Bending Titanium (repeatedly)

Daniel,

good question. until a more knowledgeable individual replies, let me just mention a few points.

while it’s true that Ti alloys, generally speaking, have superior fatigue characteristics to many other metals, any time any metal (other than perhaps “memory metals”) is flexed/bent and yields that piece of metal is weakened. [yield = basically, when the material which is bent does not return to the same shape when the bending/buckling stresses/forces are removed]

now, how many times this can be done, i don’t know. generally speaking, you can do it more often with Ti than with Al (very poor fatigue characteristics cp to Steel & Ti). if you can bend the metal without it yielding and insert it into a pot/cup, with the pot/cup keeping it from springing back, then you might never fatigue, crack, and break the spork. if you yield it & never straighten it out, then, once again, you will never break it. don’t know how much it needs to be bent, so don’t know if you can reasonably use it in its bent state.

thin, flat Ti can be deflected/flexed/bent many times. as long as it does not yield, it should, generally speaking, be able to experience many, many cycles. each differently shaped specimen has different fatigue characteristics even if made of the identical Ti alloy. we often test aircraft components, that go on our helicopters, out to 100 million cycles. the basic rule in fatigue is the higher the stress applied to the part, the fewer cycles at that stress level the part can experience before a crack develops & then propagates to failure (breakage) under additional cycles. the lower the stress applied (resulting in less deflection/strain), the higher the number of cycles at that load level. the results of such testing is “plotted” on a graph/plot, known as an SN curve. . you could do a web search on “SN Curves” for more info. there is more to it than just these facts, but these suffice to give a rudimentary understanding of fatigue. i’m trying to err here on the side brevity, instead of putting most readers to sleep – as i fear i usually do. [hey…is that snoring i hear?]

sorry can’t be more help here. hopefully, a more knowledgeable person will reply and educate us both.

Seems to me repeated bending of any alloy will eventually exceed the ability for it to re-align its crystal structure.

One interesting thing I’ve noted though is the effect of repeated heat/cool cycles on my homemade aluminum windscreen. I used 6″ wide flashing to make it and it fits into my cookpot with little effort. I’m beginning to notice that when I take it out of the pot it just “springs” into the proper hole alignment now (no overshoot, no memory from being rolled up tighter in the pot). I wonder if this is due to tempering of the material (much like disc brakes)?

John,

as i understand it, “any bending” will NOT necessarily cause damage to the crystalline lattice of a a metal/part/piece of material. the stress/load must be above what is sometimes called the “endurance limit”. if the stress/load is above the endurance limit, then some damage will result. this does NOT mean that the part will immediately break (known as static failure – the part is essentially just ripped apart by the humongous – a little known techinical term – load). the endurance limit is different for a different number of cycles applied to the part. the endurance limit for a higher number of cycles is lower than the endurance limit for a smaller number of cycles.

a stress applied causes the metal to “stretch” under load (called strain and often measured in the units micro-inches per inch). if the stress and strain do not exceed certain levels, then no damage is done to the crystalline lattice of the metal. on the molecular level, bending stresses must be above a certain level for damage to occur to the crystalline lattice. even brittle metals like elemental Ti possess some “elastic” ability. each metal has this characteristic – “Google” the following phrase: “Young’s modulus of elasticity” (which relates stress to strain). all metals have this property.

in fact, Google Thomas Young – one of the most varied and fascinating individuals – a genius of the highest order – perhaps on par/nearly on par with Newton or Euler (pronounced “oi-ler”, not “you-ler” as is commonly heard). His contributions are many & varied in several different distinct fields. most articles on him fail to mention his photographic memory and his contributions to Biblical scholarship (using his photographic memory wrote down every word in the King James Version of the Bible and how many times it was used & assigned the appropriate Hebrew/Greek/Aramaic word to each word each time it was used – see “Young’s Analytical Concordance to the Bible”, did his own translation of the Bible from the original languages – hebrew, aramaic, and greek , as well the nearly unbelievable number of ancient languages he could read (all by the age of 16; taught himself to read english at age 2 & calculus at age 5. …and still there’s much more about him, discovery of the “cartouche” in Egyptian Hieroglyphics, he discovered the wave properties of light published in his paper “The Undulating Wave Theory of Light”…oops…i diverge from the intent of this post – there’s more, check him out).

hope this info helps. if i have muddied the waters any for you (or anyone else), just post back & i will try to clarify.

also, any ME, metallurgist, or structural engineer out there, please feel free to help out here (i’m floundering & going down for the third time!!!) if you are so inclined.

