Roland asked about the wisdom of bead blasting aluminum...
Not a bad idea. Typically "bead blasting" implies that the blasting
medium used is glass bead. Its low mass means that little energy is
transferred to the part being blasted, so the plastic deformation that
leads to strain hardening is minimal. In other words, blasting with glass
won't do much to help or hurt.
Shot peening is another matter. Same process, but with steel shot.
It's used to induce compressive stress on the surface of a part (for lbcs,
typically things like connecting rods). This residual compressive surface
stress is balanced by a tensile stress on the interior of the part. Since
fatigue failures originate at the surface, shot peening improves fatigue life.
The amount of shot peening is controlled by peening test strips (Almen strips)
along with the part being peened. (The strips are shot peened on only one
side, and the amount of curvature induced is a measure of the peening.)
The concept is essentially the same as tempered glass or prestressed concrete.
Anyway, blasting with glass cleans the surface without doing much to it.
Blasting with alumina, silicon carbide, or garnet is likely to remove material.
Blasting with silica sand causes siliconiosis (sp? --anyway, bad for lungs).
Blasting sheet metal with almost anything risks turning an lbc fender into
an Almen strip.
Shot peening heavy parts subject to fatigue improves life if done in a
controlled manner.
Mike (having a blast--part by part by bloody part--with my MGA) Habich
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