THE ITEM in the June 2013 edition of Sea Breezes on the Mosaic project set me thinking about the relative merits or otherwise of welds versus rivets in the integrity of ships’ hull structures.
A rivet was a unit of attachment occupying a round hole whose shape was an ideal way to avoid local stress concentrations in the plate. The material of the plate and rivet are matched and little if any diffusion occurs between the two providing a purely mechanical joint. Failure of a single rivet would be a mainly localised effect since those in the surrounding areas which remain intact continue to maintain structural integrity.
Contrasting these characteristics with those of a welded joint gives some cause for concern, a worry that has been around since the start of shipbuilders moved to welded hulls. Welds are highly dependent upon the materials to be joined; the control of weld parameters, environmental conditions at the time and not least the skill of the welder. Cracks in fusion welds may at first be microscopic and relatively harmless provided they remain below a critical size and stress level but time and cyclic stresses can initiate the growth of the cracks and result in catastrophic propagation over great lengths of weld leading to rapid loss of structural integrity.
The Mosaic proposals suggest that the introduction of higher strength steels in critical hull areas could reduce the risk of corrosion and failure while lightweight composite materials could bring weight reductions to improve performance. To introduce new materials and add mixed steel specifications into the welding requirements brings an even greater need for monitoring the weld quality and identifying any hitherto unforeseen failure modes. These thoughts exclude the obvious and monumental task of reliably monitoring the through-life integrity of welds and attachments over enormous hull areas to identify and repair cracks before they become critical. I am sure that mariners who rely on the integrity of the structures which transport them and their cargos in fair weather and foul will hope for a positive outcome to the Mosaic project with no unpleasant and unforeseen side effects.