Nyhus, Bård (SINTEFMaterials and Chemistry) | Dumoulin, Stephane (SINTEFMaterials and Chemistry) | Nordhagen, Håkon (SINTEFMaterials and Chemistry) | Midling, Ole Terje (Marine Aluminium AS) | Myhr, Ole Runar (Hydro Aluminium) | Furu, Trond (Norsk Hydro ASA) | Lundberg, Steinar (Hydal Aluminium Profiler AS)
Aluminium is known as a safe and suitable material for offshore installations. Factors that favour aluminium are low weight, no need for surface treatment and low maintenance costs. Though aluminium has a high strength-to-weight ratio, it suffers from strength reduction in heat affected zones when welded. The strength of the soft zones is often dimensioning in design, and the ability to predict the strength reduction is important for fully utilizing the potential of aluminium as a structural material. In the current study, the cross weld strength of EN AW 6082-T6 and EN AW 5083-H321 as a function of wall thickness at room temperature and at −60°C (”arctic temperature”) was tested. The main objectives were to verify that the materials and the weldments are not deteriorated at low temperatures, and to check if using additional reduction factors for the heat affected zones for plates and extrusions thicker than 15 mm as specified in the design standard EN 1999-1-1 is correct. The results show that there is no reduction in strength for low temperatures, nor for plates and extrusions thicker than 15 mm. Based on the results in this study, changes in EN 1999-1-1 are recommended.
Unlike body-centred cubic (BCC) metals, the yield and strength temperature sensitivity of face-centred cubic (FCC) materials, such as aluminium (Al) alloys, is negligible when lowering the temperature below room temperature (Hertzberg 1996). Because of the high specific strength, good corrosion resistance and good mechanical properties at low temperature, Al-alloys are often used for low temperature conditions such as cryogenic applications (e.g. Liquefied Natural Gas (LNG) tanks and space/aeronautics). Thus, the low temperature characterization found in the literature focus on test temperatures far below −60°C.
In BCC materials, such as steels, the dislocation width is narrow and the Peierls stress increases rapidly with decreasing temperature, thus the yield stress will increase strongly with decreasing temperature. An important consequence of this for BCC materials is that the yield stress can rise to such high levels that only a very limited plastic zone ahead of a crack will occur before unstable (brittle) fracture results. This material brittleness will not occur in FCC alloys (Aluminium), and a ductile fracture mode will prevail.