Spectacular structural collapses sometimes happen due to inadequate strength of the material used to make the structure. But machinery components, subject to stresses from rotational motion and/or vibrations, usually break due to fatigue.
What does this mean exactly? Metal and plastic parts do not “get tired” in the same way humans do. Being in a state of fatigue, for humans and other animals, implies that we are as good as new after resting. But metal components subject to fatigue do not get their strength restored. They are permanently impaired, unless they are somehow repaired.
Fatigue loading is repeated, cyclic loading. The loading may be uniform, random or some other pattern. A truck going down a smooth highway may experience fairly uniform loading on the axles once it is loaded with whatever it is carrying. It may carry a heavy load for one trip and a lighter load for another trip. A vehicle on a dirt road with many potholes will experience a non-uniform load, even if the exact same weight and distribution of cargo is hauled every time.
What is unique about fatigue and what confused early railroad engineers trying to understand why locomotive axles were breaking is that a fatigue crack initiates and grows at stress levels below the nominal yield strength. Why would a crack initiate and propagate at such low stresses?
Eventually, people figured out that although the nominal stress (i.e., average stress) was low, a sharp corner, a corrosion pit, a scratch or dent, damage from weld heat or many of a long list of other factors could result in a local area with the actual stress approaching the tensile-stress value. Thus, with repeated load cycles creating a stress above the yield strength – even within an area as small as one or two grains – a crack may initiate. If you are unfamiliar with the idea of grains, you may wish to look up ASTM grain-size numbers.
The bottom line about fatigue cracks is that they are progressive. They happen over time. They do not happen all at once due to a single load cycle. Fatigue cracks in most common structural steels will look very different from cracks that happen all at once. Figure 1 shows a shaft from a stamping press. The crack initiated from an area roughened due to contact with the inner bearing race, which was supporting the shaft. Fatigue cracks commonly have beach marks, or alternating stripes of lighter and darker colors. The different colors result from different loading levels, different atmospheric conditions or both. Therefore, cracks that grow in uniform loading and environmental conditions do not have beach marks.
How can we tell if it is a fatigue crack if the most well-known feature of a fatigue crack is not present? Figure 2 shows a tie rod from an injection-molding press. While there are a few faint and indistinct beach marks, we have another indicator: ratchet marks. Ratchet marks are (generally) small steps along the part surface. Ratchet marks tell us that there were multiple crack initiations on both sides of the ratchet. Higher stress levels and sharp stress concentrations often result in a series of ratchet marks.
Figure 3 shows another shaft. When viewed directly, the fracture surface is very smooth, almost featureless. The overview image at left already has the contrast enhanced. The detail view at right has the contrast enhanced more to show some slight variations in color. Very smooth fatigue cracks are generally associated with low stress.
If the stress is so low that the fatigue crack kept propagating when there was barely any material left “holding on,” why did the crack start in the first place? There might have been a shock load that was not big enough to create a complete separation. There might have been some damage to the surface, as previously noted. In fact, we see some severe damage to the surface. But the area where the crack appears to have started, left and right of the one obvious ratchet mark, looks pretty clean. A mystery!
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.
All images/graphics provided by the author