Case Studies - Aircraft Tie Down Points
Cracking was found on aircraft tie-down link plates on board HMS Ark Royal after post installation proof load testing. Imes took part in an investigation conducted by the Ministry of Defence (MoD) in order to generate a safety case into the continued operation of such test given the presence of these cracks.
These link-plates were a new design, manufactured to support the Merlin helicopter and consisted of a duplex stainless steel cup casting and forged shackle, with a carbon steel annular ring. The annular ring is to facilitate installation into the deck without having to weld dissimilar materials.
The cracked link-plates were examined using a number of different techniques including Acoustic Emission (AE), strain gauging and visual inspection. Fatigue and destruction tests were completed by Imes on both new and cracked link-plates.
Two link plates were strain gauged and monitored with AE as they were subjected to incremental loadings up to 150% of their safe working load (SWL).
By conducting this investigation it was possible to prove that the link-plates' operational life was not affected even with inherent defects. Destruction and fatigue testing proved that there was considerable remaining life and strength within components even with surface breaking cracks. It was determined that the primary cause of the cracking was the residual stresses within the casting generated by the heat treatment during manufacture.
The installation of the link plates, welding them into the deck, also contributes to the areas of high stress within the casting. When load tested, the stresses within the link plate reach sufficient levels to cause cracking. These cracks propagate with use before they eventually arrest the annular ring interface where they do not threaten the safe operation of the link plate.
AE was instrumental in pinpointing the precise location of such high stresses and understanding the mechanism causing them to occur. In addition, fatigue was discovered not to be of consequence as the link plates withstood 330,000 cycles at their working load of 56kN and still, when tested to destruction, failed at over 4 times their safe working load.
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