Forgotten old PT processes

June 2010

Mike King, one of our American readers wrote us by the end of April 2010:

‘‘I just read your article on the "wink" PT inspection.

Around 1970, I was a young sales engineer for an ET equipment manufacturer.

The airlines had repair facilities in Miami, among them Pan Am, National, and Delta Airlines. I believe it was at Delta that I was shown the inspection they called "wink Zyglo®" (Editor’s note: ZYGLO® is a trademark by MAGNAFLUX). It was being done on a large disk, which I think was a Pratt and Whitney part. The disk was mounted vertically and at the top, there were two "hooks", for lack of a better word (Editor’s note: ‘‘jigs’’ is a more appropriate word). These hooks held the top of a pair of adjacent christmas trees (blade retainers). One of the hooks was powered to load and release its christmas tree horizontally, so that the bottom of the space between the christmas trees (blade root area) was alternately loaded and released. This is where they were looking for cracks.

After PT application and cleaning, the loading and unloading cycle was started. If a crack were present, the residual PT would suck back into the crack and disappear when the load was applied (crack opened), and when the load was removed, the PT would squeeze out of the crack and be visible.

Afterwards, my boss and I wondered: How many cracks have they induced this way?

This from the best memory of a 66 year old NDT ET and UT guy.’’

Our answer:

Thank you very much for your e-mail and for your interest in our Website.

As told in our paper, since you are 66 years old, you are in the right age-range to have heard of this process.

You write: ''How many cracks have they induced this way?"

We may understand that some of those who performed this process were not sure of the tensile stress they had to apply on the part! Nevertheless we think that there was a huge factor between the figure they used and the figure needed to induce a crack in the blades!

Inducing cracks by this process would mean that the applied tensile stress would have been unnecessarily too high, therefore inappropriate.

We wait for your comment on this last point.

As an experienced sales engineer in ET, do you think ET has replaced the “wink PT”? We are not sure, as the area which was so checked is almost impossible to check with ET, due to the shape and the curve radius.

His answer:

I was at Allied Signal Engines and I did run a program of inspection for cracks in the bottom of the dovetail slots on fan disks of biz jet engines.  We used ET with a shaped probe. So ET can be used in some odd geometries if you are willing to spend the money on special probes. We got the inspection in place after three fan disk failures. We found a LOT of cracks, and had no more failures.

Also, Pratt and Whitney had a long-term program using ET on 4th stage turbine blades in the F-16 engine. It was a small probe that looked something like a tuning fork, mounted on an 18 inch extension handle and fit between two adjacent blades.

Our anwer:

Mike, we would like to phone you. Could you, please, send your phone number by email?
Thank you.