April 2010 - An answer to the last month's riddle

In our last month’s Editorial we asked for help by our readers about our mini-story “Forgotten old PT processes’’ published in the “OLDIES BUT GOODIES” series.

We were trying to understand why the ‘‘Wink fluorescent PT’’ is no longer in favour.

One of our readers sent us a message stating the winking method is not obsolete as a manufacturer markets a blue light searchlight with a blinking mode easing the indications detection, as written in our DPCNewsletter N°021 of February 2010 titled "Tomorrow's penetrants".

As per our reader this is quite similar to an aircraft flashlights at night: a steady light does not generate "signals" enough for the brain to notice it. A blinking light draws the attention more because many ‘‘ON/OFF’’ impulses arrive and titillate the eye and the brain.

Our reader did perform tests by himself and he is fond of this blinking mode.

Our opinion is that this method is ingenious; it is a real breakthrough as there is no need for alternated flexions of the part! Penetrant bleeding out of the discontinuity is the only requirement!

"Psychology", or at least, the way the brain reacts, is important in PT: there is a reason why colour contrast penetrants are red, or purple, or orange, when a black indication against a white background gives a better contrast. This is an important point that we explain in training courses to Levels 3.

We explain our reader that, though the light is blinking instead of the indication, if such a source had then been available, it would have been interesting indeed to make comparisons to find out which one of the two methods is the most sensitive and the most reliable: the blinking light or the blinking indication ? It would be possible to perform this comparison nowadays ... if cost effective.

We think the right answer to our question lies elsewhere: during the span of time between the “death” of the blinking method and the blinking lights marketing another “lambda” method filled the gap.

We have to ask the question:

Is the blinking source technique simpler, more sensitive and more reliable than the lambda method?

But first we have to identify this method.

We think we put our finger on the "lambda" method. In the previous paper, we gave several clues to our readers.

We think the most interesting one is:

‘‘Do you think another NDT method, more sensible, more reliable, easier, quicker, may have replaced wink PT?’’

It should be another NDT method able to detect surface discontinuities: Eddy Currents Testing (ET) comes immediately to mind!

Indeed … in the ‘70s Eddy Current flaw detectors were far from what they are today! They came with a dial and a pointer set on a moving-coil galvanometer. The pointer deflection may be due to different parameters, almost impossible to differentiate: alloy, heat treatment, edge effect, flaws, conductivity, corrosion, etc. Far from nowadays digital Eddy current impedance plane flaw detectors!

Then one of us had even an opportunity to try and find out forgotten sheets of paper between the carbon fibers layers on helicopters blades … without achieving any result! Well Eddy Currents was a completely new world and many people did not imagine what could be done with this method!

Could we say that almost everybody thought that ET could help for everything … a situation we still face today!

Nevertheless it had been easy to detect cracks due to filiform corrosion under the paint scheme (before flexible polyurethane paints schemes were designed) on wings of the aging CARAVELLE 12 aircraft.

We now know that detecting near-surface flaws under compression is very easy when using Eddy Currents, which, it must be said, are easier to use than the “Wink fluorescent PT”.

Maybe some readers have their own opinion and would like let us know.

Sorry, once again, nothing to win, except for our respect, admiration and esteem. A reminder of the American ‘60s pinball machines on which the warning “no prizes, no wagering” was written!

In our next month’s Editorial, you may read the ideas of the reader(s) whom we will have got suggestions from - if any.

Many thanks in advance for your help.