Dear P08IP708,
What you are saying seems logical. But, it should be tested for a wider range of values. Those curves are only valid for nu* = 0.4.
PV Elite/CodeCalc uses the actual formula instead of the charts, so they can go for the extended range and are not limited to the charts.
But, it would be interesting exercise to find out at which value of x gives the biggest Fm.
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Best Regards,
Mandeep Singh
CADWorx & Analysis Solutions
Hexagon PPM