Tuesday, June 08, 2010

Should Corrosion Preventive Fluids be used on Riveted Joints?

Spraying a corrosion inhibitor into the wings of aircraft results in it's leaking out at seams and rivets and sometimes creating a 'smoking" rivet" appearance. Two possible explanations for the "smoking rivet" are:
1. the lubricant has carried dirt and debris to the outside surface or,
2. the wet skin around the rivet and seam attracts dirt.
Some mechanics have suggested a third possibility:  that in highly loaded riveted lap joints (such as the Cessna 310 wing outboard of the nacelles), the lubricant might be creating a loose joint. In other words the lubricant is creating the "smoking rivet by interfering with the load transfer of the joint. I have done a literature review to see if this idea has any merit.

Transfer of the applied  load through the joint is shared between frictional action at the faying surface and contact between the hole boundary and rivet . Lubricating the faying surfaces might create partial slip and transfer all of the applied load to the rivet. Slip also creates fretting and this leads to cracking. (Farris, T. N., Szolwinski, M. P., and Harish, G., “Fretting in Aerospace Structures and Materials,” Fretting Fatigue: Current Technologies and Practices, ASTM STP 1367).

"Comparative flight-simulation tests on four types of joints of 2024-T3 sheet material were carried out with and without application of the penetrant LPS-3.  Types of joints were double strap joint with hi-lok bolts, asymmetric strap joint, lap joint with countersunk rivets and lap joint with dimpled holes.  Observations on slip during the fatigue tests were made, also in static tests of failure. A reduction of fatigue life was observed in two joints. The effect depends on the design of the joint, while the maximum load in the test may also be significant in view of the occurrence of slip." Effect of an Anti-Corrosion Penetrant on the Fatigue Life in Flight-Simulation Tests on Various Riveted Joints  Schijve, J | Jacobs, F A | Tromp, P J Natl. Lucht. Ruimtevaartlab. Vol. NLR TR 77103 U, pp. 34. 31 Aug. 1977

"Some aircraft manufacturers and operators have attempted to control in-service corrosion by the use of water-displacing organic inhibitors which can be either brushed or sprayed onto corrosion-susceptible areas of the structure. However, because of the low surface tension and lubricating properties of these preparations, concern has been expressed as to their potential side-effects on the fatigue performance of bolted and riveted joints. Fatigue tests were carried out in repeated tension under both constant-amplitude and multi-load-level sequences on several types of 8-bolt double-lap joint specimens of 2024-T3 alclad aluminium alloys sheet.
Tests were made on joints assembled with either ‘dry’ components or components coated with the corrosion inhibitors LPS-3 or PX-112. Contrary to the findings of previous investigations into the effect of inhibitors on riveted joints, the two corrosion inhibitors used were found, in general, to have either no effect or a beneficial effect on the fatigue lives of bolted joints. It is concluded that the specific effects of a water-displacing organic corrosion inhibitor on fatigue strength of joints are likely to be dependent on the type of joint, its configuration and on the severity of the load spectrum involved.Water-displacing organic corrosion inhibitors—their effect on the fatigue characteristics of aluminium alloy bolted joints A.S Machina and J.Y Mann

"The results showed that the lives of the treated shorter than those of the untreated specimens." A Short Study of the Effect of a Penetrant Oil on the Fatigue Life of a Riveted Joint by P. l-f. O’Nei/l and R. I. Smith Structures Dept., R.A.E., Farnborough

"It is concluded that joints fabricated with lubricated rivets, like those fabricated with lubricated threaded fasteners, have lower bearing yield and bearing ultimate joint strengths than when fabricated with clean fasteners." EFFECT O LUBRICATION ON THE JOINT BEARING STRENGTH OF RIVETED LAP JOINTS DEPARTMENT OF THE NAVY  NAVAL AIR DEVELOPMENT CENTER AIR VEHICLE TECHNOLOGY DEPARTMENT REPORT NO. NADC-72055-VT

" It was found that in joints with high fastener-clamping force, the application of lubricative corrosion-prevention compounds increases the fatigue life, whereas the use of CPC is detrimental to the life of joints with low fastener-clamping force.Fatigue behaviour of aluminium alloy 7075 bolted joints treated with oily film corrosion compounds

"The results showed that the presence of CICs had a significant influence on the fatigue life, and also on the failure mode of the joints. At high load levels, the application of CICs caused a reduction in the fatigue life of the joint by more than a factor of two. In this load range, the CICs appeared to cause the failure mode to change from tensile failure of the sheet (the prevalent mode at medium load levels) to shear failure of the rivets (observed at the highest load levels). Specimens that failed by rivet shearing showed some fatigue cracks propagating along the critical rivet row. In treated specimens tested at medium load levels, a reduction in the fatigue life still occurred, with all specimens failing in the sheet. At low load levels, there was little difference in fatigue life for the three conditions, although specimen test run-outs meant that further testing will be needed. The results are believed to have significance for managing the small aircraft in which these joints are common." Corrosion treatments and the fatigue of aerospace structural joints Aditya Jaya  Ung Hing Tion , Reza Mohammed, Cees Bil and Graham Clark School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Bundoora, VIC, 3083, Australia

Structural Vibration (engine and propeller induced vibrations)
Joint  friction between the metal surfaces transfers most of the high frequency acoustic vibration across lap joints; especially important for engine cowling and tail surfaces. High clamp loads (solid rivets typically have better sheet pull-up than "pop" style rivets) that increase joint friction are beneficial. The introduction of lubricants or anti-fretting agents at the joints will be obviously be accompanied by a loss of structural damping, which in turn means an increase of vibration amplitude and stress. The Damping, Stiffness and Fatigue Properties of Joints and Configurations Representative of Aircraft Structures by D.J. Mead, University of Southampton, England

1 comment:

  1. Brad Heinitz AP IA12:11 AM

    I've spent many hours repairing a few cessna 310's riveted wing structure. First the construction of the 310 wing skin to rib joint is at best barely legal. From my experience the thinnest sheet metal that can be counter sunk is .040 . Had Cessna dimpled these fasteners (at great expense) we wouldn't be having this trouble. A properly designed joint with a satisfactory number of fasteners and within it's design load capacity whether lubricated or not will not "SMOKE". Smoking in my experience is caused by complete looseness of the fastener or a bad hole combination. If the joint remains tight and uniform there would be no leakage through said joint (rivet hole interface). But as you all know nothing is perfect. (like the finely machined parting halves of a 985 Pratt, then wont leak, BS... There is always going to be a small amount of suface rms at the rivet hole interface. So if lubricating these ares is not good than I suppose we should stop using zinc-chromate paint in the joint too?