Maybe a half a dozen times in my life I have learned something that, in a minute, totally transformed and rearranged everything I thought I knew about a subject that I thought I had previously understood.
On a trip with so many highlights, it’s hard to pick only a few. But if pressured, I would give first prize to the A.E. Seaman Mineral Museum, which is part of Michigan Tech. I literally had to pry my feet from the floor and force myself to move on to the next exhibit. After three hours, I had only seen half of the items. We went back on the same $8 ticket the next day it was open. My colleague was very patient and enjoyed the benches, while I barely noticed my aching feet.
One of my colleagues graduated from Michigan Technological University (MTU) in 1986 and had not been back since. He’s a mechanical engineer but has worked his entire career in the foundry industry. As an MTU alum, he gets invited to the school’s events, and that included a geology tour.
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.
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.