# The Basis for Reliability-Based Mechanical Properties of Structural Aluminium Alloys
## 1 Introduction  
The first edition of the recommendations for the design of aluminium structures was published in 1978 by the ECCS committee T2 which was chaired by [PERSON] [1]. Up until the publication of the final version of Eurocode 9 in 2007 [2], substantial work has been conducted by the Technical committee CEN-TC 250/SC9, and the limitations imposed by the absence of standards and regulations for the design of aluminium structures have finally been successfully surpassed. The revision of the structural Eurocodes is now in the final phase, and soon the second generation of Eurocodes will be published, along with a revised and upgraded version of EN 1999, again by CEN-TC 250/SC9, chaired by [PERSON]. Nevertheless, a formal link between Eurocode 9 and Eurocode 0 regarding the achieved level of reliability, in terms of probabilistic analysis of design rules given in Eurocode 9 (partial factors on the resistance side), is still missing. This is additionally stressed by the requirements of EN 1998 [3] and especially with the upcoming upgraded version of EN 1998, regarding minimum and maximum mechanical properties and planned plastic mechanisms. To bridge this gap, the TC 250 SC10 ad-hoc group on reliability isworking actively on the 'Technical report for the reliability background of Eurocodes', and all TC 250 SCs are supposed to support this activity with appropriate data. It is a well-known fact that various factors influence the reliability level of structures designed according to (Euro)codes. In a limit-state equation, the action side variables are generally dominant, but resistance side variables are also important and should not be ignored. Generally, from the resistance side, mechanical properties are variables with the highest weights and should be the first aspect to consider. Additionally, by separating the model uncertainty in structural resistance and the uncertainty in material properties, a more straightforward approach to determining the partial factor with uniform reliability can be achieved. Namely, model uncertainty in structural resistance, especially regarding stability issues, includes multiple influential variables being researched [4], and the separation of the material variability as a variable can provide an easier and superior calibration process. In the current Eurocodes (as in previous national standards) for metals, the guaranteed (minimum) mechanical values according to relevant product standards are used as characteristic values. Although from the aspect of a semi-probabilistic approach guaranteed values are not characteristic values, they are the most reliable substitute as it is proven that the material fulfils the guaranteed (minimum) values. This approach can be regarded as non-economic, given that nominal values are much closer to design values than characteristic values. Even though aluminium is a homogenous material, the type of treatment (temper) and the content of alloying elements affect the uniformity of mechanical properties [5]. To properly quantify the extent of variability of mechanical properties, its influence on the material level itself has to be considered using uniaxial tensile test results, excluding possible variability introduced by the testing method [6; 7]. So far, limited research has been conducted regarding the variability of mechanical properties of aluminium, in contrast to structural steel [8] with which aluminium is often compared. Several authors from the Royal Institute of Technology in Stockholm as part of wider research programmes [9; 10; 11; 12; 13] conducted a number of tensile tests to determine the mechanical properties of aluminium. However, the purpose of those tests was to obtain mechanical properties for the development of the numerical model, with not enough results to statistically characterise the mechanical properties of aluminium alloys.