NDE on a Rope

by Kelly Streeter, PE

I joined Evan and Keith at the Confederation Building in Newfoundland in May to complete a nondestructive evaluation pilot project designed to challenge our conclusions from our hammer sounding study of the limestone units. While in some cases it may be easy to detect delaminations by hammer sounding alone, the variable installation details at the Confederation Building made it very difficult to confidently predict by ear whether or not a stone “sounded” delaminated.

Delaminated stone used for calibration - on the ground

By using ultrasonics we were able to evaluate the stone’s response to a hammer hit in both the time domain and the freqency domain.  As Evan mentioned in his post, we had the added advantage of easily-accessible, known-delaminated and known-sound units which had already been removed for the renovation of the building’s west wing.    Not only were these units easily accessible, we could clearly see the delaminations in some of these removed units.  We tested those first to calibrate our observations.  It was fairly easy to see the multiple reflections from the delaminated stones, marked with arrows in the plot below, as opposed to the one clear reflection from the sound stone.

We then plotted the power spectral density, or PSD of the recorded hammer hit.  The PSD allows us to convert the power of a signal, as measured over time, into the frequency domain, showing us clearly in numerically and visually (on a graph) what our ear is trying to parse out.  Using this method we can analyze the way the frequency content changes from one stone to another.

The calibration went very smoothly. We found that we could easily identify the differences in both the time and frequency domain plots between the sound and delaminated stone units. We took these observations to the wall to test two different areas on the tower.

And then came the hard part.

Completing Ultrasonic testing - on the wall

It was a challenge (to put it lightly) to manage the rope access equipment, the tablet computer and the hammer and receiving transducer and all of the associated cables.  I found myself wishing I had two more arms.

Another significant challenge was the weather. We had to battle the frequent, low-level mist and rain throughout our visit. I was attempting to get the data but not expose the electronics to excessive moisture.

In the end, it was an excellent and rare opportunity to apply different nondestructive evaluation theories derived for more modern structural materials like concrete and pavements, to dimension stone. We discovered that we could quite easily detect delaminations once we had calibrated the equipment to a known condition.

One Response to NDE on a Rope

  1. tom mcdowell says:

    Very interesting – wish I was 30 years younger and 50 lbs lighter to be there with you.

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