Thread: Getting from equations 1 to equation 2

Hello all,

I am curious about how the author of an engineering paper got from equation 1 to equation 2 (see file attached) just by taking 'to' ('t' subscript 'o') to be the instant at which the plastic strain delta epsilon (subscript 'ij' and superscript 'c') occurs.

Is it purely mathematics to get from equation 1 to equation 2? If not I am missing some information from the paper to understand how to get from equation 1 to equation 2.

P.S. sorry I am new to the forum and havent worked out the maths scripting yet

Thanks