Some Notes on the Influence of Manufacturing on the Fatigue Life of Welded Aluminum Marine Structures

Abstract

Designers are constantly looking for ways to reduce the structure weight to lower the overall displacement and hence the cost of fast ferries and other high-speed vessels. The easiest option for the designer is to choose a lightweight material. Aluminum has become the adopted choice of material for high-speed vessels owing to its high strength to weight characteristics. Unlike steel, aluminum is more prone to fatigue cracking and has no fatigue limit. In order to minimize weight, the designer will make use of finite element methods to optimize the scantlings and perform fatigue checks against established codes. This can lead to a structure that has the empirical margins of safety reduced owing to the accuracy of mathematical modeling. However, what is often overlooked is the effect the manufacturing process has on the fatigue life of the fabricated structure. This aspect is excluded from the designer's fatigue calculations, which assist in reducing the scantlings. Currently, there is no guidance for fatigue life reduction for the designer that establishes good and bad workshop practice, other than experience, or the implications of basic shipyard fabrication. It is shown that whereas strain-hardened alloys improve mechanical strength, they reduce ductility. This has consequences when forming the hull plate by potentially introducing crack like flaws into the alloy matrix if the plater overrolls the plate. If there is misalignment or there is too much gap between the plates, the weld will create localized stress concentrations. If the welder has poor joint preparation or gas shielding, porosity can be introduced into the weld. Porosity has a significant effect on the fatigue life of the weldment. This paper brings together a collection of data on such issues that the designer needs to be aware of to prevent an unwanted fatigue failure in the fabrication process.