a 0	initial crack length
a c	critical crack length
B	specimen thickness
Ci	initial compliance of load–CMOD curve
$CMOD$	crack mouth opening displacement
$CMO{D}_c$	critical crack mouth opening displacement
$CTOD$	crack tip opening displacement
$CTO{D}_c$	critical crack tip opening displacement
d	mid-span deflection
$E$	modulus of elasticity
f t	splitting tensile strength
H	depth of the specimen
$H_0$	thickness of clip gauge holder
$K_{Ic}^{ini}$	initial fracture toughness
$K_{Ic}^{un}$	unstable fracture toughness
$K_{Ic}^c$	cohesive fracture toughness
$K_{Ic}^{c,E}$	cohesive fracture toughness by experiment
$K_{Ic}^{c,A}$	cohesive fracture toughness by analytical method
P u	maximum load
P ini	initial cracking load
S	span of the specimen
${\sigma }_s$	cohesive stress corresponding to the kink point of bilinear softening law
$\sigma \left(x\right)$	cohesive stress corresponding to crack length x
$\sigma \left(CTO{D}_c\right)$	critical value of cohesive force at notch tip
$w_s$	crack width corresponding to the kink point of bilinear softening law
$w_0$	crack width corresponding to the stress-free point