Bergmann method

In: local elastic-plastic strain methods->uniaxial methods

Because of the mean stress effect application is the same for all materials in the SWT criterion, Bergmann tries to introduce a further parameter to be able to get better approximation:

Note that the mean stress effect projects only into the Basquin's part of the e-N curve (i.e. to the high-cycle fatigue above all). Negative mean stress is expected to be non-damaging and its value is reset to zero. The conversion to an equivalent value allows a use of the common uniaxial rain-flow decomposition. The choice of the appropriate decomposition method is the point in Calculation Methods window, where the way of the tensor's reduction is chosen.

Note: The results of the method are similar to Erdogan & Roberts method, where also another parameter was introduced to the SWT criterion.

Nomenclature:

Mark	Unit	PragTic variable	Meaning
	[MPa]		stress amplitude
	[MPa]		mean stress
	[-]	C_BERG	Bergmann's mean stress coefficient
	[-]		strain amplitude
E	[MPa]	E	tensile modulus
	[MPa]	SIG_F	fatigue strength coefficient
	[-]	EPS_F	fatigue ductility coefficient
b	[-]	EXP_B	fatigue strength exponent
c	[-]	EXP_C	fatigue ductility exponent
N	[-]		number of cycles to crack initiation

Decomposition

- Rain-flow with von Mises reduction

- Rain-flow with Tresca reduction

- Rain-flow with von Mises (signed) reduction

Elasto-plasticity

- No

- Neuber elastic-plastic accommodation

- Glinka elastic-plastic accommodation

Material parameters

E	[MPa]	tensile modulus
NU	[-]	Poisson’s ratio
SIG_F	[MPa]	fatigue strength coefficient
EPS_F	[-]	fatigue ductility coefficient
EXP_B	[-]	fatigue strength exponent
EXP_C	[-]	fatigue ductility exponent
C_BERG	[-]	Bergmann's mean stress coefficient

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