Questions and discussion for this lecture live here. Fire away by hitting Reply below 
Hi Sean, I am enjoying this Lesson even though I have a background in plates and shells, I am finding newness in your lesson, thanks. Comment, at the end of Lesson 12, you have sigma stresses but not Tau stress, (other than the zero) and you start Lesson 13 as if we knew Tau (in the integrand).. I thought I missed Tau, but went back and couldn’t find it… just a pedagogical note that it would have been good to button up all the stresses in Lesson 12.