The importance of corrosion in the dynamics of fatigue resistance of metallic structures. What preventive actions can be adopted?
RUST: THE POWERFUL ACCELERATOR THAT REDUCES THE LIFE OF STRUCTURES
di Enrico Fabbri
The accident in Ohio (USA) last July shocked due to the raw images captured by a spectator, yet also represents an important precedent to reflect upon. In fact, rust is one of the most dangerous risks to the strength of structures, as it advances slowly yet inexorably. These risks need to be taken into consideration both during the design stage and during annual inspections, even in the absence of specific manufacturer requirements.
The first photo here below shows a detail of the ride involved in the accident at Ohio State Fair (USA), while the second shows the breakage of a ride in France. Two very different rides, different manufacturers, but with the same common problem: a breakage caused by rust.
A good manufacturer designs rides to withstand at least 35,000 operating hours, which generally corresponds to approximately 20/25 years of operation. The design and calculation stages take into account many factors, such as vibrations, dynamic forces and wind, but they never account for the effects of rust. Generally, the rust that appears on the outside of metallic structures is the least dangerous: it is visible and the operator can perform the required maintenance when needed. What is potentially more dangerous is the rust that is created inside important metal structures – the rust that builds up silently and continuously, without the operator noticing it.
How does this rust form? Both cases here clearly show that water has probably entered from one side of the structure, accumulating in the bottom/outer part. In both cases, the water did not have the chance to drain outside and therefore started to slowly yet inexorably trigger corrosion. I can say that it is quite difficult for a designer to hypothesize such types of event; water can indeed infiltrate the metallic structure both during transport and during installation or operation of the ride. There are also rare cases in which moisture forms inside well-insulated structures due to large temperature fluctuations between day and night.
Is it possible to notice what’s happening? Well, in both cases reported here, the operator noticed the problem only when it was too late. The only difference is that in France, no harm was caused to people. We can conclude therefore that it is quite difficult to spot the problem simply by visual inspection. Many operators carry out NDT on the most significant welds, yet this technique may potentially not detect the problem with enough advance warning. This is because rust can fracture the metal in places away from the welds. So, it can be concluded that the normal experience of a good operator is often not enough to identify the problem.
What can be done then? To see what’s happening inside a structure, the
following are needed:
- analyse the design to identify critical points;
- visually inspect the attraction and the inside of the structures with the help of powerful lamps and, where necessary, with a small video camera. By this I mean a small camera fixed to flexible tubing and connected to a monitor to help see what’s going on inside a structure (endoscope);
- if necessary, make a small hole in the structure. This operation is very delicate; choosing the best position where to make the hole needs to be agreed on with the manufacturer, or alternatively with a very experienced engineer;
- check welds using non-destructive testing;
- check the thickness of metal structures using special equipment. It should be stressed that all this equipment must be operated by professional personnel, who have been qualified by attending special courses. Please check with your engineer for all these procedures.
How are the results evaluated? And what happens next? These techniques detail the updated status of component strength. If everything is in good condition then there are no problems. If the inside of the structure has corroded, then precautions must be taken: if corrosion is minor, then the affected areas can be cleaned, locating where the water may have entered and implementing a solution. If, on the other hand, corrosion is considerable, then the solution is repair or replacement. In all these cases, before proceeding always contact the manufacturer, meticulously collect and analyse all the required information to decide on the best solutions. Repairs are always very complex operations. Indeed, these must provide a series of reinforcements that restore s tructural strength with respect to the manufacturer’s original design. Often repairs do not provide an optimal solution and therefore it is necessary to replace the entire oxidised component. It is clear that replacement cannot be performed with a component that is exactly the same as the old one, therefore a modified component should be installed, taking into account new solutions that prevent the rust from occurring as in the original.
So how important is it to check for rust? No professional technician can neglect the importance of rust, as this can cause enormous problems. To give an idea (but this is purely an example), rust inside a structural component can reduce its working life by about 30%-50%. This means that a ride (or a set of components) initially designed to last at least 20-25 years may break after 13-16 years, and in severe cases even after 10-12 years. This helps understand how important the issue is.
What on the other hand should be avoided? There are a number of actions that should be avoided, such as:
- drilling holes in metal structures to let water out. This is wrong because even the holes in the metal sheets can cause a breakage in the component (especially if they are close to welds). Holes can only be drilled following very precise analysis to decide where to place them. It is much better to trust experts;
- trying to solve the problem on your own, without being assisted by experienced engineers or the manufacturer;
- believing that nothing will happen because nothing has ever happened to others who have the same attraction. This reasoning is wrong, because not all attractions are used in the same environment or climate;
- having these aspects checked by employees or managers who may not have understood the importance of the problem.
What are manufacturers doing? As I mentioned at the beginning of this article, recent accidents have left their mark. Some manufacturers have thus begun to review some of the components of already completed attractions more critically, issuing service bulletins that require more frequent checks on certain components.
Written by Mr. Enrico Fabbri firstname.lastname@example.org
Article originally published in Games Industry (Italy) magazine
Original date: September 2017
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