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Proof stress

  • The stress that will cause a specified small, permanent extension of a tensile test piece. Commonly the stress to produce 0.2% extension is quoted in N/mm2 for steel. This value approximates to the yield stress in materials not exhibiting a definite yield point. 
  • The load per unit area that a structure can withstand without being permanently deformed by more than a specified amount. Stress that will cause a specified permanent deformation.
  • Proof stress can be found by referring to the stress/strain curve at the point where strain is = 0.2% original volume (the material has grown 0.2% in volume) proof stress will be given as a measurement of energy (MPa,KPa etc.) as it specifically refers to the amount of energy required to stress the material to 0.2% its original volume.
  • Proof  stress, by definition, is the stress to cause some level of permanent deformation.

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