Off Topic NDT: What Lies Beneath?
Mast and rigging specialist Marine Results has found another niche for its services by getting involved in the magic behind nondestructive testing (NDT).
The first "level" of testing is basically one up from tapping an area with a hammer, as it’s really another, more sophisticated tapping mechanism.
Appropriately called a Woodpecker, it is a handheld bit of kit useful for testing, thin composites or cored materials with thin skins; bulkheads and booms for example. With it the Marine Results team can check a large area reasonably quickly. Further, it makes it possible to pick up delamination to the inaccessible rear of the composite, which is difficult to find with basic tapping.
However, with thicker skinned structures, hulls and so on, you have to move up a level. The Bondascope still works by resonance, but shows the result on a small screen. It picks up local changes in stiffness in even quite thick structures, so the team can translate the wavering spots on the small oscilloscope display and relate them to damaged areas or to reinforcements such as bulkheads or stiffeners. The unit can locate damaged areas as small as 20 millimeters across fairly easily, “including the kind of bumps you get from logs and floating debris,” explains Ben Wood, who manages the Dye Penetrant facility of Marine Results.
Ultrasound technology takes it one stage further-and here it becomes very, very useful for boatbuilding. The zigzagging lines on the screen can be translated into "strata"-which are reflected pulses of ultrasound from the structure and if there’s an unexpected jump in the pulse position, its probably indicating a discontinuity such as a void or delamination.
“This means you can identify the joins on dense composite structures-carbon or glass fiber-even in multiple layers, and you can see how well they have bonded,” says Mr. Wood.
“So, the mast manufacturers come to us and ask us to check the bonding, for example where two layers have been overlapped to form the tube of a mast, and the areas of stiffening where the designers have put in additional laminate to strengthen the composite.”
But the process is also being used to check the integrity of the material before it starts to be bonded into a mast, daggerboard or rudder. “If you can pick up an area where the resin and fibre have not consolidated properly before you start to work with it, you save a lot of time, money and effort,” explains Mr. Wood.
The biggest part of NDT is calibration of the apparatus. He explains: “Usually the mast manufacturer will give us offcuts and test panels, but we have also built up a huge library of testing pieces, so that the equipment can give a good read out as possible.”
Obviously, all this is easier on a new build since the mast and rigging is laid out in front of you, but if there’s been damage at sea, then you want to get an idea of the structure’s integrity before the crew sets out again.
Which meant one day last month Mr. Wood was to be found dangling from rigging 25 meters in the air, with an iPhone in one hand, an ultrasound device clipped onto his belt and a white marker in his pocket.
“The mast manufacturer was in New Zealand, the boat was in Greece. However, the ultrasound unit meant I could map out the area of damage to the composite, mark it up with the white pen, take a picture of the mapped area on the phone and email it through to the designer. Then came the calls to determine whether the boat was good to sail or not-it was, and the crew made it out by sunset.” There’s another side to NDT that’s usually more allied with metal structures.
This relies, not on acoustic and ultrasound signals, but on fluorescent dyes that can make even the most microscopic of cracks glow like a luminous party string under ultraviolet light.
Ben Wood, who manages the Dye Penetrant facility, gives a guided tour of the process in the Marine Results basement. It is not a straightforward "kit" test by any means.
“Firstly it has to be done in an extremely controlled environment,” he explains. “It’s not just the chemical composition. The temperature of the drying ovens, the water pressure and the step by step process is extremely well regulated, so we can get duplicable, reliable results time and time again – no matter which member of the team does the inspection.”
The parts to be tested are cleaned carefully in solvent, before being dipped into a bath of luminescent yellow dye. The parts are then washed down, getting rid of all the excess, except that which has crept into a crack or discontinuity. Oven drying follows, and then "developing" in a dust storm cabinet for 20 minutes, which allows particles to adhere to the sticky dye at the edge of any fault lines – these pick up some of the dye, giving a wider silhouette to possible damage.
Now, it is probably a time consuming and expensive task for one-off checks, but, says Mr Wood, “if you are renewing a boat’s rigging and hardware at the start of another season, it probably saves money because we can pass the parts through the tests in series, and say, for example, ‘this tip cup needs replacing, but this one’s OK with a clean up’. An owner isn’t stuck with such a big replacement bill.”
The nice thing about all this is that there is, despite the very well regulated environment at Marine Results’ premises, a portable version. This can be taken and used on metal parts in situ, and although it doesn’t give such a fine result, will point out if more action is needed.
Mr Wood does add a note of caution though: “We do get people who want to know if the structure is safe to sail with or not – but that’s not actually our job. What we do is hand the results of the investigation over to the designer of the mast who will have to make the ultimate decision. We only point out where and what the material failures actually are and give detailed information on the size and location and nature of the defects.”
But this data feed back to the designers does have its benefits beyond identifying the immediate problem: “The mast designers actually take on the information and work it into their products,” says Mr Wood. “For example, there were a number of material failures where the halyard kept on banging into the mast-later mast designs got another layer of strengthening just at that point.”