You can probably use fourier series to represent this graph over the different intervals
ext press coeff.pdfHi,
I am trying to find the equation used to generate this graph shown in the attachment (any one of the lines).
I suspect it is some type of log function, but I have been away from higher math for many, many years.
Any help would be greatly appreciated.
ext press coeff.pdf
That looks to be some sort of Bode plot.
For instance a Bode plot of looks like this (link included).
It is typically approximated by a straight-line-plot as you have, with corner points at s=1 (down) and s=10 (back to horizontal).
None of the standard functions, except for the absolute value, has a graph consisting of straight line segments. Such graphs can be produced by piecewise-defined function. The linear function that goes through points and is given by . The top graph goes through (10, -3.2) and (500, -2.3). Therefore, the middle portion of the top graph is given by , and the full top graph is given by
Another way to define f is using min and max functions.