Hackle zone

Figure 9a shows a typical load-loadline displacement curve of a nylon compact tensiiin specimen. The slight deviation from linear elastic behaviour prior to fracture relates to the presence of a plastic deformation zone confined near the pre-crack tip (Fig. 9b). This stable crack propagation domain is adjoining a wide hackle zone (rapid crack growth domain) characteristic of a brittle fracture.

Figure 9.34 Scanning electron micrographs showing the tensile fracture surfaces of (a) neat polymer, (b) 10 phr NT-Al O, nanocomposites, and (c) 10 phr APTES-AI2O3 nanocomposites. A - mirror zone, B - mist zone, C - hackle zone. Reprinted from [97] with permission from Elsevier.

Figure 9.36 shows the hackle zone of neat epoxy and nanocomposites. Neat polymer shows large hackle markings and nanocomposites show fine hackle 3D-markings. 

In poorly bonded NT- Al O /epoxy composites, particles are clearly visible and the crack seems to have propagated aroimd their equator. Ihe fracture surface of nanocomposites consists of hemispherical holes (A), top surface of the debonded particles (B) and particles covered by epoxy matrix (C). The crack may propagate above or below the poles of the particles through the matrix. Interfacial debonding seen in the mirror zone is not seen in the hackle zone for treated alumina-epoxy nanocomposites. Another toughening mechanism noticed in the hackle zone is particle pullout, which is seen in both NT- Al O /epoxy nanocomposites and APTES-Al Oj/epoxy nanocomposites, whereas micro-cracking is noticed only in APTES-Al Oj/epoxy nanocomposites. 

The void growth is noticed in the mirror zone for both the nanocomposites, but in the hackle zone, it is noticed only in the nanocomposites having NT- Al Oj particles and hence it can be suggested as the main toughening mechanism for NT- Al O /epoxy nanocomposites. 

Fig. 7.24 Fracture surface of a fiber showing mirror and hackle zones [39]. With kind permission of Elsevier...

Figure 3.492 shows details of the damage mechanisms ahead of a mod II crack of CF/PEEK composite. Under the action of shear stress ahead of the crack tip, tensile cracks form in the matrix at 45°. In the regions within the damage zone where the stress intensity is highest, i.e. very close to the crack top, these cracks extend completely between fibre/matrix interface and open up. Finally, the material separation between the cracks leads to the full development of the hackled mode II fracture surface. The damage zone in CF/PEEK under low velocity/mode II conditions is extended over four to six fibre layers. 

Fig. 4.34 SEM images of a matched fractured PET fiber show a defect at the locus of failure (arrows). The region surrounding the locus of failure, the mirror, is the slow fracture zone. As the fracture accelerates across the fiber, ridges, or hackles, are formed in the outer fracture surface.

SE images of a matched pair of tensile failed PET fibers are shown in Fig. 4.30. A classical slow fracture zone, or mirror, is seen adjacent to the locus of failure. A typical ridged or hackle morphology is exhibited as the crack propagates and accelerates away from the failure locus. In this study, an inorganic residue from the polymer process was shown to be the cause of failure [295]. The value of such a fractography is that... 

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