tested their cryogenic LH2 composite tank demonstrator, it was fabriated from T700 fibers robotically wound around a mandrel with pre-preg (resin applied to the Carbon Fiber at the Carbon Fiber factory, and shipped in refrigerated containers to prevent the resin from curing) Carbon Fiber laser sintered / "glued" into place, before being placed into an autoclave and "baked" or "cured" at relatively low temperatures (200 to 300 degrees Fahrenheit) for a given number of hours or days. Doing that ensured a very high fiber-to-resin ratio with the fewest possible voids or weak points throughout the entire composite LH2 propellant tank structure. This was not a fast or particularly cheap process, but it has been proven to work, repeatably. Repeatability with composites is far more difficult to achieve than with use of metals, but it does work and there are testing methods that can detect and characterize any void spaces (air bubbles in the composite work piece, where there is neither Carbon Fiber nor resin present in that particular area of the structure) or weaknesses in composites using X-rays, sonar transducers, and light.

Now, back to basic material properties...

"Yield Strength" / "YS", which is different than "Ultimate Tensile Strength" / "UTS", frequently abbreviated to just "Tensile Strength", for any kind of metal, is the point at which it starts to deform like plastic when a mechanical load is placed upon it, and it will then continue to deform until it fails (reaches its UTS), unless that load is removed. Tensile Strength is measured by taking a specimen of the candidate material of set dimensions, and then trying to literally pull it apart using a machine capable of applying force sufficient to tear it apart (usually a hydraulic load cell that can precisely apply force using hydraulic power, and precisely measure the force being applied). YS and UTS can be measured in terms of "psi" (pounds of force applied per square inch of material surface area), or "MPa" (MegaPascals), a unit of force equal to 1,000,000 Newtons applied over 1 square meter.

Most metals will yield, or begin to deform, long before they are completely torn apart. Yielding is permanent deformation of the material, whereby if the load is removed, the piece of material is permanently altered in shape and strength from the act of deforming it. This is different than elastic deformation, whereby the material will "spring back to its original shape" after the load is removed. When a sheet of steel is stamped in a stamping press, and it retains the shape imprinted into it by the stamping process, that is an example of permanent deformation that both strengthens and work-hardens the steel in an irreversible way. You can make the steel softer again by annealing it with heat, or you can remelt it, draw it into a sheet of steel again, and start the process over, but ever after, unless melted back down into liquid / molten steel, that piece of steel has experienced a permanent or "plastic" deformation. In other words, the steel has "yielded", and is not going to "spring back" into a flat sheet of steel. Yielding can be a good or bad thing, dependent upon what we're trying to do with the material. When we want to shape the material into a structural spar for an airliner wing, yielding is a good thing. When the aircraft is flying around, if the spar yielded when the plane banks sharply to avoid an obstacle, that would be a very bad thing, an indicator of faulty design or engineering practices or a pilot exceeding the structural limitations of the airframe.

Unlike most metal alloys, for a material like Carbon Fiber, the YS and UTS are very close to each other, almost on top of each other. There is very little warning of an impending CFRP failure. It does not start to deform or bend like metal until it's very near to the point of failing completely. When Carbon Composite materials do fail, they break like a glass rod, quite unlike a soft and structurally weak steel, such as the one used by SpaceX, namely 304L stainless steel. That said, the amount of force that must be applied for a properly built high modulus CFRP structure to fail, is anywhere between 2X and 6X of the absolute strongest metal alloys in existence.

The strongest metal alloys, all of which are ferrous or Iron-based (steel alloys of some kind), have yield strengths of up to 400ksi, or 400,000 pounds of force applied per square inch of surface area. There are no Titanium or Tungsten alloys I'm aware of that come within a country mile of the strongest steel alloys, but I will accept any correction in this area, as my knowledge of this class of alloys is somewhat limited. Titanium can be lighter for a given strength, when compared to steel, and Tungsten can be much harder than steel or Titanium, but hardness and yield strength are not the same thing, even though hardness is closely related to tensile strength in steel. Very hard Tungsten alloys are also very brittle. None of these ultra-hard / ultra-high strength steels are suitable for service at cryogenic temperatures. They become even harder and stronger at cryogenic temperatures, but they also become so brittle that they can and will shatter like a pane of glass when struck by a rubber mallet or subjected to a local or differential temperature shock / transient. There are a variety of epoxy resins which may be used to chemically bind Carbon Fiber together, for which that simply does not happen.

