Crack tip blunting

The above observations enable a qualitative relation, based upon the role of crack tip blunting, to be postulated between the fracture behaviour and the viscoelastic yield behaviour of the materials1 8- 44,45). The basis for this relationship is the idea that crack-tip blunting decreases the local crack tip stress concentration and thus higher applied loads are required to cause failure. 

The basis for the relationship between fracture and yielding is related to crack-tip blunting which causes a decrease of the local stress concentration. So, higher applied loads are required to produce failure. A decrease of cry will increase the values of KIc and GIc. This is the general trend that is observed therefore, it is not possible to pretend to increase both the yield stress and the fracture resistance of a neat thermoset. However, this may be accomplished by including a modifier in the formulation (Chapters 8 and 13). The resulting heterogeneous structure promotes new deformation mechanisms that increase the fracture resistance. 

A. Luque et al Molecular dynamics simulation of crack tip blunting in opposing directions along a symmetrical tilt grain boundary of copper bicrystal. Fatigue Fract. Eng. Matls. Struct. 30, 1008-1015 (2007)... 

Electrode potential and pH at an active crack tip may be significantly different from those on boldly exposed surfaces of a material. Low-pH conditions can lead to local dissolution of metal and crack-tip blunting, which reduces stress concentration effects. In contrast, low-pH conditions favor hydrogen generation and, consequently increased risk of HEC. Reduction in local ductility associated with HEC is more likely to produce sharp crack tips, which, in turn, can exacerbate stress concentration effects for any synergistic SCC or CF. (Phull)5... 

Crack tip blunting is attributed to localized yielding at the crack tip. Localized yielding may result from shear deformation, or normal stress deformation. Unlike shear deformation, which occurs at constant colume, normal stress deformation involves a volume dilatation and is considered to be responsible for the formation of crazes in thermoplastics. Since crazes are not observed in highly crosslinked epoxies, it is generally assumed that plastic deformation at the crack tip takes place via a shear yielding process. 

Initiation of fracture, however, was not observed in these experiments. Moreover, to initiate fracture in specimens with a wide craze at the crack tip K,-values were needed which were distinctly higher than Kjp This may be due to the crack tip blunting which is associated with a wider craze width. 

For materials which undergo large scale elastic-plastic deformation, macroscopic crack propagation is preceded by an amount of crack tip blunting. The macroscopic crack initiation point is thus not clearly defined. Many standards address this question ... 

Fig. 12.34. Results of the lattice Green function approach in the context of the problem of crack tip blunting (adapted from Schiptz et al. (1997)). Frame (a) reveals the blunted crack tip geometry, (b) shows cleavage, while (c) shows crack tip dislocation emission.

Cracks that enter the elongated microvoids are rounded at their tips (crack-tip blunting) and prevented from premature propagation. 

Studies of the dynamic crack propagation behaviour of commercial RTPMMA materials [52,53] showed that crack propagation occurred by a stick-slip process. The toughening was attributed to crack-tip blunting by shear yielding. Unfortunately, the use of thin bhmtly-notched specimens means that the stress state in the samples cannot be specified with certain. ... 

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