eFunda: Low-Cycle Fatigue
engineering fundamentals Low-Cycle Fatigue
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The S-N curve eastimate the service life of materials above 103 (often >104) cycles. The corresponding stress level is usually below 2/3 of yielding stress. The "low-cycle fatigue" model, on the other hand, is made for 104 cycles and below. The stress level usually steps into plastic range.
Strain-Life Equation

When plastic strain occurs, the service life of material decreases, often no more than 104, a.k.a., in low-cycle fatigue range. The research of low-cycle fatigue was traditioanlly done for pressure vessels, power machinery that are exposed to a heat source/sink which induces thermal expension (thermal stress) to the structure. The low-cycle fatigue is usually presented as the plastic strain in log scale against cycles to failure N also in log scale.

To add Fig 12-13, low-cycle fatigue

The result of low-cycle fatigue is near a straight line for common metal meterials such as steel and is often referred as Coffin-Manson relation:

where is the amplitude of plastic strain.
is fatigue dutility coefficient defined by the strain intercept at 2N = 1. For common metal materials, .
N is the number of strain cycles to failure and 2N is the number of strain reversals to failure.
c if called fatigue ductility exponent. For common metal materials, -0.7 < c < -0.5. A smaller c value results in a longer fatigue life.

Bridge between High and Low Cycles

The Coffine-Manson formula describes the relationship between plastic strain and fatigue life in the low-cycle high-strain fatigue regime. Basquin's equation, on the other hand, describe high-cycle low strain behavior

where is the amplitude of alterning stress
is the amplitude of elastic strain
E is Young's modulus
is fatigue strength coefficient and is approximately equal to the monotonic true fracture stress .
N is the number of strain cycles to failure and 2N is the number of strain reversals to failure.
b if called fatigue strength exponent. For common metal materials, -0.12 < b < -0.05. A smaller c value results in a longer fatigue life.

Universal Strain-Life Equation

Manson proposed a simplified formula known as the mothod of universal slopes

where is the amplitude of alterning stress.
is the true strain at fracture intension.
E is Young's modulus.
N is the number of strain cycles to failure.

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