Design Allowables for Composite

[Bob H]

Just a quick comment about hybridizing a carbon laminate with Kevlar- don't! Kevlar is fine in tension but very poor in compression so adding Kevlar plies will produce planes of weakness in a laminate under compression and cause premature failure.

[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.

[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

[bdk]

So much discussion here on so many interesting topics, it is hard for me to collect my thoughts!Tool drop (and for the most part hail) is not a problem with a thick enough laminate. Two plies over foam or honeycomb just won't cut it for impact damage! Boeing (and others I'm sure) had big problems with moisture intrusion causing skin to core disbonds when the water froze and expanded at altitude. Some aircraft were found to be carrying hundreds of pounds of water. Typically three to four plies of 5HS prepreg carbon is enough to prevent typical damage.Kevlar is very hygroscopic meaning it sucks up moisture like a sponge which seriously degrades its mechanical properties. It is also nearly impossible to sand with good results.Strain allowables can be quite low for wet layups. 250F cure prepreg is better while I've seen as much as 6,000 micro inches per inch used with 350F cure Carbon prepreg.On the subject of material properties, you can use toughened resins, but you can also use notched allowables which assume a (typically 1/4 inch diameter) flaw in the laminate.Another thing yet to be mentioned is ply orientations. They are very important! Sounds like a good topic for another thread though. P.S. My comment regarding a smoking hole was in reference to lawn dart type crashes. No question we can do better to design in crash worthiness and make many more accidents survivable.