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

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.

A Geometric Definition of Short to Medium Range Hydrogen-Mediated Interactions in Proteins

We present a method to rapidly identify hydrogen-mediated interactions in proteins (e.g., hydrogen bonds, hydrogen bonds, water-mediated hydrogen bonds, salt bridges, and aromatic π-hydrogen interactions) through heavy atom geometry alone, that is, without needing to explicitly determine hydrogen atom positions using either experimental or theoretical methods. By including specific real (or virtual) partner atoms as defined by the atom type of both the donor and acceptor heavy atoms, a set of unique angles can be rapidly calculated. By comparing the distance between the donor and the acceptor and these unique angles to the statistical preferences observed in the Protein Data Bank (PDB), we were able to identify a set of conserved geometries (15 for donor atoms and 7 for acceptor atoms) for hydrogen-mediated interactions in proteins. This set of identified interactions includes every polar atom type present in the Protein Data Bank except OE1 (glutamate/glutamine sidechain) and a clear geometric preference for the methionine sulfur atom (SD) to act as a hydrogen bond acceptor. This method could be readily applied to protein design efforts.