Aramid tube

Discussion in 'Materials' started by thebruce, Jul 28, 2016.

  1. thebruce
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    thebruce Junior Member

    Hello All
    I’m hoping to discuss methods for vacuum forming and subsequently testing (in relatively small quantities) structural aramid round hollow tubing using 6 oz plain weave old stock. I’m thinking >50” and between ½“ and 1 ½“ diameter plain cylinders as a starting point. I have some experience using West System 105/205 in a shop environment hand laying one-offs but none with Aramid or mold/mandrel release...so this is a bit of a stretch...particularly the testing.
    I have gone so far in my thought process as to entertain an air pressured mandrel (possibly tire tube) inside sewn aramid sleeves (at least for a starting point) but having no experience with such I thought I might prevail on the group's collective experience.
    Thanks in advance to any and all who offer advice,
    thebruce
     
  2. gonzo
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    gonzo Senior Member

    You will have to find a testing facility. Some universities can do that at a lower cost than private contractors. The testing will still cost you several thousands of dollars. Also, tests only provide raw data. An engineering analysis must be done after to make sense of it. The analysis is often more expensive than the testing. However, the testing parameters will be determined by the type and scope of the testing. It will be more efficient if you first hire an engineer to design the testing. That will also give you guidance on what to build as samples. Sample coupons should be standardized also. Using a tire inner tube is really crude. The advantage of using aramid fibers is that the structure can be lighter than a comparable steel one. To accomplish that manufacturing standards have to be tightly controlled. Otherwise you will have a heavier, more expensive and possibly weaker structure.
     
  3. thebruce
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    thebruce Junior Member

    Aramid tubing

    Thanks Gonzo,
    Absolutely right and sorry I didn't make myself more clear...thought I shouldn't begin with a lengthy treatise.
    Should have also mentioned I wanted to approach this experimentally/experientially. I am more interested in "anecdotal" results a la ... if if doesn't break...it's too heavy" kind of thinking...and I would act as my own testing facility through personal use (albeit likely not in a competitive environment).
    Actually, I was thinking the inner tube would likely be easily removable since I wanted to avoid drafting the tubing (say into a cone) over a hard mandrel. I was actually thinking about trying a silicone sheet lining and attempting to balance the internal pressure of the inner tube against the external pressure of the vacuum in an effort to end up with hollow tubing that might then ultimately be spray foam filled (burning is not a primary consideration) as an exercise in "pre-stressing" selected components (a question that has lay on my brow for years)...but I digress.
    I would be very interested in knowing how (or even if) you guys would approach building small profiles such as safety rail stantions or hobie-sized catamaran or proa beams. My thinking is that the openings might be too small for much in the way of internal spars or structures...but might be in the zone where a little extra modulus would suffice without too much weight penalty. I can envision a lot of uses for small-ish, high strength, light profiles in the boatbuilding community.
    In an effort to save anyone else the trouble, I am aware that aramid is absorptive and UV sensitive.

    Thanks again Gonzo for the advice.
    thebruce
     
  4. gonzo
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    gonzo Senior Member

    I would make those profiles out of metal. I don't see any advantage on weight, price or strength for a hand made small profile. If you don't test them impartially, it will be easy to convince yourself that they are better than commercially available materials. Forums are full of people that make claims but won't test them scientifically. However, if you are going to have fun, go for it. I spend a lot of my time on things that don't really make sense; like owning boats ;).
     
  5. thebruce
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    thebruce Junior Member

    Aramid profile tubing

    Thanks again for the advice and right again. I’m pretty sure I read somewhere that you have to pick 2 of the weight/strength/price triad...I'm selecting the first two...emboldened by the mitigating advantage of having a few yards of aramid, long since absorbed in the cost of previous work (so, upon further analysis, I’m going for all three I guess). I figure I can absorb a little foolishness without any real consequence outside of my own time/risk...particularly if I am my own client for the time being. Just guessing but...you must be an engineer...don’t remember funning across that in your prior postings. I, OTOH, am not an engineer and have only just begun to get a feel for the strength and utility of this material. I may end up using carbon fibre when I run out of this testing stock but I have to admit that I like the failure mode of Aramid in this particular setting. I plan to test my work in action (where the rubber meets the toad, as they say) and I am hoping for a little advanced notice where I can get it...because I am bound to make mistakes (the nature of trial and error). Clearly, dependent on the application, there could be sufficient advantages of composite structures over many commercially available metal products to make the endeavor worthwhile...even if it ultimately fails. So I guess this exercise has helped me further refine my question to - “as diminishing diameter effectively precludes handbuilt internal structure, is there a point (prior to rod) where tubing wall thickness (with foam or other applied retrospectively) might compensate for rigid internal (compression) structure as posited earlier?” Clearly section profile would essentially control structural characteristics...but here I am consciously staking out round tubing as my first playground with common building profiles.
     
