For those of you who have been following along with this blog recently, or the magazine, you have seen my previous writings about how to recognize fatigue cracks when they do not have obvious beach marks. This time we will start with a couple of images where the classical fatigue features known as beach marks are very obvious (Figure 1).
At left there is a large screw with more or less parallel or slightly curved bands of alternating dark and light coloration. These bands fill a section about a quarter of the total cross section at the bottom of the image and a little over half at the top. The crack itself follows the screw thread root, which is as expected, although we don’t really have any way to know whether there was anything special about this particular thread root that allowed the crack to initiate at this location. Going back to the beach marks as well as the crack, we see that they are more or less perpendicular to the axis of the screw’s cylindrical surface.
The beach-marked areas are separated by a rougher-looking area, which forms a step. The ends of this step are deformed (left is marked “DT” for ductile tear). There are some scrape marks (dark upside-down broad “V” and “hockey stick” shape across the top), and someone drilled a hole near the center. This is one of the most classical-looking fatigue crack images I have.
The only visible deformation is at the end of the band that represents the final separation. This is one indication that the stress was below the yield strength as the crack was propagating. Remember that fatigue, by definition, is a crack propagating at a local stress that is above the tensile strength (otherwise it would not crack), while the overall (“nominal”) stress is below the value where permanent shape change can happen (yield strength).
At right we see a much smaller fracture surface. The wide dimension of the rectangular cross section is approximately 1 cm. This was an augur used to convey granular material inside of a steel tube. The outside surface of the augur, shown at the upper right (short side), would be expected to be more likely to have the crack initiation if the “cause” were wear against the inside of the tube. That is not the case. The curvature of the beach marks, shown by the black arrows, indicates that the initiation is at upper left. It is possible that there is a wear streak at that inner (slightly rounded) surface.
Again, the crack is very flat and perpendicular to the inner coil surface. This crack orientation hint is often a strong indication of where the crack started. The much wavier crack surface at upper right is a result of the two masses of steel on either side of the crack starting to “realize” that they are no longer unitary. The stress state becomes more complex as the two sides of the crack flop and twist.
Figure 2 again shows a crack surface that has readily noticeable beach marks, although these bands of lighter and darker color are, relatively speaking, much wider than usual. There are only a total of seven gradations of gray from along the entire crack surface. The dramatically different width of the dark gray along the horizontal diameter when comparing the left to right sides is another feature that seems unusual to me. The crack is, generally speaking, very smooth. It has two large ratchet marks that appear to blend into one (long, horizontal purple arrows) as the leftmost main crack extends under the rightmost crack.
Is this a rotating shaft? The crack-growth direction, perpendicular to that of the beach marks, does not have the typical variation that we see in rotating shafts. It could be a fine threaded screw. I did not take good enough photos of the part surface to see what is going on along the part surface and what the flat rim features are.
The two cracks (left and right as shown) do not actually merge into a single crack front until it has propagated from what is shown as the lower edge up to the row of black vertical arrows. The row of green arrows below it is still technically indicating two separate cracks. The matching colors of the beach marks indicate that the same loads and environments were driving both cracks.
By the time we get to the red-arrow beach mark, the crack front (beach mark) is starting to reverse its curvature. This might make the beginning student of fractography think at first glance that the crack started at 12:00 rather than at the 5:00 and 7:00 positions, as shown. In this case, however, the stress is so low that the fatigue crack has essentially propagated all the way across the cross section of the shaft. As the crack nears the moment of final separation, the local stress intensity goes up at the pointy ends of the crack. The crack starts to propagate faster at the ends and creates what looks like a thumbnail, but it is the final separation (orange arrows).