Introduction to structural integrity
Introduction to structural integrity

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Introduction to structural integrity

2.6.1 Stress corrosion cracking in stainless-steel structures

On 9 May 1985 the roof of a swimming pool at Uster near Zurich collapsed, killing 12 and injuring several others.

The concrete roof had been held up by a set of stainless-steel tie bars, which were found after the accident to have cracked transversely (Figure 21). Chlorine is added at quite high levels to swimming-pool water supplies in order to control bacterial contamination from swimmers. It is a very powerful oxidising agent, and can attack a very wide range of materials, typically by forming hairline cracks in components that are in tension. Traces of chlorine gas in the general atmosphere of the building were found to be the cause, having attacked the material chemically. Stainless steel and many other types of steel are known to be susceptible to chlorine attack, as are certain plastic materials.

Figure 21
Figure 21 Transverse cracking in stainless-steel tie bars

After that tragedy, yet further failures occurred in swimming pools. In the Netherlands, for example, a ceiling collapsed during the night of 8/9 June 2001 at a pool in Stenwijk. It was discovered the next morning by a party of visiting swimmers; fortunately there were no casualties. The pool had been open only nine years, and chlorine was again used to disinfect the water of the pool. The design of this swimming pool was slightly different, however. The ceiling above the pool was reasonably well supported, but there were air ducts above weighing several hundred kilograms. These were attached to the outer roof above by stainless-steel threaded bars. Upon examination, the threads were found to be deeply cracked. When the cracks reached a critical size, the air ducts fell onto the ceiling, which in turn failed owing to the extra load and the shock, or impact, loading.

Stainless steel is used extensively in swimming-pool fixtures such as handrails, ladders leading from the pool and diving boards. It has been found to perform well, without cracking, for many years, even with constant immersion in warm water dosed with low levels of chlorine. The metal has a polished surface and so can be cleaned easily, and would normally have a long and uneventful life. But the stresses applied in these fixtures are only intermittent, not continuous. Except perhaps for diving boards, the fittings are over-designed, so the imposed loads from the users are relatively low.

However, when similar grades of stainless steel are used as tie bars, they are under continuous tension, which is when stress corrosion cracking becomes a relevant failure mode. The air circulating in the ducts above the Stenwijk pool will have been saturated with water vapour as well as carrying traces of free chlorine gas, and will have attacked the protective oxide film on the steel. This attack will have produced brittle cracks, which then grew slowly under the tensile load, with new metal being exposed as the crack advanced. The high stress concentration at the advancing crack tip, together with the imposed tensile load, will have encouraged further crack growth until criticality.

Falling objects will impose much greater loads than their nominal weight owing to the momentum gained by the drop. The exact force of the impact will depend on the distance through which they fall.

Exercise 7

Explain why the cracks were initiated at the roots of the threads.

Answer

The root of the thread of any screwed joint represents a stress concentrator, where the applied stress can be magnified many times at the corner. The exact value of Kt will depend on the radius of curvature at the root, sharper roots being more severe than shallow roots.

Even low tensile loads will have been enough to stimulate crack initiation, owing to the importance of stress concentrations in magnifying the imposed load. The critical threshold for the initiation of SCC is normally very low, but must be a tensile stress. The other interesting feature of these failures is the continued attack by very low levels of gaseous chlorine. The chlorine levels are lower than in the pool water, so the extent of the attack probably relates to the much greater ease with which gas molecules can penetrate a corrosion film.

The incident in the Netherlands led to a survey of several thousand swimming pools in the country. It emerged that 14 pools were at immediate risk of sudden failure owing to deeply cracked suspension bolts of a similar design to those at the original pool. They were closed immediately for repair. A further 18 pools were considered dangerous and needed extra support of the ducting. Most of the pools were very new, one being only a year old. The stainless-steel parts were replaced with galvanized steel equivalents or with stainless steel with a high molybdenum content (~6%), both of which are much more resistant to SCC.

The accident led to other surveys in Germany and the UK, where a warning notice was circulated by the Health and Safety Executive. It highlights the dangers arising from using what appear to be corrosion-resistant materials in situations where brittle cracks can develop quickly in safety-critical components.

Stainless steel is also sensitive to chloride ions (such as those present in sea water and brine), and especial care is needed in designing the material for use in ships and boats where exposure can occur.

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