  6. tom28571
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    tom28571 Senior Member

    I'm not sure what purpose the tubes you wish to mold are to be put to. If they are to be spars that will need to be stiff, you will not get the real value that Aramid offers by using plain weave stock material. All composite spars that are meant to have high bending modulus (stiff) use unidirectional fiber as are similar pressure vessels.

    The Aramid fibers that are woven around other fibers do offer a very strong material but not a stiff one with minimal elongation when stressed. That is true even for the best carbon which has to be laid up in the direction of the tension stress to provide a stiff structure.

    The woven material you see when looking at many composite carbon tubes is there to provide "hoop" strength to resist stress around the circumference. Hoop strength is often provided by winding unidirectional fibers in spirals along the length of the tube or pressure vessel.
     
  7. gonzo
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    gonzo Senior Member

    Most of the carbon we see is there for looks. They use coarse weaves because it wouldn't be too appealing to simply see the black surface unidirectional carbon has.
     
  8. thebruce
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    thebruce Junior Member

    Thanks for the reply Tom...if that's your zip we are close.

    Actually purpose is an open question. At this point I just want to form and test tubes as an exploration of fabrication...purposely implying fabrication of tubes into structures. As designers, elongation (flex with memory) is a property to be explored/exploited...thinking lashed beams (early cats/proas), bows and suspension frames. Is it not the difference between monoliths and instruments at its heart (simply?) a mastery of the characteristics (memory/frequency) of stress transitions? In terms of (my vernacular here) organic materials (say...wood) and inorganic materials (carbon, kevlar, etc). the biggest difference I see is that inorganics offer us a level of consistency that is difficult to "select for" in organics...in the same vein as laminating. Consequently, wood is incredibly de-rated to provide adequate safety margins given potential flaws. My goal is to produce consistent high modulus profiles so a designer can move on to design without the fuss and expense of the likes of filament winding. As I stated, I think prestressed (a characteristic of filament winding and concrete) is an area of great promise. Likely the benefits of prestressing would extend to panels as well if we can learn to handle the frequencies.
    I admit to being intrigued by the ease of joining composites...thinking of a thread I read here about cutting and re-joining a mast (aluminum I recall)...seeded my thinking.
    I am absolutely not material prejudice...so any advice you offer is greatly appreciated. Clearly my forte would be wood joinery...from sawyer to instruments. I could use discussion/advice on anything composite. I have this aramid and it is burning a hole in my pockets...so to speak.
     
  9. gonzo
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    gonzo Senior Member

    Pre-stressing works for reinforced concrete because is has a high compression strength and very low tensile strength. For Aramid/epoxy composite that is not true. I can't follow what you mean about monoliths and instruments etc. The sentence makes no sense to me. Wood is no de-rated. Margins of safety are not necessarily higher for wood than composites.
     
  10. thebruce
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    thebruce Junior Member

    Sorry Gonzo,
    I have been accused of being cryptic before...the difference between instruments and monoliths refers to the refinement or acumen reflected in our use of materials...think stone age architecture compared to the architecture of modern wings. I would consider a modern wing an instrument where Mayan temples are monolithic...the development of plinth arches would then be an example of a pre-stressed development/usage. As an example of de-rating let's consider a simple span table for a given dimensional wood member...the table reflects far...far less than a premium member's actual strength because safety factors generally reflect a lower quintile of the quality distribution of the timber available in the trade, the breadth of common growing conditions of a species (think of the difference between old growth and new), prevailing harvesting/handling conditions from forest to factory floor, etc.). Safety factors are, by definition, banket derating mechanisms applied to broad classes of materials. The more familiar you are with a material the closer you are able to reliably approach edge conditions of usage. I admit that I presumed that composites have reached a higher level of predictability than wood products due to their consistency...based on the zero fault usages of the aircraft industry. I'm betting it would be discernible in their regulations, but I have no desire to dig in and prove that. However I can say that I have consulted on projects that could have been substantially improved had we been able to empirically up-rate select forest products and have worked on instruments to forecast the structural quality of sawlogs or the tonal characteristics of musical instrument materials for that very reason. The ability to get a material (or combination of materials) into a desired shape and configuration is only the beginning of the use of that instrument. I hope that makes it clearer. I purposely did not address the pre-stress condition and hope you will take the time to reconsider your response as I consider it obvious.
     