In contrast, weldable austenitic stainless steel, such as 304L, which is suitable for use at cryogenic temperatures, as proven by actual testing, will begin to yield or permanently deform around 40ksi, or 40,000 pounds of force applied per square inch. Both YS and UTS go up for stainless at cryogenic temperatures, but not by an amount that will ever begin to approach CFRP. T800 fiber will fail at approximately 852ksi. That makes T800 CFRP structures 21.3X stronger than 304L, when it comes to resisting plastic deformation, aka "yielding to structural failure unless the load causing the yielding is removed", when pressure is applied inside the propellant tank. There is no metal alloy I'm aware of, suitable for cryogenic temperatures / environments, that will begin to approach that yield strength of high modulus CFRP.

The term "austenitic" refers to the crystalline grain structure of certain types of steel. All metals have a crystalline structure which imparts or dictates some of their mechanical properties. Other mechanical properties, such as density, are dictated by atomic mass. Austenitic alloys tend to be softer / weaker / more ductile. Martensitic alloys have needle-like grain structures, making them very hard and strong (very high YS and UTS), but also subject to cracking or shattering like glass.

You will see the terms ferrite, martensite, austenite, cementite, bainite, and pearlite used to refer to the crystalline microstructure of steel alloys. All of them imbue the steel with different mechanical properties that materially affect the suitability of the steel for one application or another, as well as setting limits on appropriate fabrication methods to build structures from materials with such grain structures.

tahanson43206 · 2024-03-27 12:29:07

This post is reserved for an index to posts our members may contribute over time.

The topic is inclusive enough to provide a wide range of content.

(th)

GW Johnson · 2024-03-27 16:28:09

Hear, hear! Good post Kbd512! -- GW

Calliban · 2024-03-29 20:26:07

Maybe Elon was thinking of maraging steels. These have high nickel and chrome content, so will resist corrosion much better than a plain carbon steel. They are extremely strong and quite tough. But their hardness will cause rapid crack propagation when they do fail. I had a maraging fencing sword for several years. It lasted well, but ultimately failed without warning. One thing to be wary of with high strength steels that rely on a specific microstructure, is poor temperature tolerance. As steel heats above 400°C, strength begins to decline. High strength alloy steels lose strength more rapidly. If strength depends upon cold working or a martensitic microstructure, elevated temperatures can result in permanent reduction in UTS. In alloy steels, high temperatures can also result in migration of alloying elements to grain boundaries. This can lead to stress corrosion cracking. This would be a concern for a stainless steel spacecraft that went through repeated high temperature cycles.

Last edited by Calliban (2024-03-29 20:28:31)

tahanson43206 · 2024-04-23 19:30:15

For kbd512 re topic ... thank you for providing this topic ... it is a perfect fit for the link I'll be providing below.

The article reports on development of an alloy ...

Super alloy’s stamina to bear extreme heat and cold shocks scientists
The metal alloy is composed of niobium, tantalum, titanium, and hafnium.
Abhishek Bhardwaj
Published: Apr 23, 2024 06:21 AM EST

https://interestingengineering.com/inno … 23.04.24_2

Kink bands form in a crystal when an applied force causes strips of the crystal to collapse on themselves and abruptly bend.
“We show, for the first time, that in the presence of a sharp crack between atoms, kink bands actually resist the propagation of a crack by distributing damage away from it, preventing fracture and leading to extraordinarily high fracture toughness,” said Cook.
The alloy will undergo a lot of testing and research before anything like a jet plane turbine or SpaceX rocket nozzle is made from it.
However, this study indicates that the metal has potential to build the engines of the future.

RGClark's been looking for better rocket engine performance ... perhaps (in time) this new class of materials may help.

