Fractures in the SEM

In last week’s blog post, Mattea talked about what you should do, and more importantly, what you shouldn’t do with fractured parts if you’re sending them to us for a failure analysis (i.e., don’t mash them together to see how well they fit!) This week, let’s take a look at some of the features that can be resolved if the fractures are properly preserved.

But before we do that, let’s first consider why it’s important to preserve the fracture surfaces. The main reason is because in order to figure out why something fractured, we need to first understand how it fractured….and by that, we mean the actual fracture mechanism.

So how do we do that? Well, the first step (after an initial visual examination) is to take a look at the fracture at low-power using a stereo microscope to get an overall idea of possible fracture mechanisms and identify areas worthy of further study. Once that’s done, the next step is to take a look at the fracture at considerably higher-magnification in the scanning electron microscope (SEM). Why? Because that’s where the features of interest really become apparent. Turns out, that at magnifications in the range of a few hundred times to around 10,000X or so, different fracture mechanisms have distinctly different morphologies…..thus, a ductile fracture has a morphology that’s distinctly different than a brittle fracture, which is distinctly different than a fatigue fracture, which is distinctly different than an environmentally-induced fracture, and so on.

Here are a few examples…..

 

A “cup and cone” pattern is caused by a mechanism known as microvoid coalescence and is characteristic of a ductile fracture mechanism.

 

A “river pattern” of radiating lines is caused by a mechanism known as cleavage and is characteristic of a brittle fracture mechanism.
 

A cleavage pattern of radiating lines is again visible here, just as in the previous photograph, but here, the surface is covered with a thin layer of high-temperature oxides, which shows that the fracture occurred when the part was at a very high temperature. In this case, the part had been heat treated during manufacturing, and afterwards, the service environment was outdoors–where it could not have been exposed to high temperatures; thus, the fracture must have occurred while it was being heat treated, which means this was an original manufacturing crack, rather than a service-related crack.

 

At low magnification, the crescent-shaped marks shown here indicate the fracture mechanism was fatigue, a progressive failure mechanism that occurs over time as the result of cyclic loading cycles. The crescent-shaped marks are known as “beach marks” and represent successive stages of progressive crack growth which occur due to changes in loading or other service conditions.

 

The fine, parallel features are fatigue striations and represent individual crack-growth steps. Fatigue striations are often difficult to find on industrial fractures because corrosion and/or post fracture deformation can obliterate them. Note for example, the areas of smeared metal on the left and right sides of the photograph, where the fatigue striations are not visible.

 

Identifying fatigue striations can sometimes be quite tricky, as certain microstructural features, such as layers of pearlite, can appear as multiple, fine parallel bands similar to fatigue striations.

 

This fracture exhibited a combination of transgranular and intergranular features, indicating the part had been exposed to an environment that contributed to the failure.

 

The dendritic, “tree-like” structures on this fracture are shrinkage voids, which occurred when the casting was solidifying. Thus, in this case, the crack turned out to not be a service-related crack at all, but rather, an original casting defect.

 

The rounded, dendritic structures on this crack show that it occurred when the weld was solidifying, rather than during service.

 

The rounded, dendritic structures on this fracture are the result of incipient melting during faulty heat treatment, whereby the lower-melting components melted, resulting in a brittle, low-strength structure…..which subsequently fractured during normal service.

So, as you’ve just seen, there can be a wealth of information on a fracture surface….so long as the fractures are properly preserved after the failure!

Bob Hodel