SHOT PEENING
From Wikipedia, the free encyclopedia
Shot
peening is a process used to produce a compressive
residual stress layer and modify mechanical properties
of metals. It entails impacting a surface with shot
(round metallic, glass or ceramic particles) with force
sufficient to create plastic deformation. It is similar
to sandblasting, except that it operates by the
mechanism of plasticity rather than abrasion: each
particle functions as a ball-peen hammer. In practice,
this means that less material is removed by the process,
and less dust created.
Peening a surface spreads it plastically, causing
changes in the mechanical properties of the surface.
Shot peening is often called for in aircraft repairs to
relieve tensile stresses built up in the grinding
process and replace them with beneficial compressive
stresses. Depending on the part geometry, part material,
shot material, shot quality, shot intensity, shot
coverage, shot peening can increase fatigue life from
0%-1000%.
Plastic deformation induces a residual compressive
stress in a peened surface, along with tensile stress in
the interior. Surface compressive stresses confer
resistance to metal fatigue and to some forms of
corrosion. The tensile stresses deep in the part are not
as problematic as tensile stresses on the surface
because cracks are less likely to start in the interior.
Shot peening may be used for cosmetic effect. The
surface roughness resulting from the overlapping dimples
causes light to scatter upon reflection. Because peening
typically produces larger surface features than
sand-blasting, the resulting effect is more pronounced.
Shot peening was originally developed by John Almen when
he was working for Buick Motor Division of General
Motors Cooperation. He noticed that shot blasting, as it
was called back when he was working, made the side of
the sheet metal that was exposed begin to bend and
stretch. John Almen also created the Almen Strip to
measure the comprehensive stresses in the strip created
by the ball peening operation. One can obtain what is
referred to as the "Intensity of the Blast Stream" by
measuring the deformation on the Almen strip that is in
the shot peening operation. As the strip reaches a 10%
deformation, the Almen strip is then hit with the same
intensity for twice the amount of time. If the strip
deforms another 10%, then you have the, "Intensity of
the Blast Stream."
A study done through the SAE Fatigue Design and
Evaluation Committee showed what shot peening can do for
welds compared to welds that didn't have this operation
done. The study claimed that the regular welds would
fail after 250,000 cycles when welds that had been shot
peened would fail after 2.5 million cycles, and outside
the weld area. This is part of the reason that shot
peening is a popular operation with aerospace parts.
However, the beneficial prestresses can anneal out at
higher temperatures.