(th)

kbd512 · 2024-04-24 11:39:20

Much like ceramic matrix composites (CMCs), and ceramic metal matrix composites (CMMCs), high entropy alloys (HEAs) like the Nb45Ta25Ti15Hf15 alloy investigated in the article that tahanson43206 provided a link to, are the next generation of metal alloys for extreme high and low temperature performance.

The CMCs and CMMCs are either heat shielding materials for hypersonic vehicles or extreme temperature components within the hot sections of jet and rocket engines, especially combustor cans, afterburner components, convergent-divergent nozzle components, and uncooled rocket engine nozzle extensions. CMCs and CMMCs exhibit extreme resistance to oxidation at high temperatures. HEAs are interesting for sCO2 turbines and RamGen engines (supersonic inlet velocity gas turbines for Mach 2 to Mach 3 operation), as well as the cooled portions of rocket engine nozzles, turbopump components, main injector plates, injector pintles, hot section expansion turbine blades, engine casings, and fasteners in jet engines, etc. The nozzles and combustor cans don't require extreme tensile strength, but they do require extreme thermal shock and oxidation resistance.

Initial testing of the aforementioned HEA indicates good creep strength, which is mandatory for sCO2 gas turbine components operating at both high temperature and pressure, but I want to see the results of other basic tests such as oxidation resistance, stress-corrosion cracking, resistance to chemical attacks, etc. There are a myriad of different test results we require before judging these novel materials as suitable for aerospace applications. The issues associated with the use of D6AC high strength steel in the F-111 wing boxes and fuselage immediately comes to mind about what can happen if you assume too much and know too little. There was nothing intrinsically wrong with using D6AC, but forging, heat treatment, and corrosion control all mattered greatly, but were little understood and little appreciated until they became real problems during both manufacture and long-term service.

Calliban already indicated what can go wrong with high strength steels. I have no way of knowing how well made his rapier was, but heat treatment is critical to high strength steels. The stronger and therefore harder the steel, the lower its impact resistance. A rapier's blade is subjected to repeated sharp impacts over a very small surface area, analogous to a lifetime of impact strength tests, so perhaps not the best application for a very hard steel. 1095 and other low alloy plain carbon steels, with appropriate heat treatments, are more typical for such blades. Steels like 1095 and 5160 are not as hard or strong, but much tougher and more ductile. This is why Mangalloy found use in forging hammers, rock crushers, and tank tracks. More recently, with the development of appropriate welding techniques, Mangalloy is now used for LNG transport and storage, because it remains tough and ductile at very low temperatures. While aircraft landing gear are a well known application of maraging steels, and also subjected to repeated extreme force impacts, those impacts occur over much larger bearing surface areas, oleos increase the length of time over which load transfer occurs upon landing, and extreme process control is applied to every aspect of their manufacture and maintenance. Knicking or cutting ("notching") high strength / high hardness steels can and does lead to fractures. Titanium alloy parts like landing gear legs or oleos and aircraft shear bolts common for engine mounting in high performance jet aircraft, behave in a similar manner when knicked or "notched".

Every so often someone within the homebuilt aircraft community has the bright idea of using Titanium alloy to reduce the weight of engine mounts, bolts, and the firewall. The smart ones who listen to others are advised against it, not because it won't work, as it absolutely will work if all the welding and handling precautions not to knick the metal anywhere are followed correctly, but mistakes do happen. The simple reason is that 4130 chrome-moly tubing is so thoroughly proven to work, even when the welding or handling of the components are less than perfect. Messing up 4130 takes real effort, even though it can be done. Other countries require that the welder be aerospace certified to weld an engine mount or gear leg. That's why you see so few tube and fabric aircraft in other countries, but lots of them in America. Here in America, we don't have any such requirement for welding. Since 4130 is the most frequently used material, even homebuilders with limited experience encounter very few weld failures where amateur welders are involved. Most have the good sense to practice their technique, try to break the weld, and to seek out advice for how to do it properly. Welding takes a few weeks to learn to do well. Riveting takes a day or two, which is why American WWII aircraft were mostly riveted Aluminum. Welding difficulties aside, the people who make the Titanium Cessna landing gear will tell you that any scratches or knicks to the gear legs must be buffed out. Their product is primarily intended to reduce the weight of the gear for bush planes, to partially compensate for the heavier tires and brakes. That's all perfectly logical, but the propellers for bush planes frequently throw small rocks when landing in river beds, some of which will inevitably strike the gear legs, amongst other parts of the airframe. A pair of Cessna main gear legs cost $2K to $5K. Titanium costs over $30K. That's quite a lot of money to drop 50lbs or so over steel. The weight savings of Titanium over 4130 are real and meaningful, but so are the increased manufacturing costs and maintenance requirements to avoid landing gear fractures in a place you may not easily get out of, such as the Alaskan backcountry.

