Mirror zone

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.

Ihe mirror zone is smooth for neat epoxy, whereas it is comparatively rough in the case of epoxy systems reinforced with 10 phr treated Al Oj and untreated Al O. Ihe increase in siuface roughness is attributed to the crack deflection during fracture. Nanocomposites show better mechanical properties due to various toughening mechanisms. 

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. 

Debonding between well-bonded APTFS-Al O nanoparticles and epoxy matrix depends on the crack propagation speed. It is observed only in the mirror zone. However, in the nanocomposites having poorly bonded NT- AljOj particles, particle debonding is noticed everywhere. 

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. 

Figure 3.2 Computerized histomorphometric approach to evaluation of the bone-implant interface. The bone-to-implant contact percentage is the linear contact area between bone and implant in the inside zone percent bone ingrowth is the ratio of bone inside to that outside in the mirror zone.

Fig, II. Detail of a failure surface of Nextel 610 fibre broken in tension at room temperature. The arrow indicates the crack initiation defect surrounded by a mirror zone. 

Fig. 104. Selected-aiea electron difOaction patterns of the icosahedial Zn55Mg Yj alley prepared by melt spinning. Incident beam along (a) fivefold axis, (b) threefold axis, (c) twofold axis, and (d) mirror-zone axes.

Figure 7.3 Origin of fracture observed at the centre of the mirror zone (white bar is 1 mm long).

Fig. 1.8. Detail of the fracture surface close to the upper center of the mirror zone, the point of craze initiation (after [14]).

This is the classic form of failure in elastic materials and consists of a relatively smooth mirror zone of crack propagation leading to a rougher zone of final failure due to multiple crack-initiation (comparable in morphology to Fig. 1.7). 

Fig. 8.36. Sections of the mirror zone of a creep crack in rigid PVC (Courtesy E. Gaube, [107 J). Oy= 40 MN m , T = 20 °C, t, = 4300 h (a) Section near the external surface where the crack originated (b) Section near the internal surface where the crack terminated.

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