You are ready to run the fatigue analysis. Open the Job Control... form, set the Action to Full Analysis and click the Apply button. The database will close momentarily as the results information is extracted. When the database reopens, the job will have been submitted. You can then set the Action to Monitor Job and click the Apply button occasionally to view the progress. When the message appears, the analysis is complete. Close down the Job Control... form when done.

Fatigue analysis completed successfully

When the analysis starts it first converts stresses to strains if stresses have been supplied. The rainflow counting procedure then takes place as discussed in earlier chapters. This results in a matrix of strain cycles with their respective ranges and means. The Crack Initiation method helps to, perhaps, better understand rainflow cycle counting.

The reason rainflow cycle counting works so well is because it actually counts the number of stress-strain cycles (hysteresis loops) in a time varying signal. So consider a signal stood on its end. As it is loaded from point A to B and unloaded from point B to point C, this converts into the section A-B-C in stress-strain space. On loading from point C to D, in stress-strain space, it actually remembers it was on the original path from A to D and the interruption B-C-B is counted as one cycle. E-F-E also counts as a cycle as does G-H-G. All cycles fall inside of one large, outer cycle (A-D-A) representing the maximum and minimum of the signal. Cycles with some noticeable area inside within this large, outer loop cause damage while those that are straight lines are purely elastic and cause no damage.

Now before damage can be determined and summed for each cycle certain corrections need to take place, the main correction being the conversion of purely elastic stresses and strains to elastic-plastic stresses and strains. Other corrections will be dealt with in later chapters.

Plasticity is accounted for in the Crack Initiation method by the Neuber method. The elastic stresses and strains are looked up on the elastic line and then corrected to fall onto the cyclic stress strain curve to determine the elastic-plastic stresses and strains. This elastic-plastic strain is used to look up damage on the strain-life damage curve. Neuber’s elastic-plastic correction (sometimes called a notch correction) is based on the simple principle that the product of the elastic stress and strain should be equal to the product of the elastic-plastic stress and strain from the cyclic stress-strain curve. Then through an iterative method, the elastic-plastic stress and strain can be determined. This is illustrated below.