Let’s wrap up our discussion on fatigue cracks.
Figure 4 shows the solid end of a die-casting machine “gooseneck.” This was basically a deep cup-shaped part. The entire cylindrical wall of the cup broke off. This was sent to me for a quote, but when I saw it, I did not recognize the crescent marks as the beach marks that I now believe they are.
To be fair to myself, I was younger, less knowledgeable and very suspicious about the fact that they had boiled the broken gooseneck in sodium hydroxide to remove the metal that had solidified onto the fragments. I think after sitting in my lab for a couple of decades, as part of the “museum of failure,” the marks have become somewhat more noticeable. The very smooth fracture is also consistent with fatigue.
Fatigue cracks are sometimes easy to recognize and sometimes much more challenging. Beach marks, ratchet marks and smooth surfaces are all keys that may point to the crack having propagated over time in fatigue due to cyclic stresses rather than due to a single, large load cycle. Beach marks are not found on cracks resulting from uniformly loading in a uniform temperature and humidity environment.