Crack initiation zones

Crack initiation zone (Zone 1) is small in size in both cases compared to the propagation zone. It shows a craze-like appearance. Five sub-zones are noticed in Zone 2 ... 

In CPN, crack initiation zone is characterized by craze-like zone. Crack propagation zone is characterized by three regions ... 

The fracture surfaces of the NCRSF specimens containing 0.75 wt% and 1 wt% nanoclay were examined using SEM (see Figure 2.18) to elucidate the toughening mechanisms responsible for the fracture behavior of nanoclay-reinforced syntactic foams. The three characteristic fracture regions, namely precrack (A), process or crack initiation zone (B), and the fast fracture region (C), are clearly visible in... 

FIGURE 2.18 Overall fracture surface of a syntactic foam reinforced with (a) 0.75 wt% nanoclay, (b) Crack process zone of syntactic foam reiirforced with 0.75 wt% nanoclay, (c) Fast fracture zone of a syntactic foam reinforced with 0.75 wt% nanoclay, (d) Crack initiation zone of a syntactic foam reinforced with 1.0 wt% nanoclay (low magnification), (e) Crack initiation zone of a syntactic foam reinforced with 1.0 wt% nanoclay (high magnification), (f) Step structures generated by agglomerated nanoclay. 

Outside this, a highly porous layer parallel to the interface. The interaction of cracks initiated in the matrix with this interfacial zone was observed. 

It should be noted that in the initiation of the cryochemical reactions observed, what is important is the very fact of formation of a new crack (or zone cut by such cracks), and not the increase in the specific internal surface of the sample. Indeed, slow heating of a monolithic sample, or of a sample... 

Fig. 7 Development of fatigue cracks in an epoxy/glass contact under gross slip condition (1Hz, displacement amplitude 60 xm) (from [97]). White arrows indicate the occurrence of crack initiation and propagation at the edge of the contact under the action of tensile stresses. The lateral contact stiffness, K, is essentially a measurement of the elastic response of the epoxy substrate within the contact zone. Brittle crack propagation is associated to a drop in stiffness due to the additional accommodation of the imposed displacement provided by crack opening mechanisms...

Fig. 9 Changes in the crack initiation times and crack depths in an epoxy resin as a function of the amplitude of the imposed cyclic displacement, a Number of cycles to the initiation of the primary cracks at the edge of the contact zone, b Measured depths of the primary cracks at various number of cycles and displacement amplitudes. Circles 103 cycles, solid diamonds 5 x 103 cycles, squares 5 x 104 cycles, c Calculated values of the maximum tensile stress at the edge of the contact using Hamilton (gross slip condition) or Mindlin—Cattaneo (partial slip condition) theories. The two curves correspond to calculations using the initial (/x = 1.0) and the steady-state (/x = 1.5) values of the coefficient of friction. PSR Partial slip regime, MR mixed regime, GSR gross slip regime...

The observation that an increase in temperature or a decrease in rate both result in the same fracture response points toward a viscoelastic influence on thermoset fracture behavior, especially crack initiation. This characteristic behavior of epoxies has been explained qualitatively by consideration of the temperature and strain rate effects on the plasticity of the material at the crack tip . In effect, test conditions which promote the formation of a so-called crack tip plastic zone, or blunt the crack by a ductile process, promote unstable crack propagation. This aspect of unstable fracture is subsequently discussed in more detail. 

Since Wf = U and A = It, Equation (14) is similar to Equation (12). It has therefore been argued [23] that the impact fracture energy at crack initiation in polymers with ductile behavior G is rather the essential fracture work we. Furthermore, the second parameter representing the variation of the impact fracture energy during stable crack growth, T, has been attributed to the work dissipated in the outer plastic zone and is not related to the fracture process [23]. 

Figure 13 also shows a series of microphotographs of microtomed sections of notched PE in which much finer sphcrulites arc dispersed. The processes involved in craze nucleation and growth are fundamentally similar to those of PP, but a craze bundle in PE is composed of smaller individual crazes in length. As the load is increased, a concentrated craze line extends into the interior of the specimen a long the midplane. A crack initiates within this craze line and propagates stably with decreasing the applied load. Unlike PP. final fracture in PE is ductile. A higher magnification view of Fig. 13c is shown in Fig. 14. The damage zone near the notch root is composed...

From the force-time traces, the work for crack initiation (Wint), the elastic component (Wint-e), and the plastic component (Wint-p) of the work for crack initiation for the unnotched PP and filled PP samples can be determined. Figures 11.21a-c illustrates Wint, Wim-e, and Wint-p versus the p-phase content for the unnotched PP and filled PP samples, respectively. Apparently, unfilled PP specimens display a linear increase of Wim with the P-phase content up to 10%. This is due to the greater plasticity of p-PP as compared to the a-PP. From the SEM observation of fractured samples, the plastic zone size of unfilled PP samples increases with increase in the P-phase content. By contrast, the Wjnt values of the filled PP show little changes over a large p-phase content range. From Fig. 11.21b, it is evident that fPint-e is insensitive to the P-phase content. However, Wint-p for unfilled PP displays a steady increase with the P-phase content. The Wjnt-p of filled PP samples remains nearly unchanged with... 

Heat-Affected Zones. The COD results are listed in Table III. At -170°C, the 9 Ni/Inconel 625 HAZ showed a high resistance to crack initiation. At -196°C, an appraisal of the 9% Ni performance is complicated by the occurrence of pop-ins. If the COD value at first pop-in is used, the 0.10 mm COD criteria was not met. If the maximum COD value is used, ignoring the pop-ins, then the level obtained is above the minimum. 

Ji N mm physical crack initiation value determined from intersection of stretch zone width and /-R curve... 

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