Crack tip opening displacement was measured by DIC.
In the case of E resin, the fact that the crack tip opening displacement is almost constant over the range of temperature investigated is in agreement with the assumptions of Williams' viscoelastic fracture theory, while this does not occur in the case of EI resin.
In this study, the critical crack tip opening displacement
(CTOD) of X80 pipeline steel was studied both experimentally and numerically.
Figure 8 presents the variations of the critical crack mouth opening displacement ([CMOD.sub.c]) and the critical crack tip opening displacement
([CTOD.sub.c]) of the three-point bending beam specimens of the concrete containing nano-Si[O.sub.2] with different values of relative notch depth, respectively.
where h is the height of the FPZ and is assumed to be constant during the test, [bar.[sigma]] and [bar.[epsilon]] are the true stress and true strain in the FPZ, [bar.[epsilon].sub.n], and [[epsilon].sub.n] are the true strain and engineering strain, respectively, at the onset of necking, [sigma] is the engineering stress during the necking process, which is a function of the crack tip opening displacement
([[DELTA].sub.1]), and [[DELTA].sub.F] is the displacement [[DELTA].sub.1] at fracture.
, [w.sub.e] is proportional to the crack tip opening displacement
(CTOD) and to the necking strain and stress, the effect of cross-linking on [w.sub.e] could be interpreted in terms of CTOD decrease, since maximum stress ([[sigma].sub.net]) is constant.
The yielding related specific essential work of fracture ([w.sub.e,y]) changed parallel with the yield strength ([[sigma].sub.y]), whereas the critical crack tip opening displacement ([[epsilon].sub.y] 0) remained practically constant in the v range studied.
In order to check whether or not this cha nge is relied on a parallel increase in [[sigma].sub.y] (see Table 1), first the critical crack tip opening displacement ([[epsilon].sub.y],0) should be assessed.
Grellmann and Seidler (3) also reported useful information assessable from the crack tip opening displacement
(CTOD) concept and the J-R curve in the study of fiber reinforced injection molded polypropylene (PP), polyethylene (PE) and polyamide (PA).
They show that the value [[Delta].sub.c] obtained by extrapolation of [d.sub.f] to zero ligament length can be identified with the critical crack tip opening displacement (CTOD).
Indeed, Eq 6, which is very similar to Shih's relationship between CTOD and critical J integral (34), indicates that the fracture toughness of a material depends not only on its ability to deform extensively before fracturing (ability represented by its critical crack tip opening displacement) but also on its capacity to bear some load.
DIC is a technique that can be performed in house giving more information about the strains present and displacements leading to information about crack tip opening displacements
. The crack tip opening displacements
can then be used to understand more about crack tip blunting should sufficient resolution be achieved through DIC.