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.

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.

The purpose of science is to reveal how the physical world works. The human world, including psychology and other social sciences, is part of the physical world. The human experience of consciousness, including our ability to experience aspects of the physical world through our sensory apparatus, is part of science.

This is the second part of Debbie Aliya’s blog “Why did the part fail?”

Speaking with greater clarity, why did the subject of the investigation deform or break? Why did some of its material disappear? Why have the strength characteristics become degraded?