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Thread: Design Allowables for Composite

  1. #1

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    Design Allowables for Composite

    I did some reading on this subject while searching for the mechanical properties of composite materials. It turns out the allowable stress/strain you can use in your airplane is FAR LESS than either the strength of fibre or the laminates. Thus the approach of "make your laminate, load it with sandbags, and see when it breaks" is incorrect and dangerous in airplane design.

    The degrading factors affecting the allowable strengths are humidity, temperature, openings and impact damage. Unlike metal, fatigue is usually not a limiting factor for composite. This explains why some certified composite airplanes claims unlimited life.

    The worst case for tensile strength usually is 1/4 inch opening at cool temperature in dry atmosphere; the worst case for compressive strength usually is impact damage at hight temperature in wet atmosphere. The carbon fibre composite starts delamination at about 3rd or 4th layers under the surface. This delamination degrades the compressive strength signficantly but is not visible. To account for tool drop, runway gravels, hails and damage during production, the compressive strength with barely visible impact damage should be used.

    These tests are expensive and the data are usually propriatory. AGATE and NCAMP attemp to share such data publicly. Unfortunately, some so-called "design allables" (both A-basis and B-basis) published by AGATE does NOT use sample with openings and/or impact damages, render them useless.

  2. #2
    steveinindy's Avatar
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    Welcome to why I decided to stay away from composites when designing my aircraft. To produce an aircraft that isn't going to crumple like a Styrofoam cup in a hard landing or crash out of composites, it pretty much requires either some obscene overbuilding (negating the primary advantage of composites) or use of materials and techniques that are out of the league of your average homebuilder (and apparently a popular manufacturer, but let's not go there).

    the worst case for compressive strength usually is impact damage at hight temperature in wet atmosphere.
    I've yet to see a composite homebuilt aircraft that exactly impresses me from an occupant protection standpoint (mostly because it's usually an afterthought if it's even a thought at all). There are some of the commercially built gliders out there that aren't bad in low speed crashes but beyond that, even in a cold dry environment the chance of you getting ejected in a real world crash is quite significant.
    Unfortunately in science what you believe is irrelevant.

    "I'm an old-fashioned Southern Gentleman. Which means I can be a cast-iron son-of-a-***** when I want to be."- Robert A. Heinlein.



  3. #3
    Quote Originally Posted by wantobe View Post
    I did some reading on this subject while searching for the mechanical properties of composite materials. It turns out the allowable stress/strain you can use in your airplane is FAR LESS than either the strength of fibre or the laminates. Thus the approach of "make your laminate, load it with sandbags, and see when it breaks" is incorrect and dangerous in airplane design.
    Tell that to the guy that pioneered many of the designs and then built an operational spaceship......
    Actually he designed first and later found it took a hell of a lot of sandbags and yet the aircraft was still light and a delight to fly.
    My goodness - can that be true?
    Last edited by flyingriki; 05-17-2012 at 02:51 PM.

  4. #4
    steveinindy's Avatar
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    Quote Originally Posted by flyingriki View Post
    Tell that to the guy that pioneered many of the designs and then built an operational spaceship......
    ...and the engineering department that backed him on most of the follow-on designs. Rutan wasn't exactly your average homebuilt designer. He has an engineering degree (graduated at the top of his class too) so I'd guess he knows how to actually do the calculations for this sort of stuff rather than just sandbagging a design to failure to get a rough estimate.

    Then again Wanttobe is painting composites with a fairly broad stroke and it's a very nuanced field.
    Unfortunately in science what you believe is irrelevant.

    "I'm an old-fashioned Southern Gentleman. Which means I can be a cast-iron son-of-a-***** when I want to be."- Robert A. Heinlein.



  5. #5
    Quote Originally Posted by steveinindy View Post
    Welcome to why I decided to stay away from composites when designing my aircraft. To produce an aircraft that isn't going to crumple like a Styrofoam cup in a hard landing or crash out of composites, it pretty much requires either some obscene overbuilding (negating the primary advantage of composites) or use of materials and techniques that are out of the league of your average homebuilder (and apparently a popular manufacturer, but let's not go there).

