A question about residual stress

  1. kelvin490

    kelvin490 130
    Gold Member

    Shot peening is a process of inducing RESIDUAL COMPRESSIVE STRESS on the surface layer of metal parts. When shooting particles are colliding on the surface of metal there is a compressive stress. My question is, how can resultant compressive stress exists after the force causing the compression is removed (i.e. after the shooting particle leaves the surface)?

    Normally we need a force to hold the object in compressive state. For residual stress, are there any parts in tension to balance this stress?
     
  2. jcsd
  3. AlephZero

    AlephZero 7,300
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    The impacts cause plastic deformation of the surface. If you could somehow remove the surface layer from the rest of the component, it would have permanently changed its shape, the same way as beating out a sheet of ductile metal like gold or silver makes it thinner and covers a bigger area.

    There is a residual tensile stress inside the component that balances the compressive stress in the surface layer.
     
  4. SteamKing

    SteamKing 8,809
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    Tempered glass is manufactured in a way such that the outer surface remains in compression. By staying in compression, the growth of cracks in the glass is inhibited if the glass should be struck by an object, preventing it from shattering.
     
  5. kelvin490

    kelvin490 130
    Gold Member

    The surface is plastically deformed, but does plastic deformation necessarily implies the presence of residual stress?

    How can a tensile stress built up inside the metal? It is hard to visualize.
     
  6. AlephZero

    AlephZero 7,300
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    Not necessarily, but it usually does create some residual stress if only part of the object is plastically deformed.

    Suppose you have a sphere covered with a thin layer of material that fits exactly around the sphere but is not attached to it.

    If you then shot peened the sphere, you would permanently deform the surface layer (plastically) so it became slightly thinner. Because plastic deformation does not change the volume of the material, the the thinner layer must have a slightly larger radius to keep the volume the same. In other words it would now be a loose fit around the inside solid sphere. (If you ever tried panel beating to remove dents from a metal plate, you will soon discover that effect - the beaten-out part of the plate always ends up "too big" for the area it has to cover.)

    In that situation, there is no residual stress anywhere. The shot peening creates permanent plastic strain that changes the shape of the surface layer, but no stress, because a gap could open up between the surface layer and the inside.

    But in the real life solid sphere, the surface and the inside of the sphere are attached to each other. That means the inside part of the sphere has to stretch elastically to a bigger diameter, causing a tensile stress through the whole of the inside. The surface layer has to be compressed slightly, giving it a compressive stress.
     
    Last edited: Aug 12, 2014
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  7. kelvin490

    kelvin490 130
    Gold Member

    How does the tensile stress made uniform in real case? For each collision the particle causes compression for the inner region at the point being collided, at the same time cause tension for the inner region around the point being collide. How can this finally be uniform?
     
  8. The depressions are made harder and stronger via work hardening. There will be an equilibrium point where the surface becomes flat (again) as the surface has become too hard for additional collisions to cause further plastic deformation.
     
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