Application of material analysis and eddy current conductivity tests to aircraft accident investigation

• Material analysis evaluated possible cause of premature flange failure.
• Linear relationship between electrical conductivity and mechanical characteristics.
• Eddy-current conductivity tests confirmed overaging of aluminium alloy in flange.
• Failed flange was likely improperly heat treated during manufacturing.

Material analysis, such as detection of causes of premature failure, is an integral procedure in solving both manufacturing and in-service problems. The case study presented in this paper describes the material analysis performed in the investigation of the Blaník L13 glider catastrophic accident caused by premature failure of the bottom flange of the right wing spar, which is made of Z 42 4203.62 aluminium alloy (equivalent to AW2024-T3 alloy). Characterization techniques including metallography, light microscopy, optical emission spectroscopy, hardness measurements, tensile testing, and electrical conductivity measurement were used to verify the basic material properties of the damaged flange.Each critical part of an aircraft can be constructed from a specific aluminium alloy whose properties are based on its composition and heat treatment. These properties include electrical conductivity, which is determined by the alloy content and prior processing. An eddy current conductivity test thus allows alloys to be sorted based on the changes in their material properties resulting from different thermal processing. The analysis results of the damaged L13 glider flange were compared with the minimum specified values of mechanical and material properties of other flanges from retired L13 gliders. Different values were obtained for the electrical conductivity and material hardness, indicating that the possible cause of the flange’s premature failure could be an imperfect material heat treatment, or material degradation.