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Residual stresses: causes, effects, relief

Short answer

Residual stresses are self-balancing stresses remaining after all external loads are removed. They arise from non-uniform plastic deformation, thermal gradients, phase transformation, or machining. Compressive surface stress improves fatigue life; tensile stress reduces it and promotes cracking. Relief methods include stress-relief annealing, tempering, stretching, vibratory stress relief, and shot peening.

What are residual stresses? Explain how they are produced in manufacturing processes, their effects on the product, and the methods of reducing or removing them.

Residual stresses are stresses that remain in a body after all external loads and temperature gradients have been removed. They are self-balancing: tensile stress in one region is balanced by compressive stress in another.

Origin

Residual stress appears whenever different parts of a body deform plastically by different amounts, or cool at different rates.

  1. Non-uniform plastic deformation: in bending, the outer fibres are plastically stretched and when the load is removed the elastic core pushes them back, leaving compressive stress on the surface that was in tension. Rolling, drawing and shot peening also leave surface compression.
  2. Thermal gradients: in quenching, the surface cools first and contracts; later the core contracts against a rigid shell, leaving surface compression and core tension. In welding, the weld region shrinks against cold parent metal and is left in tension.
  3. Phase transformation: martensite formation causes volume expansion in some regions (hardening of steel).
  4. Machining and grinding: mechanical deformation under the tool and heat generated leave a thin surface layer in tension (grinding heat) or compression (sharp tools, low feed).

Effects

  • Distortion and warping when material is machined away or heated.
  • Compressive residual stress at the surface improves fatigue life and resists crack growth.
  • Tensile residual stress at the surface reduces fatigue strength and causes stress-corrosion cracking and cold cracks in welds.
  • Dimensional instability of precision parts with time.

Methods of reduction or removal

MethodPrinciple
Stress-relief annealingHeat below the transformation temperature (about 550-650 degree C for steel), hold, cool slowly
TemperingAfter hardening, relieves quench stresses
Stretching / light cold workSmall plastic deformation (about 1-2 percent) levels the stress
Vibratory stress reliefVibration at resonance redistributes stress
Shot peeningIntroduces beneficial surface compression
Controlled cooling and proper process designReduces thermal gradient

✓ Answer checked by Claude Opus 5.5 (high effort)

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