Constant amplitude life curves

Damage parameters relate quantities (usually stress, strain, or a combination or both) to fatigue behavior. In the strain-life approach, small laboratory specimens are tested in constant amplitude, fully reversed strain control until a fatigue crack is detected. The resulting crack initiation life in reversals, 2Ni (where Ni is the number of cycles to crack initiation of technical size), and the corresponding strain amplitude is recorded.

For low levels of strain which are nearly completely elastic, the fatigue life behavior correlates well to the stress amplitude:

where is the fatigue strength coefficient and b the fatigue strength exponent which are material properties that are determined from a log-log fit of the stress amplitude and reversals to failure, 2Ni. The equation is known as Basquin’s equation, and the fatigue strength exponent is also known as Basquin’s exponent.

For high load levels, at which plastic strains dominate, the fatigue life correlates well to the plastic strain amplitude:

where is the fatigue ductility coefficient, and c is the fatigue ductility exponent, which are material properties determined from a log-log fit of the plastic strain amplitude and reversals to failure. This is known as the Manson-Coffin-Morrow relationship.

For intermediate levels of strain, at which neither elastic nor plastic strains dominate, the strain-life equation is used, which is the sum of the contributions of elastic and plastic strain amplitudes:

with the material constants determined from the elastic and plastic strain amplitudes respectively. This equation is known as the Manson-Coffin-Morrow equation, or the strain-life equation. This equation forms the basis of the strain-life approach implemented in Specialist Durability.

The Manson-Coffin-Morrow Relation

The straight lines show the Basquin equation and the Manson-Coffin equation.

Parameter	Meaning	Unit
	Fatigue strength coefficient	MPa
	Fatigue ductility coefficient	1
b	Fatigue strength exponent	1
c	Fatigue ductility exponent	1

These parameters can be specified directly in Simcenter 3D materials.

Learn more

Local stress-strain behavior

Endurance limit and static failure

Determining material properties

Mean stress and damage parameters

Axial versus torsion tests

Notch analysis

The strain-life analysis in Specialist Durability

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Constant amplitude life curves, Simcenter 3D 2021.1 Series

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