PSI - Issue 68

T.N. Examilioti et al. / Procedia Structural Integrity 68 (2025) 756–761 T.N. Examilioti et al./ Structural Integrity Procedia 00 (2025) 000–000

759

4

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Fig. 2. Elongation at fracture values of LBWed AA2198 specimens under different ageing conditions for (a) different filler materials; (b) different thicknesses. 4. Quality index background Selecting the appropriate thickness, filler material and heat treatment conditions to achieve an optimal “balance” between tensile strength and ductility in aluminum alloys is a well-defined challenge in the engineering community. To address this, various “quality indices” have been proposed for precipitation-hardened aluminum alloys, where “quality” refers to a combination of tensile stress and ductility that fulfills the design requirements for specific application Alexopoulos (2006). The fundamental idea behind these quality indices is to ensure the proper selection of materials while eliminating those that do not meet the necessary standards set by design engineers. These indices can effectively integrate relevant mechanical properties, facilitating the selection of optimized materials. In this context, Pantelakis and Alexopoulos (2008) introduced the quality index Eq (1) , contributing to the understanding of how to optimize aluminum alloy performance. ! = " + ∙ (1) where W is the strain energy density of the alloy, which can be calculated by the integration of the true stress strain flow curve derived in tensile test until elongation reaches the fracture point at A T , i.e. Eq (2) . = ∫ ( ) ∙ # ! ! (2) In Eq (1) , the coefficient is equal to 10, reflecting a typical value of the ration " / for various optimized advanced aluminum alloys commonly used in aircraft applications, Alexopoulos (2006). The choice of " over UTS in Eq (1) is intentional, as the yielding of aircraft structures equates to failure and cannot be tolerated. This index assesses material quality by evaluating the “balance” between the material properties, yield stress " and strain energy density W . The concept of quality indices was utilized in this study to identify the optimal quality among various filler materials and thicknesses, according to heat treatment conditions for LBWed AA2198, are presented in Fig. 3a-b , respectively. For both Figures the iso- Q o lines have been calculated by exploiting Eq (1) .