A quality control test to determine the propensity for Lüders lines formation in 2024-T3

Lüders lines can appear on the surface during the stretch-forming of aluminum sheets for aircraft skins. These surface strain markings, due to discontinuous plastic deformation, detract from the appearance of the aluminum skins and may be a cause for their rejection. The test method described in this paper was developed to determine whether 2024-T3 sheet material would form Lüders lines during stretch-forming. An increase in the resolution of the load-deflection curve in the vicinity of the yield point during a tension test performed with pre-determined cross head and chart speeds forms the basis of this test method. This method has been successfully used to predict the forming behavior of many heats, as well as to modify stretch forming parameters for minimizing the problem in forming of transport aircraft skins of 2024-T3 alloy.