Modified Griffith criterion

This approach was later modified independently by Irwin (51,52) and Oro-wan and Weld (53) and Orowan (54) by including a strain energy release rate term to account for changes caused by small-seale plastic deformations at the crack tip and surfaces. Below, an expression is derived using the modified Griffith criterion for the occurrence of spontaneous fracture of a brittle material depicted in Fig. 6.1. 

Rearrangement of Eq. (6.3) finally gives an expression of the critical stress limit defined generally as the modified Griffith criterion ... 

Because most materials are not ideally brittle, the Griffith criterion has to be modified to include the energy dissipated in plastic deformation around the crack tip. The energy criterion for crack formation then becomes... 

Depending on plasticity degree a polymers behavior in impact tests is described by either the Eq. (10.21) or the Eq. (10.22). Let us note, that the Eq. (10.21) was derived from the conditions of elastic energy in sample accumulation and dissipation and therefore, fracture process itself in the obvious form (i.e., new surfaces formation) is not taken into consideration at the derivation [23]. The Eq. (10.22) was obtained from the modified energetic Griffith criterion, in which a new surfaces formation at crack If ont advancement is directly taken into consideration [22]. At such treatment it is supposed, that crack edges are absolutely flat, though experimental observations... 

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