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River flow – Do you imagine peaceful blue water? Try rusty brittle material
Stress Corrosion Cracking (SCC) is a sudden and often difficult-to-predict severe degradation mode of failure in nuclear, petrochemical, chemical and other industries with pipelines, vessels, tanks and other equipment.
It resembles the branches of a river:
However, as opposed to a clear stream of water, there is nothing peaceful about it. When not taken care of, SCC can cause damage, failure and unnecessary costs. Let us describe one real-life event from aviation industry where SCC was even responsible for the loss of human lives.
An EL AL 747 freighter aircraft crash in Amsterdam, the Netherlands in 1992 was caused by the disintegration of a jet engine, and as macroscopic fractography showed, the chain of events started with stress corrosion cracking. According to a paper by Kolkman, Kool and Wanhill, the main cause of the incident was the manufacturer's choice of SCC-prone material for the pins of the levers.1
The separation of the No. 3 engine was initiated by fatigue (corrosion) in the inboard “bulkhead” fuse pin. There was no primer, cadmium plating or corrosion prevention present. This led to the loss of the No. 4 engine and pylon and damage to several systems which ultimately led to loss of control of the aircraft and a tragedy with an official death toll of 43 victims as well as 11 serious and 15 minor injuries. A better kind of this lever arm pin only costs a few dollars..
The flight originated in New York and crashed in Bijlmermeer, the Netherlands, and is now known in Dutch as Bijlmerramp (Bijlmer disaster). It crashed into flats with large numbers of undocumented immigrants, so the number of killed people is probably not definitive. To this day, it remains the deadliest aviation incident on the territory of the country.2
In 1988, seven years after the new style “bulkhead” fuse pins were put into service, reports of cracking in these pins were received by Boeing. Sadly, this Boeing was not the first 747 to crash because of the same reason. In December 1991, a China Airlines Boeing 747-200F freighter crashed shortly after take-off. We had to wait another year and another catastrophe for a change to happen.
Based solely on evidence from the El Al accident investigation focused on fatigue cracking of the fuse pins, the FAA mandated replacement of the old style fuse pins within 30 days.3
What were the actions taken? Adoption of an inspection programme, ultrasonic sensor control and a new pin design, ensuring:
- normal function, even during abnormal flight conditions
- increased fatigue life and crack growth life
- no predisposition to corrosion
- no tooling marks during manufacturing that could lead to cracking4
In order to prevent similar catastrophes, stainless steel fuse pins with improved fatigue and crack growth life replaced the “bulkhead” style fuse pins.
Our Stress Corrosion Cracking training is taking place in Amsterdam, the Netherlands; where you can engage in technical and scientific discussions and learn the best practices in SCC control, testing, detection and prevention. As Max Mayfield, former Director of the National Hurricane Center said, "Preparation through education is less costly than learning through tragedy."
1 H.J. Kolkman et al., "AIRCRAFT CRASH CAUSED BY STRESS-CORROSION CRACKING", Journal of engineering for gas turbines and power, 118(1), 1996, pp. 146-149
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