Residual stress measurements of conventional and stress peen formed 2024-T3 aluminum sheets

Abstract

Aluminum skins on the lower wings of most commercial aircraft are shaped using shot peen forming. This process, which involves bombarding the skins with hard shot, uses nonuniform plastic flow to induce curvatures---in the same way that differential expansion makes metal bilayers bend when heated. Here, we investigate experimentally how constraining conditions affect the final shape of peen formed parts. We report peen forming experiments for 4.9 mm thick rectangular 2024-T3 aluminum sheets of different aspect ratios uniformly shot peened on one face with a low intensity saturation treatment. Some specimens were free to deform during peening while others were elastically prestressed in a four-point bending jig. For each aspect ratio and prestress condition, residual stresses were measured near the peened surface with the hole drilling method. Additional residual stress profiles were also obtained with the slitting method. The residual stress measurements show that the progressive deformation of unconstrained specimens had the same effect as an externally applied prestress. For the peening conditions investigated, this progressive deformation caused unconstrained strips to exhibit curvatures 33% larger than identical strips held flat during peening. Furthermore, we found that the relative importance of material anisotropy and geometric effects did determine the bending direction of unconstrained specimens.

Simulation and automation of aluminum panel shot peen forming

“…Consequently, when the treatment was applied from the bottom side, the plates were already bent towards the top side. This created an effect of prestress that decreased the eigenstrain in the segments treated from the bottom side [4] these segments was smaller than the simulated one. The induced prestress varies locally along the plate and is also dependent on the peening pattern.…”

“…Nowadays, the market proposes a wide range of programmable peen forming robots, such as those mentioned in Ref. [3] and [4]. Nevertheless, there exists no publicly accessible numerical tool that would automatically control a peening robot, bridging the gap between the numerical model (CAD) and the robot path.…”