Hackle region

The room lemperature fracture surface of a Nicalon fiber is shown in Fig. 16. Note the initiation of fracture at the fiber. surface. The fracture surface shows a crack-initiating site and a planar minor region, a mi.sty region, and finally a hackle region in which crack branching occurs. The initiating flaw may be an impurity, a surface nick due to handling. 

Experimentally, it has been observed that, for a crack that grows in a uniform stress field, the boundaries of the mist and hackle regions are semicircular. Furthermore, if the origin is a well-defined semicircular flaw, the radii of those boundaries bear fixed relationships to the radius of the flaw. Equivalently, the apphed stress at failure and the boundary radius are related by equations with the form... 

Figure 12.29 Photomicrograph of the fracture surface of a 6-mm-diameter fused silica rod that was fractured in four-point bending. Features typical of this kind of fracture are noted—the origin as well as the mirror, mist, and hackle regions. 60X.

XPS study by Buckley and Woods (1984b) showed that freshly fractured chalcopyrite surfaces exposed to air formed a ferric oxyhydroxide overlayer with an iron-deficient region composed of CuSi. Acid-treated surfaces of fractured chalcopyrite showed an increase in the thickness of the CuS2 layer and the presence of elemental sulfur. Hackl et al. (1995) suggested that dissolution of chalcopyrite is passivated by a thin (< 1 pm) copper-rich surface layer that forms as a result of solid-state changes. The passivating surface layer consists of copper polysulhde, CuS , where n > 2. Hackl et al. (1995) described the dissolution kinetics as a mixed diffusion and chemical reaction whose rate is controlled by the rate at which the copper polysulhde is leached. The oxidation of chalcopyrite in the presence of ferric ions under acidic conditions can be expressed as... 

Figure 8.75 Fracture surfaces in brittle materials generally show a smooth region that surrounds the failure origin (mirror region) but the surface increases in roughness as the crack accelerates (mist region) until crack branching occurs. The branched region contains ridges known as hackle. (Optical micrograph courtesy of Matt Chou.)...

Figure 8. Case B4. SEM close-ups of the margin region showing hackle lines that indicate the dcp. Fracture started at the tip of the margin (large arrow on the right image). The margin is smooth and rounded and has no obvious faults, with the exception of a possible tiny crack.

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.

Fig. 5.59 SEM study of brittle failure in an impact modified nylon molded article reveals classical fracture morphology. The locus of failure (arrow) is seen (A) with surrounding mirror (M), mist and ridged hackle (H) regions propagating out into the bar. A higher magnification view of the flaw and mirror is shown (B). The flaw (C) appears to be a round fiber, likely a contaminant.

Fig. 5.64 SEM of Izod impact fractured, glass fiber filled thermoplastic test specimens show nonuniform distribution of fibers in the two different specimens (A, C, D and B, D, F). The fibers (A) appear aligned parallel to the skin and the matrix exhibits brittle failure as hackle marks (arrows) are seen. The fibers (B) protruding appear long and poorly wetted with the resin. Hackle or ridged patterns (arrows) are observed (C). Resin is also seen on the fiber surfaces in some regions (D and F) whereas cleaner fiber surfaces and less well bonded regions are also observed (E).

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