$\beta$	the mixed factor of the crack form
$\Delta u$	the displacement increment in the period of $[t_n,t_{n+1}]$
$\Delta u^{def}$	pure deformation in the global coordinate system
$\Delta u^{rot}$	pure rotation in the global coordinate system
$\Delta$	the relative displacement of points at the crack interface in this region
${\delta }^F$	the ultimate displacement
${\mathrm{\Omega }}_x^{\prime }$	virtual configuration at time x
${\mathrm{\Omega }}_x$	configuration at time x
${\sigma }_x$	stress at time x
${\epsilon }_x$	strain at time x
A and B	material parameters of PVB