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Steels - Endurance Limits and Fatigue Stress

Endurance limits and fatigue stress for steels

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  • Fatigue - when a material is subjected to repeated cycles of stress or strain and its structure breaks down and ultimately leads to fracture
  • Creep - when a material is subjected to a load for a very long time it may continue to deform until a sudden fracture occurs

Fatigue

Fractures due to fatigue is common in cyclic loaded parts like connected rods, crankshafts, turbine blades, railroad wheels and so on. Fractures occur at stress less than the material Yield Stress.

  • Yield strength is defined in engineering as the amount of stress (Yield point) that a material can undergo before moving from elastic deformation into plastic deformation.

Most steels have an endurance or fatigue limit about half the Tensile Strength.

  • Tensile Strength - (Ultimate Tensile Strength) - of a material is the limit stress at which the material actually breaks, with sudden release of the stored elastic energy.
SteelBSTreatmentTensile Strength
- σu -
(MPa)

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Endurance Limit
- σe -
(MPa)

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σe / σu
0.4% Carbon BS970 080M40 Normalized 540 270 0.5
0.4% Carbon BS970 080M40 Hardened and tempered 700 340 0.49
Carbon, manganese BS970 150M19 Normalized 540 250 0.46
Carbon, manganese BS970 150M19 Hardened and tempered 700 325 0.53
3% Chrome molybdenum BS970 709M40 Hardened and tempered 1000 480 0.48
Spring steel BS970 735A50 Hardened and tempered 1500 650 0.43
18.8 Stainless Cold rolled 1200 490 0.41
  • 1 MPa = 106 Pa = 1 N/mm2= 145.0 psi (lbf/in2)
  • Fatigue limit, endurance limit, and fatigue strength are used to describe the amplitude (or range) of cyclic stress that can be applied to the material without causing fatigue failure.

Creep

The time dependent deformation due to heavy load over time is known as creep.

In general both stress and temperature influence on the rate of creep.Normally creep strength decreases with temperature.

Allowable creep strain can be specified - typical 0.1 %/year for steel bolts and piping

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