Peen forming and stress peen forming of 2024-T3 aluminum sheets. Part 2: eigenstrain analysis

Abstract

In this study we use the theory of eigenstrains to investigate how different sources of anisotropy affected the results of shot peen forming experiments reported in Part~1. The specimens consisted of 4.9 mm thick 2024-T3 aluminum sheets uniformly shot peened on one face that were either free to deform or held onto a prestressing jig during peening. Potential sources of anisotropy included the plastic anisotropy of rolled aluminum, anisotropic initial stresses that redistribute when their equilibrium is disturbed by peenning, the geometry of the specimens, and externally applied prestress. For the alloy and peening conditions considered, plastic anisotropy had no discernable influence on the resulting shape of the peen formed specimens. Initial residual stresses, on the other hand, caused slightly larger bending loads in the rolling direction of the alloy. Although the magnitude of these loads was approximately 30 times smaller than peening-induced loads, it was sufficient to overcome the geometric preference for rectangular sheets to bend along their long side and cause all unconstrained specimens to bend along the rolling direction instead. Once the sheets started to deform, larger plastic strains developed in the bending direction. We show that this effect is equivalent to that used in the variant of the process called stress peen forming where parts are elastically prestressed during peening to obtain larger plastic strains in directions in which the material is stretched.

Simulation and automation of aluminum panel shot peen forming

“…For a plate with uniform properties and initial stresses, the proportion between ε j xx and ε j yy is constant along the whole component. It is expressed with the anisotropy coefficient χ [22]:…”

“…This value equals approximately half of the shot stream width but is specific for each particular distribution. load Γ B [22]:…”

“…The eigenstrain induced in an aluminum plate by a perfectly symmetrical impact can be anisotropic [18]. The main causes for this effect are the material plastic anisotropy [19] [20] and the initial stresses induced by the fabrication process [21] [22]. Another potential cause is the prestress, i.e., pre-bending of the plate with clamping supports during peening, which is not present if the plate is held flat during treatment.…”

“…Another potential cause is the prestress, i.e., pre-bending of the plate with clamping supports during peening, which is not present if the plate is held flat during treatment. The influence of the three causes can be quantified experimentally using an additional equipment [22]. Otherwise, the eigenstrain anisotropy can be characterized using an eigenstrain-based simulation software, so that the anisotropy is numerically adjusted to fit the simulations to the experiments.…”

“…Otherwise, the eigenstrain anisotropy can be characterized using an eigenstrain-based simulation software, so that the anisotropy is numerically adjusted to fit the simulations to the experiments. In terms of this method, a particular challenge lies in the fact that the influence of the eigenstrain anisotropy on the final shape depends on the geometry of the plate [22]. Consequently, the characterization should ideally be done using exactly the same plates as those that are supposed to be shaped afterwards.…”