Error Chain

Anodizing and Fatigue Life.

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Yes, you would like to win an award for best helicopter. Yes, you also understand judges score highly on appearance. Yes, plain aluminum is not as pretty as chrome, or even better, color anodizing.

The photos for this article show an AirVenture Grand Champion Rotorcraft winner. There’s a “before” shot and an “after” shot—before the accident and after the accident.

This gorgeous Safari helicopter was lost due to fatigue failure of a control tube after accumulating a relatively low number of hours. The control loads are relatively low. Those stresses calculate to be low.

This accident was caused by fatigue failure of an anodized control tube. Fortunately, the pilot walked away from the crash with only a sprained ankle.

So why did this control tube fail? How was this control tube different from those in other Safaris that have not experienced this failure, even though they have flown more hours? The only quick discernable difference was that the control tube had been anodized black to add “curb appeal.”

Airshow coverage sponsor:

Next question: Does anodizing affect the fatigue life of aluminum? When this question was posed to the world of metal finishing experts, I received a resounding yes! In fact, one professional said: “You never anodize critical flight components made of aluminum. Everybody knows that.” My quick response was that he needed to subtract at least one person from that “everybody.” Subsequent discussions with A&Ps, mechanical engineers, and aircraft owners showed that the “everybody” was missing a relatively large group of interested people.

The arrows point to the broken control tube. It failed after only a few hundred hours.

In a report titled The Influence of Different Types of Anodic Layers on the Fatigue Properties of 2024-T3 and 7075-T6 Sheet Material, authors W.G.J. ‘t Hart and A. Nederveen stated: “Constant amplitude fatigue tests on anodized, unnotched specimens reveal that sulfuric acid and sealed chromic acid anodic layers cause the largest decrease in fatigue strength. Phosphoric and unsealed chromic acid anodic layers do not significantly affect fatigue life. Scanning electron microscopy of fracture surface confirms that the fatigue cracks initiate at cracks in the anodic layer.”

In an excellent textbook, Fatigue Design of Aluminum Components and Structures, by Maurice L. Sharp, Glenn E. Nordmark, and Craig C. Menzemer, a chart on page 110 shows the decrease in fatigue life due to pre-cleaning, as well as the effects of Alodine and a couple of different thicknesses of anodic coatings.

Broken ends of the control tube. The part was anodized to increase the ship’s “curb appeal.”

At a stress level of 35 ksi (1 ksi = 1000 pounds per square inch), the fatigue life of a non-pre-cleaned, un-coated sam…the use of a pre-cleaning compound labeled C2X showed a fatigue life of 0.055 million cycles or a factor of 3.64 reduction in fatigue life. Again, with no pre-cleaning and an anodic coating of 0.002 inches, the fatigue life was reduced to 0.035 million cycles or a reduction factor of 5.7!

What does this mean? Note that all two-blade helicopters have 2/rev vibrations whether the pilot feels them or not. A rotor turning at 500 rpm generates a 2/rev vibration of approximately 17 Hz. This vibrating frequency impacting a device with an expected fatigue life of 0.035 million cycles would expect to fail, on average, after about 34 minutes!

Again, experience can be misleading. I’ve heard people say, “That’s a crock—I know of anodized parts that have lasted much longer.” True, but what were the stress levels? What was the vibration frequency? The pretty Safari lasted longer than 100 hours. Measurements of the spring pressure required for trimming out the collective forces on the Safari yielded approximately 50 pounds. This load was distributed over the control tube, which was a 0.50×0.083-inch aluminum tube, yielding a cross section of 0.109 square inches. The 50-pound load created a stress of approximately 460 psi.

For those of a bent other than engineering and metallurgy, an element stressed to a level less than the “endurance limit” is said to have an “infinite fatigue life.” In other words, the element will last forever at that stress level. This endurance limit is a characteristic of steels. Several sources have said that aluminum does not have a well-defined endurance limit and would sooner or later fail in fatigue no matter the stress.

However, for the purposes of quantifying and establishing estimates, The Aluminum Association published Aluminum Standards and Data in 1988, listing an endurance limit of 10,000 psi for 500 million cycles. This, of course, depends on the particular alloy.

Back to the big question: How did the control tube fail in fatigue when it was stressed to a much lower level than the above published endurance limit?

The anodizing professionals said anodizing would do this. But how did the process cause stress in a Safari control tube to exceed the endurance limit? Again the evidence clearly showed that the control tube failed due to fatigue. There was no evidence of a pre-failure existing crack or notable surface discontinuity.

Obviously, there must have been something that supported the increased stress.

Concentration Factor or Stress Riser?

The maximum stress felt near a crack occurs in the area of the lowest radius of curvature (the sharper the point of the crack, the higher the stress).

For ease of calculation, an elliptically shaped crack of length 2X is assumed.

∑max = 2∑ x Sqrt (x/p)

Where ∑max = stress resulting from the crack that has a radius “p” at the end.

∑ = the stress of the applied load computed without the crack. Applying this formula to the control load on the Safari and assuming a sharp crack such that p = 0.001X (this is a large radius of curvature for the end of a very small crack).

Or: ∑max = 2 x 460 Sqrt (x/0.001x) = 29ksi.

That is much beyond the endurance limit provided by Aluminum Standards and Data.

Near the conclusion of the research for this article, one of the biggest surprises emerged. It turns out there are very negative effects generated by a number of cleaners on fatigue life. If you are using an active cleaner that attacks or etches the material during the cleaning process, there are minute discontinuities generated by the etching. Therefore, the fatigue life of your ship will be reduced and maybe your life span also.

Bottom line? Don’t anodize or use active cleaners on flight-critical aluminum parts unless you truly know what you are doing and have some extensive experience testing anodized aluminum parts.

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