    I've yet to see a composite homebuilt aircraft that exactly impresses me from an occupant protection standpoint (mostly because it's usually an afterthought if it's even a thought at all). There are some of the commercially built gliders out there that aren't bad in low speed crashes but beyond that, even in a cold dry environment the chance of you getting ejected in a real world crash is quite significant.
    More opinions not based in fact - imagine that?
    Gee I guess Boeing doesn't have your design expertise. Maybe call them and help?
    Have seen the results of a few canard crashes where the aircraft was destroyed and the occupants, either one or both walked away, or limped away. The structures are stronger than they "look" to an amateur in the composite world. Flame on....

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    steveinindy's Avatar
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    More opinions not based in fact - imagine that?
    There's a difference between "not based in fact" and "based in evidence you aren't aware for whatever reason". The fact that you're fond of the composite designs (which I am too just so we're clear) doesn't mean that the scientific burden of proof suddenly shoots up. A foam based composite (at least in the homebuilder's variants) is not as crash impact tolerant as a steel or aluminum frame. You'd be hard pressed to find a composite materials engineer who would disagree that for certain applications, metal is still the best choice for homebuilders as the available and applicable technology stands today.

    It's a tradeoff we each make. Nothing more, nothing less. The goal of some of the research I am extraneously involved with (meaning I'm not the materials engineer obviously but I'm one of the guys who is the injury prevention side of the coin) is to identify ways to overcome the limitations and make the larger benefits of composite more applicable outside of the high end production facilities. One of the designs I am working on specifically is an advanced energy absorbing seat made out of composites.

    Without increasing the availability of ovens and advanced RTFM and other technologies, the full possibilities of composite design and construction remain beyond the grasp of the average builder. With that aspect at least, you can surely agree with me?

    If you want to look at some evidence that is readily accessible, take a look at it this way: If composites are so much better in crashes, why haven't the ag airplane manufacturers (who build probably the most crash resistant designs on the planet short of Indy and Formula 1) moved to primarily composite designs? It's because in anything but a forced landing scenario, a metal cage is a better bet to protect the occupant. It's not opinion, it's backed up with a lot of evidence from the FAA CAMI testing, NASA Langley, UT-Delfft, the Dutch aerospace research laboratories and testing within the industry, etc. Even the AGATE testing of composite airframes under lower end impacts identified several major limitations as did the testing of a couple of Beech Starships.

    The problem is not composites or a lack of capability for them to meet the needs of the situation but how they are being used and the availability of the techniques and technology necessary for their application to occupant protection in the garage or hangar of Joe Q. EAAMember.



    Gee I guess Boeing doesn't have your design expertise. Maybe call them and help?
    Two things "Riki":
    1. I didn't say it couldn't be done. It's just damn difficult. I just said it isn't in the capabilities of the average homebuilder to achieve the same strength to weight ratio you would see with a metal frame when working on a scratch built clean sheet design. 99% of us don't have the engineering know how or the equipment necessary to pull it off.

    2. I didn't realize that Boeing was equipped like the average homebuilder's garage. It makes the 787 all the more of a miracle of engineering. [/sarcasm]


    Have seen the results of a few canard crashes where the aircraft was destroyed and the occupants, either one or both walked away, or limped away
    Anecdotal evidence means what precisely? I could turn around and point to just as many (if not more) that haven't "limped away". Do you have any actual scientific evidence to back up the survival rates in composite versus metal construction? Or are we relying on your happening to be at a few crashes to prove me wrong?

    Give me a few months and I'll show you some because I have a paper in peer review on the subject at the moment.

