Crack process zones

There is usually a crack bridging zone just behind the crack tip, with a small (sub-millimetre) microcrack region at the crack tip, and the cracking process zone extends over several mm. This is the cracking mechanism in some largegrained ceramics, and in whisker reinforced ceramics. See crack bridging toughness. 

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. 

Figure 4 Schematic of the crack process zone for refractory materials.

The elastic stress cannot exceed the yield stress of the material, implying a region of local yielding at the crack tip. Nevertheless, to apply the simple framework of hnear elastic fracture mechanics, Irwin [J. Applied Mechanics, 24, 361 (1957)] proposed that this process zone size / be treated as an effective increase in crack length be. Fracture toughness is then given by... 

Rather than bearing an infinite stress at the crack tip, yielding occurs resulting in a volume of inelastically deformed material along the crack front called the process zone, as shown in Fig. 2. The size of the inelastic zone, r j , under a monotonic tensile stress, o , can be approximated by substituting o = Oj into eq. 2 for the horizontal plane, 0 = 0... 

The variability in fracture stress when small artificial flaws were controlling strength was particularly pronounced for H-451 graphite, as can be seen in Fig. 12. Here the crack dimensions and crack trip process zone dimensions are comparable to the microstructural dimensions. Consequently, local variations in microstmeture... 

The second physical quantity of interest is, r t = 90 pm, the critical crack tip stress field dimension. Irwin s analysis of the crack tip process zone dimension for an elastic-perfectly plastic material began with the perfectly elastic crack tip stress field solution of Eq. 1 and allowed for stress redistribution to account for the fact that the near crack tip field would be limited to Oj . The net result of this analysis is that the crack tip inelastic zone was nearly twice that predicted by Eq. 3, such that... 

The above results are derived from linear elastic fracture mechanics and are strictly valid for ideally brittle materials with the limit of the process zone size going to zero. In order to apply this simple framework of results, Irwin (1957) proposed that the process zone, r be treated as an effective increase in crack length, Sc. With this modification, the fracture toughness becomes... 

As discussed above, because materials often fail by the propagation of cracks, particularly brittle ones, it is difficult to measure the true yield stress, oy, of a material. Instead one measures a fracture stress, Op which is related to the size of the process zone and actual crack length by ... 

SBA-HT [Societe Beige de l Azote-Haldor Topsoe] A process for converting LPG to syngas rich in hydrogen. Two cracking processes are conducted in two zones of one... 

---