In Fatigue Analysis, Cycle Counting is the process of transforming a complex, “messy” history of stress or strain over time into a set of discrete, quantifiable cycles. Most real-world structures don’t experience simple, repetitive “up-and-down” loading (like a perfect sine or triangular wave). Instead, they are subjected variable vibrations. Fatigue laws, like the Palmgren-Miner Rule, require knowing exactly how many cycles occurred at specific stress (or strain) amplitudes to calculate the total damage. Cycle counting bridges that gap.
There are approximately 15 methods of cycle counting. The most common methods include: Rainflow, Level-Crossing, and Peak Counting. Because each method varies, their output may vary when analyzing the identical cycle histories.
To perform a fatigue life prediction, the process typically follows these steps: (1) Data Acquisition, (2) Cycle Counting, (3) Damage Calculation, and (4) Summation.
The tool below allows the user to input a stress or strain history and choose a cycle counting method. Upon calculation, it leads to the histogram of the analyzed results.
Input files should contain the raw load history used for fatigue cycle extraction. The algorithms operate on this time-series to identify stress reversals and closed cycles. The file should be structured with data under two columns, a time column (column 1), and a signal column (column 2). The time column represents the time vector or sample index corresponding to each signal value. It can be actual time (seconds) or simply sequential indices. It's used primarily for maintaining correct signal ordering, optional plotting, or time-based analysis. Cycle counting itself does not require uniform time spacing, only correct sequence order. The signal column represents the load history (e.g., stress, strain, force, displacement). This is the primary input used for extracting turning points (peaks and valleys), computing cycle ranges and means, units should be consistent (e.g., MPa, N, mm). Signal quality directly affects cycle accuracy and fatigue damage estimation. Two examples of input files are below, which can be downloaded and used ().