    [QUOTE]The structures are stronger than they "look" [quote]
    Well, it depends on the situation you're talking about. That's the problem with composites is that their vary nature makes them "stronger" but "weaker" depending upon what exactly you're looking at. I think that's the problem here. You're looking at one side of the issue and I'm looking at it from another.

    to an amateur in the composite world.
    Note, this is not said snidely...I'm seriously asking....

    And you're not? I wasn't aware you were an aerospaceor a materials engineer. If you are, I'd love to learn from you if you're willing to teach me more about your given subject.

    Flame on....
    Likewise. Like the quote in my signature says, in science what you believe is irrelevant. You're not threatening or scaring me off from discussing the subject but I do appreciate being forced to question and explain the stances I do take.
    Unfortunately in science what you believe is irrelevant.

    "I'm an old-fashioned Southern Gentleman. Which means I can be a cast-iron son-of-a-***** when I want to be."- Robert A. Heinlein.



  7. #7

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    Quote Originally Posted by steveinindy View Post

    I've yet to see a composite homebuilt aircraft that exactly impresses me from an occupant protection standpoint (mostly because it's usually an afterthought if it's even a thought at all). There are some of the commercially built gliders out there that aren't bad in low speed crashes but beyond that, even in a cold dry environment the chance of you getting ejected in a real world crash is quite significant.
    Please point us to any data on this topic. I'd be interested to see it.

  8. #8

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    Composite laminate design is normally strain limited and depending on the fiber system, carbon fiber modulus and permissible strain, you can go up to 3500 microstrain. But that is for an autoclaved prepreg toughened epoxy which gives max properties. For home design with low compaction and resin rich laminate, you would want to stay below 2000 microinch. You design to a strain limit rather than a strength limit.
    The limiting factor with most composite structures is the lack of plasticity on failure with high energy release. However by using a strain allowable with sufficient margin, composite designs are very safe and successful with no corrosion or fatigue issues. Racing cars have used carbon cages for years to protect drivers in very serious crashes so the materials do perform well when properly designed.

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    If you want to learn about the crash worthiness of composites, look to Formula 1 race cars. This is likely the only place where crash worthiness has been a primary design consideration. Some Starships were tested, but they were not optimized for crash worthiness, they just did what they did. We're they redesigned based upon the test results and retested? Until there are light aircraft crash testing requirements developed like they have been or passenger cars I don't think we will see much improvement. It is too expensive and the market will not support it.Light aircraft crash worthiness is built in via corrective actions to anecdotal evidence. Physics impose some limits however- you just can't survive a smoking hole incident and might just barely survive a stall/spin (in the back seat). The best idea yet is a parachute to avoid the sudden stoppage in the physics equation.Composites are quite similar to plywood from an analysis standpoint, although the allowables are different. You should have the same fears about wooden aircraft as composite.The important thing in a crash is to dissipate the energy. Given adequate design resources any of the noted materials should give good results. Composites are easily tailored however, especially with materials like Kevlar mixed in, so they should be more easily optimized.

  10. #10
    Treetop_Flyer's Avatar
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    Quote Originally Posted by wantobe View Post
    These tests are expensive and the data are usually propriatory. AGATE and NCAMP attemp to share such data publicly. Unfortunately, some so-called "design allables" (both A-basis and B-basis) published by AGATE does NOT use sample with openings and/or impact damages, render them useless.
    Actually, the laminate testing isn't terribly bad if you know where to go. We have a local lab we send a lot of work out to that is VERY reasonable when it comes to materials testing. You have to have the materials knowledge to put together a good B-Basis test plan, but it's not that far out of the realm for a designer to do this. Additionally, Bob H makes a great point below that basing your design allowables on a strain limit versus a stress limit is usually a more acceptable design theory for both thermoset and thermoplastic composites.

    Working with a knowledgeable engineer well versed in the materials you are designing with is key. Given the breadth of experience within EAA, I just don't think this qualifies as "out of reach" for homebuilt aircraft designers or builders. This is really where the EAA Technical Counselor program should shine.
    Dave Sterling
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