Any metallurgists out there?

[Barry Wolk]

One of my hospital customers has some light poles that are cracking and one has fallen.

We inspected all of their aluminum poles and discovered another that has a crack in the same location.

It appears that the poles are cracking at the inspection, oval-shaped, hand hole. The crack we found on the standing pole was quite obvious but I'm very concerned that this condition can't be detected with the naked eye.

My initial thought was magnaflux but then I learned that that process only works with ferrous metals.

I learned that one of the other methods is a dye that seeps into cracks making the visible. Unfortunately, it's still too cold to use that method and it may not detect cracks that are under paint.

I've learned that the only way to tell is by using an ultrasonic device to seek out voids in the metal.

Is there any other way of detecting stress cracks in tubular aluminum?

Note the short crack on the left and the long dark area on the right. These areas have been exposed to oxygen and have darkened. The bright metal was exposed when the pole fell.

[R Walling]

Most poles that I have seen have a reinforcement welded inside the pole around the hole to combat this. Must be a cheap manufacturer.

[Barry Wolk]

I thought it strange that the opening wasn't buttressed, too. Believe me the pole's not cheap. It's over $2,000 for a 30-foot pole.

[Tom M]

I would say that a saddle or something in that order should have been welded around the opening. That sure is one large opening not having any type of support around it.

[Guest DaveCorbin]

Dear Barry:

Was the pole that failed in a situation like you might get near the corner of a building, where wind forces are more intense and give stronger and faster torsional forces to the light pole thru an arm mount?

This has earmarks of a torsional failure, caused by the pole being cycled back and forth in a torsion mode. Some lights actually have shapes that are aerodynamic. If mounted on a sufficently long and strong arm, the light can impart surprisingly high forces to the pole.

Regards, Dave Corbin

[Barry Wolk]

Dave,

The pole is on the Detroit River and wind load is a consideration. It's near the building but not that close.

There are two poles in question. One that fell and another with a similar crack that we are taking down tomorrow. The one that had banners fell and the other, without banners is the one that's cracked. We've taken all the banners down but the damage could already be done.

[Guest bkazmer]

I suspect there is a stress riser due to a sharp angle or aninitiated crack at the opening if it is cut instead of cast in. How the opening is designed and made may hold the clue as to how to solve the problem without an add-on piece.

[mrpushbutton]

One method for detecting cracks in cylinder heads is to turn them upside down (with the valves and springs in place) and fill the "cups" with kerosene and leave them in the sun. The kerosene seeps into any cracks and they are plainly visible to the eye. Perhaps when the weather warms up you could paint on kerosene and see if any dark lines appear.

When I worked in the stress analysis (strain gage transducer) field (in Farmington Hills!) there was a substance called "tense-lac" basically a lacquer (smelled like old-fashioned nitrocellulose lacquer) that came in a little glass brush-bottle. You painted it onto a component that you wanted to analyze, and then loaded (stressed) the component in whatever way it was intended to be used. that's where we came in--we had calibration presses that were capable of loading up to 400,000 lbs. with NIST-traceable load cell (which we designed and built) indicating the force applied to the component (to within a fraction of an ounce of force). As the part was loaded/stressed a series of cracks would develop in the "Tens-lac" that had been painted on. The distance between the cracks indicated the force, you would compare the cracks to a card that was supplied with the product, and that would indicate the exact spot where the most stress was being seen and the severity of that stress.

Sounds like you need a registered professional engineer. After that flag pole incident in Ferndale last year and the tragedy to the parents of that poor child you would think that municipalities would be looking at all of these poles, but then again, we are talking about the city of Detroit here. Some of the $8,000,000 paid to the whistleblower police officers could have gone to safety inspections!

[mrpushbutton]

P.S. I'm with Dave Corbin, your second picture illustrates that the "fault line" is not straight across the cross-section of the pole, a classic case of torsional failure.

Look at the remains of any bolt that you have ever snapped--there is always a high opint and a low point, never a perfectly "square" break.

[Bhigdog]

It's hard to tell from the pix but it looks like the hole might have been field cut rather than factory cut with a welded collar. My guess is wind loads working on a stress riser is the culpret. A good test for the remaining poles would be to have them X-Rayed but if they are the same construction and some have already failed I think you have your answer for what needs to be done.........

[Barry Wolk]

The hand hole was factory cut.

[Ivan Saxton]

There are a couple of techniques that were used by the extraordinary Napier company. I cannot access my copy of

LJK Setright's book on the history of piston aero engines, "The Power to Fly", and therefore I cannot give a complete quotation. However, from memory, this is the essence of the relevant piece. During the WW2 production of those astonishing 24 cylinder sleeve valve Sabre engines for fighters, Napier impregnated the aluminium castings with bakelite before machining, "which completely fooled Rolls Royce when they sent a batch of castings for testing. Napier had their own sophisticated electrical instruments" for detcting flaws. I can put you in touch with Derek Grossmark in UK, who is an authourity on Napier; and he may be able to clarify what they did if you want to follow it up.

Earlier, Napier did their crack testing of aluminium castings by ear. The crankcase and gearbox housing of my 1911 Napier have myriads of little prick indentations from where they obviously went over them with a little pick hammer. (These are not so easy to notice where they sealed the surface with an aluminium paint, as they did originally , inside and out, in some places. However, if in restoration you leave these parts unpainted, dirt will gather so the testing marks show up as myriads of black spots.) So you may be able to detect faults by tapping gently around the inside of the opening with a little hammer. This is the way you test tin babbit crankshaft bearings of antique cars that you have re-poured. if the bonding is succesful you get a high frequency sound. If there is discontinuity between the lining and the tinning, you get a dull sound.

I shall try to print out you images and show them to Wayne Portelli, whose company near here makes large quantities of overhead light poles for the road system. (These are steel, tapered, fabricated and galvanised.)

Ivan Saxton

[Barry Wolk]

Thank you Ivan. I've contacted the rep for the pole manufacturer. He's submitting a warranty claim for the two failed poles. I'm of the opinion that two have failed, other will fail sometime in the future. I'd like to see them replace them all.

[GMPARTSMAN]

Barry,

PM me. I MAY have someone that may be able to direct you to a source of analysis. Also in the area of the "Hills".

Just curious, also, what lighting company do you own?

[Barry Wolk]

My company is called Bottom Line Lighting Management. This is my 30th year in business. I specialize in group lamp replacement for commercial facilities, hospitals, schools and shopping centers.

[R Walling]

Barry, A little off track but, I was reading about disposing florescent lights the other day and I wondered what is the legal way to dispose of them for the homeowner and the business’s that have large quantities of them.

[Guest DaveCorbin]

Dear Barry:

Something else you could do is to polish the side of the access holes and round the edges with a Dremel tool. That makes the stress points go away. It's exactly the same idea as polishing connecting rods in a racing engine.

The suggestion of tapping with a small hammer for "ring" instead of "thud" is also useful for a "quickie" look.

Regards, Dave Corbin

[Mark Shaw]

Barry,

Try this:

http://www.contech.com/Dye_Penetrant.htm

It works best to find cracks in machined surfaces & should work even in the cold.