Let’s wrap up our discussion on fatigue cracks.

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.

Eventually, people figured out that although the nominal stress (i.e., average stress) was low, a sharp corner, a corrosion pit, a scratch or a 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. 

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.

If you ever have the chance to walk in the woods after a major storm, you may amuse yourself trying to figure out why a particular tree was blown down while a nearby tree remains upright. You may see trees revealed to be rotten at the base, which tipped over, but took apparently healthy trees with them.

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.

Figure 3 shows another crack that has only ratchet marks to tell us it has been propagating in fatigue. See the white arrows at lower left. There are a few more ratchets to the right of the ones that are marked.

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.

Detailed inspection of the upper portion of the crack (Figure 2, rotated to an orientation about halfway between the left and right views of Figure 1) revealed a ridge pattern with finer steps merging to coarser ones from bottom to top (equivalent to left to right on Figure 1).

Tooling failure analysis can be very challenging. Frequently, we have to deal with complex loading and multiple simultaneous damage processes.