River patterns

Fracture surface replicas viewed by TEM are shown in Figure 2.26. The figure shows river patterns of brittle fracture failure of a steel fastner and fine equiaxial dimples on the surface of a tensile fracture of high-strength steel. 

A crack in a ceramic material upon initiation may accelerate and interact with microstructure, stress field and the generated acoustic vibrations. These interactions may lead to some fractographic features such as fracture mirror, hackle or river patterns and Wallner lines. 

Figures 7.64 and 7.65 provide a view of the fracture surface of a bolt recovered following a fall of ground from a hardrock mine. This is at magnification x 10. A close-up of dimpled areas is presented in Figure 7.66, at a magnification x 1500. The dimples are shallow and inclined characteristic of ductile fracture. No sign of cleavage or quasicleavage, is observed (absence of river pattern that would be associated with brittle fracture). In the upper right part of the photo dimpled areas are masked by oxidation.

As an aside to the polymethacrylates discussed In this section, mention must be Included of earlier and much more striking results obtained with a polycarbonate. The fracture surfaces reported were more akin to ones characteristic of a quite brittle polymer. In contrast to results for other linear polymers. A grain structure was observed which appears closely similar to that discussed In the present work. Its formation was attributed to a mechanism of crack Intersection In river pattern areas. 

Fig. 4 Example of the results of crevice corrosion. Type 304 SS exposed to 6 wt.% ferric chloride for 48 h at room temperature with castellated crevice washer applied around center hole. Pitting also occurred at several sites outside the crevice. The draining of the occluded solution from the crevice sites (and pits) led to the river pattern of...

The final area of the fracture showed the presence of a shiny granular appearance, indicating a sudden brittle (cleavage) fracture, which was also confirmed by the SEM photographs showing cleavage steps and river pattern. 

The close-up of dimpled areas is shown in Figure 5.61. The dimples are shallow and inclined characteristic of ductile fracture. No sign of cleavage or quasi-cleavage is observed. The absence of river pattern excludes the possibility of brittle fracture. 

Fig. 6. Fracture surface of the ZrC/Ti3AIC2 composite. The interface of ZrC to TI3AIC2 matrix is identified with backscattering image inserted. The river patterns, ridges and prongs of ZrC particle ate indicated with arrows. The letters T and Z represent T3AIC2 and ZrC, respectively.

Fig. 8.4 The river-pattern emitting from a pore on the fracture surface [56]. With kind permission of editorial ofBce of lOP Science...

A river pattern consists of steps between parallel cleavages on parallel planes, that usually converge in the direction of the local crack propagation. This is mainly observed in metals, but also in ceramics (see Fig. 8.4) ... 

Wong et al. [67] measured the enantiomeric ratios for eight polychlorinated biphenyl enantiomers in aquatic sediments from selected sites throughout the United States. Nonracemic enantiomeric ratios (ERs) for PCBs 91, 95, 132, 136, 149, 174 and 176 have been found in sediment cores from Lake Hartwell. Nonracemic ERs for many of the enantiomers have also been found in riverbed sediment samples from the Hudson and Housatonic Rivers. Patterns in enantiomeric ratios among congeners were consistent with known reductive dechlorination patterns in both river sediment basins. The enantioselectivity of PCB 91 was found to be reversed between the Hudson and Housatonic River sites. Moisey et al. [24] determined the concentrations of a-, and y-hexachlorocyclohexane isomers... 

Figure 4. Discharge of suspended sediment from world rivers in 10 tons ear. Sediment yield in tons/km ear for major drainage basins is shown by pattern (see key).

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