icular ly ▁a ▁she ar ▁stress , ▁since ▁it ▁is ▁easier ▁to ▁analyze ▁she ar ▁de formation ) ▁in ▁static ▁equilibrium . ▁In ▁this ▁sense , ▁a ▁solid ▁under going ▁pl astic ▁de formation ▁is ▁a ▁fluid , ▁although ▁no ▁vis cos ity ▁coefficient ▁is ▁associated ▁with ▁this ▁flow . ▁Gran ular ▁r he ology ▁refers ▁to ▁the ▁continu um ▁mechanical ▁description ▁of ▁gran ular ▁materials . ▁ ▁One ▁of ▁the ▁major ▁tasks ▁of ▁r he ology ▁is ▁to ▁empir ically ▁establish ▁the ▁relationships ▁between ▁stra ins ▁( or ▁rates ▁of ▁stra in ) ▁and ▁str esses , ▁by ▁ade qu ate ▁measurements , ▁although ▁a ▁number ▁of ▁theoretical ▁develop ments ▁( such ▁as ▁ass uring ▁frame ▁invari ants ) ▁are ▁also ▁required ▁before ▁using ▁the ▁empir ical ▁data . ▁These ▁experimental ▁techniques ▁are ▁known ▁as ▁r he ometry ▁and ▁are ▁concerned ▁with ▁the ▁determ ination ▁with ▁well - defined ▁r he ological ▁material ▁functions . ▁Such ▁relationships ▁are ▁then ▁am enable ▁to ▁mathematical ▁treatment ▁by ▁the ▁established ▁methods ▁of ▁continu um ▁mechan ics . ▁ ▁The ▁character ization ▁of ▁flow ▁or ▁de formation ▁origin ating ▁from ▁a ▁simple ▁she ar ▁stress ▁field ▁is ▁called ▁she ar ▁r he ometry ▁( or ▁she ar ▁r he ology ). ▁The ▁study ▁of ▁ext ensional ▁flows ▁is ▁called ▁ext ensional ▁r he ology . ▁She ar ▁flows ▁are ▁much ▁easier ▁to ▁study ▁and ▁thus ▁much ▁more ▁experimental ▁data ▁are ▁available ▁for ▁she ar ▁flows ▁than ▁for ▁ext ensional ▁flows . ▁ ▁Vis co el astic