Rainflow Counting

Rainflow Counting

Rainflow Cycle counting is a method to determine the number of fatigue cycles in a load time history. For a load of constant amplitude, it is easy to calculate the fatigue due to cumulative damage (https://www.fea-solutions.co.uk/cumulative-fatigue-damage/) using the S-N curve (https://www.fea-solutions.co.uk/s-n-curves/) and Miner’s Rule. In a real world component, however, the load history is often complex, with the number of cycles and amplitudes being difficult to determine.

A load time history graph usually shows load versus time, or strain versus time. If a stress based fatigue analysis is being performed (https://www.fea-solutions.co.uk/stress-based-fatigue-analysis/), this load or strain history is converted to a stress time history, so that Rainflow Cycle counting can be used.

Rainflow Cycle Counting is made up of 4 stages:

  1. Hysteresis Filtering – This removes very small cycles that contribute a negligible amount of damage.
  2. Peak Valley Filtering – Only keeps data points which are reversals in slope of stress time graph.
  3. Discretization – Maps data points to discrete ‘bins’ to enable counting procedures.
  4. Four Point Counting Method – Produces a usable Rainflow Matrix from the discrete bins.

The outcome of the Rainflow Cycle Counting is the Rainflow Matrix. This displays the following properties of the fatigue cycles:

  • The ‘From’ Stress – the stress level that the cycle originates from.
  • The ‘To’ Stress – the stress level the cycle finishes at.
  • Number of cycles. This is often indicated through the use of colour.

From this matrix, the fatigue damage of these fatigue cycles can be calculated, using Miner’s Rule and an S-N Curve of the material. This allows different complex time histories to be compared in terms of the fatigue damage they do to a component.

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