Paul,John,
Thanks for the comments. From a theoretical standpoint, bending the titanium beyond it’s yield strength (where you cause permanent deformation, ie it does not spring back), and then bending it back to it’s original shape will weaken the metal, umong other things. The spork in the picture I mentioned earlier was bent this way (to fit a small cup) and then bent back straight for use, I would guess repeatedly.

Realizing that this is “not good” for the spork, I guess what I was wondering is from a practical standpoint how often you could do this before the spork became unuseable. For example, if you could bend it and straighten it out 50 times before you damaged it that to me would be acceptable enough that I might consider doing it just for neat packability.

can’t really say how many times. this is why we would take a spork & place it in a test fixture in our fatigue lab & bend it repeatedly until it breaks. from the experimental measurements done by instrumenting the spork with strain gauges we could then process the data and produce an SN Curve which is a graphical depiction of the spork’s fatigue characteristics.

there are analysts/designers who could mathematically calculate the expected results. i’m NOT one of them. sometimes they are right. sometimes they are close. other times they are hiking the wrong way on a one-way trail. that’s why we verify analytical/design expectations with real fatigue testing.

a simpler solution would be to buy another spork & conduct your own experiment. better still would be to get such info from the manufacturer, but for a spork such testing may never have been performed.

sorry, i can’t help more here.

Paul,

Is that related to shear strength? I’m not a metallurgist so I can’t really picture it. It seems to me if a material has a finite duty cycle then there must be an additive effect of successive cycles on the chemical stability of the material (the lattice). Otherwise, we’re talking about a critical value of force needed to overcome the total chemical strength across the material (cross-sectional shape discluded)?

I’m not familiar with Thomas Young… but my non-science hero has always been Friedrich Nietzsche. I find his cynical aphorisms as refreshing as a morning can of Red Bull… which is why as a child I slept through Church service when the New Testament was discussed but was wide awake for the Old… more operatic value I guess.

Oops, now I’m straying ;-)

John,

i’m but a lowly software engineer. i’ve left research and now work in a Test Engineering group at a major helicopter manufacturer. my knowledge of fatigue and damage to materials comes from writing computer programs that embody our specific fatigue methodology which enable our Test Engineers to analyze the results of fatigue tests and make decisions on the “life” of parts (essentially, how many cycles before they break). As such, I have no formal training/education in this field. This means that there are many “holes” in my knowledge in this area.

“Shear strength” is not a term i have heard thrown about. I will check with some of my co-workers (Aeronautical & Mechnical Engineers) and ask them. My gut feeling is that “Shear strength” is related to static failure (ripping the part apart instead of fatiguing it by cycling it to failure) and has to do with the “ultimate strength” of the part – i could most certainly be wrong though.

also, i know that, “shear” is a term used of fluids and, i believe, relates to how cohesive the fluid is and, in part, its ability to reduce friction, or, as i understand it, how easily the fluid separates into layers. in very viscuous fluids, sometimes used in dampers in large mechanical systems, a loud “snap” can be heard as the fluid shears into two pieces/layers. i’m going to stop here because i know very little about this. hope someone more knowledgeable chimes in soon.

Paul,

Am I correct then as I had concluded that you work for Sikorsky in Stratford, CT?

Also, as I recall, Young’s Modulous is used for stress/strain relationships for Structural Geology. But it has been over 20 years since I had Structural Geology. Also, rock as well as metals, and most other materials are subject to compression, tension, and torsional stresses. Certainly with torsional stress there would be a shear strength at which the metal would shear. We used to demonstrate torsional stress by twisting a piece of chalk. The break would form a helix like shape. Therefore there was shear at the point of “tear” or rupture.

Rich

Rich,

yes.

know nothing ’bout Structural Geology, so i can’t say how it’s used in Struct. Geo. can state unequivocably that Young’s Modulus of Elasticity does relate stress to strain as you stated.

thanks for the add’l info. i’ll make some mental notes for future use.

With all scientific analysis put aside. I have a Snow Peak Ti spork. I bend it slightly to where each end is about 2.5 to 3 inches apart so it will fit in one of my pots.
I have owned this spork for almost 4 years now and go out on at least one weekend trip per month and at least one extended (5 – 7 day) per year.
That being said, I have bent and returned the spork about 225 times just for use, not including demonstration to scouters. I have yet to see any visible signs of stress along the handle. I expect I will continue to be able to bend the spork for another year or two. Besides, when it finally breaks, I figure I got my anualized money’s worth out of the tool (about $8.00 over 6 years, granted I’ve only seen 4 of them). BTW, I’ve melted more lexan spoons (I tend to leave in pots)to cover that cost in half the time.

