regarding stress calculation in taylor cylinder impact test

hi all,

          there is a formula for calculating stress in taylor cylinder impact test. but that formula only depends on final length of cylinder. once cylinder hits the rigid target cylinder will have elastic and plastic deformation. we can extract plastic deformation data through experiment. my question is is can we apply that formula for every plastic deformation reading rather than just applying that for final cylinder length.  formula is given here. here l is initial height. L1 is aftet imapact total final length, H is height of cylinder it means after impact cylinder shape is preserved for some portion so H is dat height. after impact cylinder takes mushrom shape and after mushroom shape the cylindrical shape starts. rho is material density and v is impact velocity. 

 formula is  sigma= rho*v^2*(l-H)/(2*(l-L1)*ln(l/H)