See:
Merrill, D. R., Bikson, M., & Jefferys, J. G. (2005). Electrical stimulation of excitable tissue: design of efficacious and safe protocols. Journal of neuroscience methods, 141(2), 171-198.
Full Text PDF available from link at: https://scholar.google.com/scholar?hl=nl&as_sdt=2005&sciodt=0%2C5&cites=10867594465599094979&scipsc=&q=Electrical+stimulation+of+excitable+tissue%3A+design+of+efficacious+and+safe+protocols&btnG=
See pp.189 onwards on "Mechanisms of Damage" for a discussion of how electrical stimulation can damage nerve tissue. The mechanisms are still debated but the damage does occur. There are some equations for calculating the safe charge density (in micro Coulombs per square centimeter per phase) below which damage does not occur.
For an application of this to e-stim see Diglet48's e-stim safety page:
https://github.com/diglet48/restim/wiki/estim-safety#stimulation-induced-tissue-damage
in particular the section on stimulation induced tissue damage. His examples are for his particular set-up using his equipment which delivers a certain amount of current into certain types of electrode. Based on his experiments and calculations he states certain safe stim signal frequencies for particular electrodes that he uses.
What I would like to do is come up with a spreadsheet with cell forumlae which can calculate the minimum safe frequency for a given k value. This would of course also depend on the amp and circuit being used and probably require some measurement of current and voltage output.
Would anyone with more math and spreadsheet skills than me be able to set up such a spreadsheet, maybe as a google sheet, and share a link to it here? It should have cells where you input things like your amp power and current and desired k value and it should then calculate a minimum safe carrier wave frequency to not exceed the safe charge density. This would help us design safe stim signals and prevent tissue burns and nerve damage.