(Source: Boating Business)
A small blister on a hull can be just that, or it can be hiding something nastier, like impact damage to the core behind the skin. But how do you find out if a repair’s needed without actually sawing a chunk out? Mast and rigging specialist Marine Results has found another niche for its services by getting involved in the magic behind Non Destructive Testing (NDT).
|Marine Results NDT uses fluourescent dye|
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 20mm across fairly easily, “including the kind of bumps you get from logs and floating debris”, explains Mr Wood 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 fibre – 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 metres 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.” Contiune reading HERE.