If you visit theme parks early in the season, you might have noticed that on some rides there are peculiar-looking patches of white spray paint.
Particularly noticeable on Blackpool Pleasure Beach’s veteran Wild Mouse rollercoaster, they are the remnants of off-season inspection activity.
During a long season and thousands of trains completing laps of the circuit over the year, the welds of the rollercoaster track and other ride components are subject to a great deal of stress.
It may come as a surprise that welds cracking during the season is not an unheard of occurrence, indeed on some rides it is expected.
In much the same way that it would be no catastrophe if a single wheel was to crack on a (non-rollercoaster) passenger train, it is not particularly dangerous if one of the hundreds of welds present on a rollercoaster track were to give.
However, it soon becomes clear which areas require the most attention, and sometimes they are tested during the main season, not just during the close months.
From Nemesis’ helix to the Runaway Mine Train’s dive into the tunnel, from The Ultimate’s axles to the bases of 1970s flat rides, they are all regularly subjected to magnetic particle testing.
The white spray paint is a contrast aid, and it is applied once the weld in question is suitably cleaned and free of the ride’s normal paint of other contaminants such as oil, grease or water.
With the contrast aid applied, a magnetic field is applied across the weld. A solution of black iron filings suspended in a clear liquid is then sprayed across the test surface.
Cracks break the generated magnetic field, and as the iron filings are pulled towards each pole of the magnet, a clearly-defined black line appears across the profile of any cracks present.
Engineers at the park can then arrange welding repairs as required, and this is a very routine process on some of Blackpool’s older rides.
Magnetic particle inspection – a method of non-destructive testing (NDT) – is not the only way of inspecting rides and their welds for fatigue, but it is the method that leaves the most noticeable trace to the theme park enthusiast.