Picking up where we left off in part 1, this latest crack was also a fatigue crack.
In this case, I could see that the crack started at a discontinuity. I thought I clearly explained that there was a discontinuity most likely present at the time the item was manufactured. It probably affected the details of the crack timing and location within the component. But without knowing where the high-stress location on the part was, I couldn’t confirm that the discontinuity was a defect. More work would be required to confirm that. One thing that I would need to know for sure is exactly where the highest-stress location is on that part when in use.
But it turned out that my client had never thought about the fact that stressed parts are almost always stressed unevenly and almost always have a particular location where they will crack IF THEY crack. I needed to step back and explain the concept of expected crack location. And this crack was NEAR but not AT the location that “probably was” the highest-stress location. Hence, I called it a discontinuity, not a defect.
In the popular (among failure analysts) book by Don Wulpi Understanding How Components Fail, we are shown a broken shoelace. For those of us not too young to have lots of experience with shoelaces, especially mid-20th-century shoelaces, we know that one of the upper eyelets (closest to our ankle) is usually where the lace breaks.
That’s because the combination of stretching and bending creates the highest stress at this location. Furthermore, the old shoelaces used to wear out, and the most wear was also at one of the upper eyelets because that’s where the opening went through the greatest change, resulting in the greatest sliding distance. In addition, it USUALLY breaks when we are trying to tie the shoelace because that’s when we’re stretching it most strenuously. Of course, if we are in a rush, we’re more likely to yank harder. We have thereby just increased the chances of inconvenient fracture just when we were already running late. Now we definitely missed the darn bus!
New shoelaces don’t wear out as often, but that advance came at the expense of the shoelaces staying tied. The new laces are more slippery, and I’m not sure which is a greater annoyance: Having to go around with an ugly knot in my lace until I have time to change it or having to constantly retie my shoes.
