Low-cycle method

In: Fatigue calculation methods

The group name corresponds to the fact, that all methods gathered in it should be used for description of low-cycle fatigue (up to 1e5 number of cycles approximately) above all. All of these methods result in a value of total damage D induced by the load history. The reciprocal value of damage corresponds to number of cycles of given amplitude and mean value, that the component can withstand:

The evaluation is often much more complex than it is by fatigue limit oriented high-cycle methods in PragTic. The load history is usually constituted from cycles with different amplitudes, which induce different levels of damage Di. The final summation of all partial cycles is done via Palmgren-Miner rule:

,

where each i-class corresponds to a block of ni cycles with the same amplitude and mean value of loading inducing the same unit Di damage.

To break the continuous load history apart into separate cycles within the i-classes, some method of load history decomposition is necessary.

The low-cycle methods can use two different fatigue curves:

* S-N curve (Wöhler curve) is usually related to local elastic stress analysis. Its only representative in PragTic is the LESA method. The number of cycles is related to some form of final break. The following rule is used:

* e-N curve (BMC curve - Basquin curve for description of elastic strain, Manson-Coffin curve for plastic strain) is usually related to local elastic-plastic strain analysis. These methods obey the rule combined from the two parts in the parentheses above:

Clearly, the e-N curve offers the resulting value of number of cycles in an implicit manner, which requires use of some incremental solution. PragTic uses invariably the Newton-Raphson iterative scheme. The number of cycles is related to an initiation of a technical crack.

© PragTic, 2007

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