Cycle counting is the prerequisite for any fatigue or service durability assessment method dealing with arbitrary operational load sequences. Consequently, an appropriate cycle counting algorithm is required.

Cycle counting methods in general are characterized by the following features [10]:

• decomposition of a given course of load (stress) — time history into a sequence of reversal points

• definition of a relevant elementary event (e. g. hysteresis)

• formulation of an algorithm for the detection and processing of elementary events.

The superposition of transients according to the design code (ASME Code, NB 3222.4, see Figure 12) is based on the peaks and valleys method. The largest stress ranges are usually determined from "outer combinations" (e. g. load steps across different transients respectively events). The associated frequency of occurrence results from the actual number of cycles of the participating two events with the smaller number of cycles. This event provides the associated contribution to the partial usage factor Ui. The summing up of all partial usage factors according to Miner’s rule delivers the accumulated damage (usage factor U) or cumulated usage factor CUF.

2007 SECTION III, DIVISION 1 — NB NB-3222.4 Analysis for Cyclic Operation

(5) Cumulative Damage.

■Usual counting method according to ASME-Code

■Search for the largest stress range across events („external combinations11)

■e. g. implemented in ANSYS®- postprocessing

Additionally, a counting of sub cycles within the events should be carried out according to the rain-flow cycle-counting method [e. g. 10] although it is not explicitly addressed by the design code [1]. This is standard practice in the framework of the AFC. The Hysteresis Counting Method (HCM) according to Clormann and Seeger [10] is applied for this purpose. Additionally, the introduction of so called basic events allows a more realistic consideration of the load time sequence [13].