Design Allowables for Composite

[wantobe]

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.

[steveinindy]

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.

[flyingriki]

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....

[steveinindy]

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.

[Bob H]

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.

[wantobe]

[QUOTE=Bob H;16146] 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.

I think Bob intended to say " you would want to stay below 2000 microstrain." I agree with a number of this range. However, if you do the "load the sandbags up" approach, depending on how you do it, you may get a number well above this, such as 5000-8000 microstrain. And that is dangerous.

When I talked about the degrading effect of impact damage on compressive strength, I was talking about those that were barely visible, such as a tool drop on the wing skin. I was not talking about crash, that was not "barely visible". The reason that you can only use compressive strength with barely visible impact damage is that you can not see it, so you have to assume it is in your composite structures due to production process, gravels, birds, strangers, etc.

[bdk]

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.

[Kyle Boatright]

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.

[steveinindy]

Please point us to any data on this topic. I'd be interested to see it.

Like I said, if you're willing to wait a few months, it's being reviewed for publication currently so I'm more or less under an agreement not to distribute it. Suffice to say that the data is "out there" but the problem is that it took the better part of five years of part-time effort to gather it all together since no one has bothered to gather it up from all the various sources (local LE, NTSB, coroners/MEs, etc).

Racing cars have used carbon cages for years to protect drivers in very serious crashes so the materials do perform well when properly designed.

When properly designed and manufactured. That's the problem is that we're not talking about the same level of manufacturing capability in a homebuilt aircraft versus a Formula 1 racer.

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.

That's where I learned what I do know about the subject. I'm literally 10 minutes (fifteen during rush hour) from one of the premier tracks on the planet and I spent a lot of time down there talking to and learning from folks. This is one of the reasons I get so frustrated with the approach that experimental aviation takes towards composites. The issue though is the cost associated with the equipment needed to produce the Indy/Formula 1 level of composite. It's out of reach of all but a select few and even then most of those guys are going to just buy a biz jet and be done with it.

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). T

"Smoking hole" events in general aviation are rather rare in the sense that the term was originally implied (the plane just freaking buries itself or completely disintegrates on impact). Most of the "smoking hole" crashes we do have are not actually of that nature but are rather simply the plane either burned or melted during the post crash fire. The effective elimination of post-crash fire- which is quite possible from an engineering standpoint even in the hands of non-professional designers and builders- would do away with 40% of the mortality associated with general aviation crashes.

As for stall/spin incidents, the amount of force isn't all that great in most of them because of the lower speeds associated with them. The issue is dissipation/direction of the energy associated with them. Most aircraft aren't designed to dissipate the energy instead of transmitting it to the occupants. This is one reason why I cringe when I see an aircraft with the seat bolted literally on top of the spars with no stroke distance whatsoever.

As far back as the 1940s, Hugh DeHaven and his colleagues (to later include John Stapp of the rocket sled tests fame) pointed out that with a little design effort, that most (if memory serves, the number was plugged at 70%...if anyone really cares, I'll go find the article and share it) currently fatal crashes are not beyond the capability of the human body to survive it. The numbers the FAA currently uses to set its "standards" are drawn from research that came out of the end of WWII and are not inline with the numbers that were later demonstrated by Stapp and others. A lot of those numbers were taken from human volunteers so the margin of safety for those studies was pretty broad and probably builds in an artificially low number.

The best idea yet is a parachute to avoid the sudden stoppage in the physics equation.

The ballistic parachute is a great idea (probably one of the top five or six safety improvements in aircraft over the past 25 years) but unfortunately it's only amenable to a very narrow set of circumstances. It's not really much of a solution to the sorts of crashes that account for most of the fatalities for a number of engineering and human factors reasons.

Composites are easily tailored however, especially with materials like Kevlar mixed in, so they should be more easily optimized.

They can be and the work that came out of the European CRASURV projects showcases this beautifully. The biggest issue that those of us who work in the crash survivability are not the technical issues but the economic ones (as previously mentioned) as well as the resistance from folks who have a loyalty to previous designs because they tend to take a suggestion for improvement as a frank criticism of their beloved aircraft when that is not usually the case. It's only the case in a few extreme situations where one looks at the design and wonders what the hell they were thinking.

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.

One of the labs here locally actually does some testing for cost (which often is negligible) if we're willing to ask nicely and let them do it when they aren't busy with "paying customers". They realize we're not doing this for a profit (actually we will probably be a formal not-for-profit organization by the end of the year) and so they help us when they can with things like our fuel tank and parts of our seat designs.

When I talked about the degrading effect of impact damage on compressive strength, I was talking about those that were barely visible, such as a tool drop on the wing skin.

Oops. I do apologize if I derailed your thread.

You might look at Mike Niu's books on composite design. If I recall there is some discussion about overcoming this issue and how to identify it. I have one of them that I can let you have if that would be helpful (PM me)

[Bob Dingley]

I hope that I'm not out of line, butI like composite airframes. I flew the Sikorsky 76 variants for about 25 years and I am impressed with the strength and durability of the "Kevlar Comet." The forward 2/3 of the airframe is kevlar honeycomb with fiberglass doors, cowls, etc. Fine copper mesh is embeded for lightning protection so you can penetrate squall lines and CBs with peace of mind. The aft 1/3 is semi mono aluminum. The composites have no structural issues. The aluminum structure is where all the cracks and smoking rivets are found.
Example of durability: S76 SN #2 was delivered to my carrier about 1976 or 77. 1980 or so, some ace ran it dry, almost made the traffic pattern and dead sticked it in the bay. It turned turtle and sank in 25 ft of sea water. It was recovered and parked in the back forty for two years where it was cannabalized for parts. Critters and birds moved in. Then it was refurbed and went back in service. Note: composites are more rat urine tolerant than aluminum.
All the company "Igors" spent their lives tied down outside in cold/hot humid conditions from the Gulf Coast to Latin America to Alaska and points east. No composite issues. I flew old number 2 for most of the 90's. She was sold in 2004 with 23K hours on her. 3K were mine. Only issues were that she still had the Kapton wires that she was born with. Like flying a haunted house.
I saw many S76 roll overs, hard landings, etc over the years but the Kevlar took it nicely. I recall two accidents that 76s were flown into the ground both in night zero/zero weather. One ran out of fuel on the ILS at 2 AM. It shattered like a big light bulb. The other was flown into the ground after take off. They hosed off the mud, replaced belly antennas, windshield, radome and of course the entire power train. The crew had career ending injuries. The bird went back in service in months. Tough.

Bob

EAA759930