Saturday, June 13, 2015

COMMENTARY ON COMPOSITE-METAL JOINTS

Update 11-8-15:  I am surprised that this article has not drawn more readership and comment than it has.  This topic does really kill people.

Initial article:  I first wrote this little commentary in July 2006,  in response to questions from friends and colleagues,  regarding the Airbus crash in NYC,  where the vertical fin came off under hard rudder application during climb-out from takeoff.  I used to send it by email as a small file.

This topic of properly joining composite structures to metal (or to other composite structures) with fasteners,  just keeps coming up,  again and again.  I have noticed that a lot of outfits still fail to do this job properly.  So,  I thought I would put this article up for interested persons to use.  Knowing the truth might save a life.

Original 12 July 2006 article ----

Here is how we used to build composite rocket motor cases with pinned closures at the old McGregor rocket plant (see Figure 1,  where the "shims" are thin sheet steel stock).  Weight was at a premium,  as was reliability.  We used these for up to 5-inch diameter motors up to 4000 psi maximum expected operating pressure with wall thicknesses away from the joint under 0.10 inch.  We even used these externally un-insulated in situations with Mach 5 aero-heating by friction.  (Typically,  these had around 0.1 inches of rubber insulation on the inside to hold the 5000-6000 F fire away from the carbon-epoxy that degraded at about 300 F.)  None of our pinned joints ever failed.

    Figure 1 -- Pinned Joint in Composite Rocket Motor Cases


This (Figure 2) is typical of general industry practice joining composite (or plastic) to metal.  This concept has long been used in automotive and consumer products,  because it is cheap.  It has also long been known to be very failure-prone.  The idea is to try to spread the loads for lower stresses with a big washer,  but it never really works that way.  It always starts at a single point on each washer,  unzips around it,  then unzips from washer to washer.


   Figure 2 - Typical Industry Practice,  Subject to "Unzip" Failure

The recent (as of 2006) fatal crash in NYC of an Airbus airliner operated by American Airlines,  with a composite vertical fin,  points out the need to do this joint correctly.  It is necessary that there be some sort of metal attachment fittings for a bolted joint.  These fittings would be “glassed-in” or otherwise made part of the composite fin structure. 

Such reports as are given to the public do not indicate exactly how this was done in the Airbus design.  That design must lie somewhere between the extremes (“good” vs “bad”) pictured above. 

However,  reports do indicate that these attachment fittings tore out of the composite fin structure,  and were found still bolted securely to the fuselage,  sans fin.  Reports also indicate that one of the pilots applied full rudder deflection to correct an upset at low altitude climb-out conditions. 

The ability to use rudder,  aileron,  or elevator controls at full deflection,  anywhere in the flight envelope,  without fear of losing tail surfaces,  has been taken for granted in American aircraft design practice since the early days of commercial travel.  (This stricture does not apply to wings,  of course.)

Reports since the crash tell of a dispute between Airbus,  American Airlines,  and the pilots’ union over who was supposed to tell the pilots they could not use full rudder deflection during climb-out,  and whether or not they were even told at all by anyone.  This is proof of the foreign design not meeting generally accepted US expectations.  It thus may not strictly meet FAR Part 25,  either:  a reciprocity issue needing swift resolution.

The attachments tore out under side load to the fin.  These conditions put tension on the interface between the composite structure and the attachment fittings somewhere.  That it failed,  and that it needs an unusual flight restriction,  together are proof that the attachment design had insufficient means of spreading a tensile attachment force into the composite structure of the fin.  Thus it is probably not the right joint design.

The right way to attach a composite fin to a metal fuselage is pictured below (Figure 3).  This is an adaptation of the well-proven layered interface used in the rocket motor pinned closure joint.  The key criteria are (1) provide adequate adhesive shear area to spread the tensile (and compressive and shear) loads into the composite structure by shear,  and (2) size the members and the adhesive bond areas to take full deflection forces everywhere in the expected flight envelope,  and a little beyond it,   for safety.


   Figure 3 - Correct Way to Join Composite Structure to Other Structure with Fasteners

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