Fracture dimples

Stress-assisted transformation permits some p alloys to achieve a degree of low-temperatvire ductility otherwise unexpected in a bcc structure. For example, Ti 50Nb, whose deformation at low temperature is discussed in Sections 8.1.2 and 12.14.1, when tested to failure at 4.2 K, reveals the finely dimpled fracture surfaces characteristic of microscopic ductile fracture (Fig. 6.2). 

B. The direct identification of such microscopic fracture modes as transgranular cleavage, radial fracture, intergranular separation or ductile dimpled rupture, was shown to be considerably more dependable by SEM than by OM... 

Fracture modes of the forged steels were dominated by the dimpled rupture process in unexposed, hydrogen-exposed and tritium-exposed steels and welds. Heavily sensitized steels had a similar fracture appearance as tritium-exposed-and-aged steels... 

Microfractography consists of either directly using scanning electron microscopy (SEM) or examination of replicas of the sample by SEM or transmission electron microscopy (TEM). In contrast to usual light microscopes SEM has a great depth of field. The SEM view of the fracture surface of a low-carbon steel sample broken in tension is shown in Figure 2.25. It is useful to note if details such as ductile dimples, local quasicleavage and MnS inclusions are apparent. 

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 dimple is a concave depression on the fracture surface resulting from microvoid growth in coalescence. As a result of the state of stress during fracture, the dimple may be elongated, oval or equiaxed. 

Figure 7.64 shows a failed bolt where the failure surface is investigated by SEM at 1000 magnification. The observed dimples are relatively shallow with respect to the surface and oblique. The presence of inclusions as in (A), characteristic of this type of steel that initiate dimples. The shape and orientation of dimples can be used to indicate the direction and application of load as well as the degree of ductility of the samples. In this case, as the dimples are parallel to the direction of fracture, this was classified as a shear fracture. 

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.

Fig. 17a—d. Details of the fracture surface on a CT-specimen of crystalline PETP a general view of notch (N), subcritical crack growth through crazes (A), transitional region (B), and brittle fracture part (C) b dimple pattern in the central part of the broken craze (region A) c patchwork structure in the transition zone to crack instability (region B) d brittle fracture part (region C)... 

SEM observations showed that fracture surface of the HPT-processed Ti tested at room temperature has the dimpled surface typical for many nanomaterials, but after annealing the dimples looked lager and their orientation in the direction of shear fracture was observed [9],... 

In Al-Zn-Mg-Cu alloys alloyed only with Fe, fracture had a mixed character it was ductile and semibrittle, transcrystalline with the formation of pores around the spherical second phase particles. The plasticity of these alloys was EL = 13-20 % depending on the content of Fe. As was shown by X-ray microanalysis, the phase that is situated in dimples and has spherical form contains Zn, Mg, Cu, and Fe. 

Figure 5.59 shows a failed bolt, and the dimples are shallow with respect to the surface. The dimples are parallel to the direction of the fracture and thus denoted as shear fracture. 

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. 

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