Consultant David Jesson considers how non-destructive testing can help us bridge the gaps in our knowledge.
There is a Calvin and Hobbes cartoon, by cartoonist Bill Watterson, in which Calvin asks his dad how they came up with the weight limit on the bridge they’re crossing. Easy – they run progressively heavier trucks over until it collapses, and then they rebuild it exactly the same as before... It must be remembered that: i) Calvin’s dad is a lawyer, and ii) he’s a dad. A big part of the science of engineering is to avoid the need to destroy big, expensive structures. Instead, we understand their behaviour by using well understood models, and a bit of experience.
That’s great for setting things up, with pristine components, and well characterised materials, but what about when we need to predict when something will fail, or how much life it might have left? What about the effect of damage? Or a need to change the duty of an asset, perhaps by increasing an operating pressure?
We can’t drive progressively heavier lorries over a bridge to check its health, or to decide if we can run heavier lorries over it. But we do need to perform some kind of evaluation – a non-destructive one. There are a range of tools that can be used to get information in this way. Different techniques are suitable for different materials and different situations. Sometimes we need to look at surface defects, and sometimes we need to look inside a structure; we might need to characterise a crack or determine the extent of corrosion.
Whilst the foundation of the knowledge we use in designing structures and other assets is based on destructive tests, which give us fundamental properties, successful asset integrity is made easier when the current condition of a structure can be evaluated non-destructively. No bridges were harmed in the writing of this blog…