Mike,

Thanks for the info! I’m going to start bending my Snow Peak Spork. Sounds like it’s totally feasible to do so based on your experience.

Cheers

Mike,

thanks for the input. that info is very useful. i for one really appreciate you taking the time to share this info. i’ve been wondering how many more times i can bend my spork before it breaks. i think you’ve answered that question for me.

Mike, looks like you have a future in Test Engineering (the guys i work with love to experiment, test, and break things). I believe my boss is hiring, get me your resume. thanks again for the valuable info.

Jim,
i for one wish Mike had dealt the death blow after my first post. no…make that before my first post. there sure is no arguing with experience. as my boss always says, “One test is worth a thousand expert opinions!”, and as Igor Sikorsky used to tell new engineers, “Young Gentlemen, when the facts differ from the theory. I urge you to trust the facts.”

[i’ll throw another one in here, since there was been some humor in your post. Igor Sikorsky is sometimes quoted about the early days of aviation, as supposedly saying: “In the early days, designers were often the first to pilot their designs. This had a way of eliminating both bad designs and bad designers.” guess this might be Russian humor – dark, but true.]

The nerve of some folks! Spoiling (perhaps killing!) a perfectly good theoretical discussion by interjecting real life experence. What’s this world coming to? he-he

Fatigue Stress Failures depend on more that just “whether or not it is Ti”

the following things should be taken into account:
1) The actual Alloy Used (6al4v vs 3al2v Ti, or 7000 vs 6000 series Al, or 304 vs 316 vs 400 vs 900 SS)
2) The finishing or annealing process used (cold or hot annealing, cold or hot rolling, hard or soft annealing – i belive one can have hot/cold hard/soft… if I remember my materials science class correctly)
3) The force of the stresses… I would bet that the ti spork in question would fatigue quicker if you flexed it rapidly causing the bend point molecules to heat up and not have a chance to dissipate heat

Point of my random interjection of information? The particular spork that experience has proven “flexible” apparently works well, that doesn’t mean that every ti spork will “flex” readily.

Might as well try it for whatever spork you’re trying.

I personally like my Lexan Foon (gsioutdoors) but that’s cause it’s rather long (same theory as BMW’s Long Spoon, but it’s a spork)

right you are. you remembered your class well. thanks for the info. it’s much appreciated.

my programs actually deal with over 50 types of materials (including ~8 Ti alloys).

also shape of the material. angles. radii. etc. come into play in fatigue. now we use many flaw tolerant designs that dissipate stress after a crack develops so that it does not rapidly propagate to failure.. the idea is somewhat the same as CF and composites, but the mechanism is diff.

i hope i remember this Thread in the future. this subject will undoubtedly come up again in a few months. at that time, i hope to remember to direct everyone to Joshua Mitchell for an answer.

thanks again for helping out here.

btw – I plan on seeing how easy it is to bend my snowpeak spork when I get home from being out of town for work…

Paul – I am currently a software engineer, among other tasks. Thanks for the offer though. No offense, but I think Connecticut is a bit too expensive to live there and it’s much to close to my ex for my comfort level.

Jim – The original post asked “Can thin titanium be bent repeatedly…” I have enough theory in my life that I just went right to facts as I have experienced. I like the K.I.S.S theory as well. (That’s – Keep It Short and Simple rather than the more rude version)

Joshua, and others – When I bend my spork, I usually do so in a gentle curve rather than an abrupt change in shape. The smallest pot I have bent it to fit is the Snow Peak 700 Trek mug; Fits quite well over my Brunton Crux and small canister.

I was flipping channels last spring, and I swear I saw our very own Dr. Jordan on a TV show about ultralight backpacking. (Maybe on the Travel Channel? Not sure.) Anyway, he was showing the host how he bends his Ti spoon to fit in his mug. Just bent it right in half and shoved it in.

Ryan, can you confirm this? Or was I hallucinating again?

–Ken B

It is true. I saw the episode too. I actually have it on Tivo because I could not see it at the time. I still have it on there and, yes, Ryan does bend the spoon to fit in the mug.