  11. tom28571
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    tom28571 Senior Member

    Bruce,

    I think I understand your posts although the terminology is alien to me and I suspect most others on this forum. You are correct about wood ratings being derated to some value much lower than the best of a species, as obtaining the best is rarely possible. Span tables are based on deflection, which loads the member well below its rupture stress limit. The US Forest Products Lab gives the various modulus values for different species but does not, as far as I see, give any indication of the range of values or which level they specify.

    Wood is the ultimate composite material with high strength fibers bonded in a matrix of weaker material to distribute loading and avoid point stress failures. Bamboo is a prime example. FRP is our copy of nature.

    I am, or once was, an engineer but an electrical one. Mechanical studies have been made mostly following retirement for designing a post and beam house and several boats. All engineering follows the same path. I'm on the ICW midway along the NC coast.
     
  12. gonzo
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    gonzo Senior Member

    Arches are not pre-stressed. They are a typical example of a structure in compression.
     
  13. tom28571
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    tom28571 Senior Member

    Gonzo,

    I know what a plinth is as well as what an arch is but have no idea what a plinth arch is. Maybe we are handicapped by a common language here. Arches are naturally not pre stressed as that is their stability secret.
     
  14. thebruce
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    thebruce Junior Member

    Wow...as disclosed, I am not an engineer...and I am having big communication problems. Seriously...neither of you recognize a masonry arch as a stressed system? The master bricklayer I apprenticed as a teenager called the center stone in the arch (where the stress “changes direction”) a plinth. So if compression is not a stress would a arched bridge be stressed if, for example, a car drove over it? Or would we refer to that as loaded? Either way, I consider it obvious that such an arch stands (over time) under constant compression (stress) by virtue of adhesion/cohesion of mortar (or pure geometry in dry lain examples) enabling the distribution of the load rather than the blocks pushing out tangentially to the “stress” of compression and the eventual demise of the structure...as cautioned by that same master mason. So what shall we call those forces? I remain flexible.

    In that same vein, pre and post stressed concrete structures exhibit, in cautious vernacular, enhanced load carrying capabilities (with some compromises) and I consider it obvious that pre or post stressed composite structures will, to a greater or lessor extent, behave accordingly...to what extent...I do not know but I would delve into that mystery.

    As for lumber rating, if we do not recognize this then we have gotten too far removed from the materials we depend on.

    In an effort to re-direct this conversation to something that will land me back in the shop, I have read a bit on Polyumac foam which is a bit cheaper, I think, than CoreCell. If we are agreed it would be a reasonable starting point for laying up these tubes then I will proceed. Suggestions as to what testing would be germaine would be appreciated. I fully expect to test these first examples to destruction before I am through.

    I will be more careful with my terminology in the future and I apologize for any confusion I may have caused in my ignorance. Thanks for your patient efforts.

    I’ve heard the ICW is great inland cruising...I’m a river person myself...so now I'm a bit jealous.
     

  15. tom28571
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    tom28571 Senior Member

    Ah Bruce,

    I think we have happened on the main problem with the communication. Definitions are meant to be defining. That is their purpose and we can't change them for personal whim, or lack of knowledge, with out miss directing the conversation. You used pre-stressed in describing an arch and that simply sent the wrong message. Everything in a gravity field is stressed in some way, so yes, an arch is stressed, which is how it works and is why Notre Dame Cathedral is still standing.

    A plinth is the base that an arch or column stands on. The piece that connects the two sides of a arch is the keystone.

    When communicating, words are important if the idea behind them is to be understood. In sailing a lot of otherwise gibberish is used which is called jargon so as not to confuse what is meant with everyday life. Same in any technical field or even music and poetry. I suspect you are way more knowledgeable about what you want to do than we are. I also know you must already know this but I just wanted to set it in place here.
     
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