The information that is presented here is generic, in the sense that fractographic features are greatly affected by microstructural variation, composition and environment but,
usually, the following comments apply and the features illustrated will be apparent.
a)
Macro-appearance
The overall macro-appearance of a fast fracture may be bright and facetted (cleavage and intergranular fracture), dull and fibrous (microvoid coalescence - MVC), or smooth and fairly shiny (shear). Fast fracture is generally rougher than fatigue fracture surfaces and is more likely to show greater evidence of ridges. These ridges represent the boundary where different planes on the fracture surface have met up [1-2]. The ridges on the fracture surface can be used to locate the crack initiation site, as indicated below. In relatively thin plate, the tear ridges appear to form chevrons (or a herringbone pattern), and the apices of the chevrons point back towards the origin (
Figure 1). The chevron pattern arises because fracture initiates at the centre of the plate thickness where the constraint on plastic deformation is the highest. The fracture then progresses on slightly different planes in a fan shape to the edges of the plate, and this process repeats itself as the crack re-initiates ahead of the current crack tip position. The chevron markings in
Figure 1 are a little obscured by zinc on the fracture surface. In thicker sections the ridges occur by the same mechanism, but may be much more pronounced (
Figure 2) and, again, can be used to locate the fracture origin.
Figure 3 shows the fracture surface of a large casting, which comprises a mixture of fatigue and fast fracture. The ridges point back to the origin in the square box marked 1.
Figure 1 Chevron markings on 12 mm thick plate
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Figure 2 Fracture ridges on a round shaft
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Figure 3 Fracture ridges on a large casting
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Ridges sometimes occur on fracture surfaces through intergranular separation of coarse columnar structures in cast components. An example of such a case, arising from aluminium nitride precipitation at grain boundaries is shown in
Figure 4. The fracture surface represents intergranular fracture in the outer regions and interdendritic fracture in the core of the casting. For the sake of completeness, the microscopic appearance of aluminium nitride precipitation is given in
Figure 5. This occurred on the fracture surface of Charpy specimens machined from the steel. The aluminium and nitrogen content of this steel were 0.063% and 0.015% respectively - these values are sufficient to give
AlN precipitation if cooling rates through the temperature range 1150
o - 800
oC are too slow.
Figure 4 Intergranular ridges on a casting
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Figure 5 Needle-like AlN precipitation
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Tensile testing of round bar or wires often
produces a so-called cup-and-cone fracture, an example of which is
shown in Figure 6. The shape reflects shear at about 45º
around the periphery of the bar or wire which occurs under biaxial
constraint, once the inner region (which experiences triaxial
conditions) has failed by planar fast fracture (usually microvoid
coalescence - see Part 4 of the Fractography Resource).
Tensile testing of round bar or wires often produces a so-called
cup-and-cone fracture, an example of which is shown in Figure 6. The
shape reflects shear at about 45º around the periphery of the bar or wire which
occurs under biaxial constraint, once the inner region (which experiences
triaxial conditions) has failed by planar fast fracture (usually microvoid
coalescence - see Part 4 of the Fractography Resource).
Figure 6 Intergranular ridges on a casting
Go to Part 4 of the Fractography
Resource
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Failure Analaysis
