Flexcom > Theory > Analysis > Frequency Domain Fatigue Analysis

Overview

Histogram is a frequency domain postprocessor to Flexcom and is used as a general cycle counting tool. The input is taken directly from the results of Flexcom random sea analyses, and the results are output in the form of response histograms. The program can combine results from a number of analyses taking percentage occurrence of the corresponding seastates into account. The module produces tabular and graphical output.

One important point to note is that response histograms in the frequency domain are presented with respect to amplitude. This is inconsistent with the corresponding time domain cycle counting tool (LifeTime), which presents response histograms in terms of parameter range.

Operation

The steps involved in performing a general cycle counting analysis with Histogram are as follows:

•Step 1. Perform a Flexcom random sea analysis for each fatigue seastate.

•Step 2. Specify the relevant input data into Histogram. Fatigue seastate data is specified in terms of a list of Flexcom random sea analyses, along with percentage occurrences of the corresponding seastates. General data includes the parameters from the Flexcom output which you want cycle counted, plus the “bins” or divisions for the Histogram output.

•Step 3. Run Histogram. When running, the program loops over all seastates and opens the Flexcom postprocessor file for each. It reads the parameter values from these files and then calculates the response histograms for each parameter requested

The main analysis results, histograms in this case, are then output to a file named jobname.out. This file also contains “debug” output, which presents results of the program’s intermediate calculations for each individual seastate (Flexcom analysis). This “debug” output may be used for checking and interpreting the analysis results.

For each histogram you request, Histogram also creates a separate plot file. An example is shown in the figure below. As you can see, on the X or horizontal axis is Parameter Amplitude, while the Y or vertical axis has No. of Cycles.

Histogram_Operation_Sample Histogram Plot

Relevant Keywords

Step 1

•Numerous keywords are contained in the $LOAD CASE section, including data such as environmental parameters (e.g. current and waves), boundary conditions of various kinds, internal fluid loading, and the analysis type and solution parameters.

Step 2

•Numerous keywords are contained in the $HISTOGRAM section, which relates to general cycle counting in the frequency domain.

o*BINS is used to specify bins or divisions for histogram output.

o*PARA is used to request response histograms of restoring force, stress, reaction, or relative rotation.

o*PDF is used to specify the probability density function to be used in the calculation of histograms.