Peen forming and stress peen forming of rectangular 2024–T3 aluminium sheets: Curvatures, natural curvatures and residual stresses

Abstract

Aluminium 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 aluminium 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.

Eigenstrain‐based analysis of why uniformly shot peened aluminium plates bend more in the rolling direction

“…Recall that our aim is to clarify the contribution of different sources of anisotropy on the final deformed shape of the aluminium sheets peen formed in Faucheux et al [3] This analysis must be carried out from the only experimental data…”

“…Deflection magnified by a factor of 4. Reproduced from Faucheux et al [3] available, namely, residual stresses and curvatures measured on coupons removed from the specimens. Because of stress rebalancing, these stresses and curvatures differ from those in the original specimens.…”

“…T A B L E 1 List of specimens peen formed in Faucheux et al [3] for which residual stress measurements are available. where ε are the total strains, ε el are the elastic strains and ε * are the eigenstrains.…”

“…Experimental studies have been widely used to understand the peen forming process mechanics since the 1950s. Faucheux et al [3] conducted a detailed literature survey on publicly available experimental research works related to peen forming. This study revealed that the vast majority of publicly available data was acquired on mm strips submitted to peening conditions inducing deflections that remained small, when compared with the strips thickness.…”

“…Residual stresses measured with the slitting method in 51 Â 51 mm coupons removed from an as-rolled sheet. Reproduced from Faucheux et al [3] recorded at 51 μm intervals down to a depth of 1 mm, and the data were fit with a sixth-order polynomial to obtain continuous residual stress profiles.…”