▁can ▁show ▁both ▁instant aneous ▁de formation ▁as ▁el astic ▁material ▁and ▁a ▁delayed ▁time ▁dependent ▁de formation ▁as ▁in ▁flu ids . ▁ ▁Pl astic ity ▁is ▁the ▁behavior ▁observed ▁after ▁the ▁material ▁is ▁subject ed ▁to ▁a ▁yield ▁stress : A ▁material ▁that ▁behav es ▁as ▁a ▁solid ▁under ▁low ▁applied ▁str esses ▁may ▁start ▁to ▁flow ▁above ▁a ▁certain ▁level ▁of ▁stress , ▁called ▁the ▁yield ▁stress ▁of ▁the ▁material . ▁The ▁term ▁pl astic ▁solid ▁is ▁often ▁used ▁when ▁this ▁pl astic ity ▁threshold ▁is ▁rather ▁high , ▁while ▁yield ▁stress ▁fluid ▁is ▁used ▁when ▁the ▁threshold ▁stress ▁is ▁rather ▁low . ▁However , ▁there ▁is ▁no ▁fundamental ▁difference ▁between ▁the ▁two ▁concepts . ▁ ▁Dim ension less ▁numbers ▁ ▁Deb or ah ▁number ▁▁ ▁On ▁one ▁end ▁of ▁the ▁spectrum ▁we ▁have ▁an ▁in vis cid ▁or ▁a ▁simple ▁Newton ian ▁fluid ▁and ▁on ▁the ▁other ▁end , ▁a ▁rig id ▁solid ; ▁thus ▁the ▁behaviour ▁of ▁all ▁materials ▁fall ▁somewhere ▁in ▁between ▁these ▁two ▁ends . ▁The ▁difference ▁in ▁material ▁behaviour ▁is ▁character ized ▁by ▁the ▁level ▁and ▁nature ▁of ▁el astic ity ▁present ▁in ▁the ▁material ▁when ▁it ▁de forms , ▁which ▁takes ▁the ▁material ▁behaviour ▁to ▁the ▁non - New ton ian ▁regime . ▁The ▁non - dimensional ▁Deb or ah ▁number ▁is ▁designed ▁to ▁account ▁for ▁the ▁degree ▁of ▁non - New ton ian ▁behaviour ▁in ▁a ▁flow . ▁ ▁The ▁Deb or ah ▁number ▁is ▁defined ▁as ▁the ▁ratio ▁of ▁the ▁characteristic ▁time