When the Navy switched from HY-80 to HY-100 / HY-110 / HY-120 for submarine hulls and now the Ford class aircraft carrier hulls, the corollary was increased welding costs, increased weld rejection or failure rates, and increased inspection an maintenance requirements. The stronger steel was used in the more modern Sea Wolf and Virginia classes to increase dive depth. The Ford class uses the stronger steel to avoid significant weight increases related to its much larger flight deck area and heavy EMALS catapult equipment. HY-110 allows the Ford class to retain the same basic super carrier hull design as the Forrestal / Kitty Hawk / Enterprise / Nimitz classes. All of the super carrier hulls, from Forrestal to Ford, are based upon the Forrestal class hull design, with minor increases to beam width to accommodate increasingly greater weight of installed equipment / fuel / aircraft. The Ford class is "maxing out" the Forrestal class hull design. Any further weight increase would require a hull redesign. HY-100 / 110 / 120 avoided the need to do that. Forrestal class ships started at 80,000t at full load, Kitty Hawk and JFK were heavier, Enterprise significantly heavier, Nimitz modestly heavier than Enterprise due to reactor improvements, whereas the new Ford class is now at about 110,000t with a full load. Any "heavier-than-Ford" super carrier design would require a lengthened hull with an internal redesign for weight and balance purposes as well as that all-important optimal hull form to attain 30 knots for launching jet aircraft. The Forrestal class was the fastest of all the super carrier designs- faster than all the nuclear powered carriers. Internet folklore aside, every super carrier since Forrestal has increased beam and draft, but the same installed geared steam turbine engine power, which means they've become progressively slower, with the Ford class being the slowest of all the super carriers. Ford is the slowest because it has to shove more water out of the way as the beam width crept up. You'll notice that the later Nimitz and the new Ford classes have bulbous bows to delay the bow wave riding up and slowing them down. Earlier carriers didn't have them, because length-to-beam and installed engine power alone was sufficient with their more slender hull forms and reduced draft.

In general, the more sophisticated a material is, or the more extreme the performance requirements become, the tighter the process control involved in manufacture, maintenance, and the more lengthy the list of caveats regarding appropriate uses and problems to be aware of. That said, there are ideal applications for all of these materials. These new HEAs are showing great promise for our next generation engine components to make more power while minimizing weight, thus improving overall efficiency of energy usage.

kbd512 · 2024-07-23 09:17:04

This mechanical engineer provides an explanation as to why SpaceX is switching from 301L to 304L stainless steel:

Why Is SpaceX Switching to 304L Steel for Starship SN8?

The toughness of 304L at cryogenic temperatures is higher than 301L, as well as the corrosion resistance of the welds is higher, is markedly higher for 304L, despite the fact that 304L is a weaker metal (lower yield strength / lower ultimate tensile strength).

GW Johnson · 2024-07-23 10:20:18

Just goes to prove that other properties than simple tensile strength get into real-world engineering design.

GW

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#1 2024-03-27 12:13:49

Materials Mechanical Properties Terminology

#2 2024-03-27 12:29:07

Re: Materials Mechanical Properties Terminology

#3 2024-03-27 16:28:09

Re: Materials Mechanical Properties Terminology

#4 2024-03-29 20:26:07

Re: Materials Mechanical Properties Terminology

#5 2024-04-23 19:30:15

Re: Materials Mechanical Properties Terminology

#6 2024-04-24 11:39:20

Re: Materials Mechanical Properties Terminology

#7 2024-07-23 09:17:04

Re: Materials Mechanical Properties Terminology

#8 2024-07-23 10:20:18

Re: Materials Mechanical Properties Terminology

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