Help!!! How to calculate the consistency parameter by consistency condition (potential function) of plasitc flow

hi, there!

I have read a "Manual for LS-DYNA Wood Material Model 143"(website: http://www.tfhrc.gov/safety/pubs/04097/sec150.htm#toc_1_5_1 and http://www.tfhrc.gov/safety/pubs/04097/append_e.htm).

I have two question:

1. how to calculate the consistency parameter (formula (180)) by potential function, I'm quite not understant how the formula (19), (20), (25), and (26) came out. Could anyone explain it to me or tell me what material I should get and read?

2. the effective strain-rate increment is given by formula (37), I wonder if the strain-rate increment is equal to strain increment  divided by time increment?

thank you!