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. The long, horizontal white arrow shows hand-drawn brackets (I’ve had this photo for many years!), indicating an area of increased roughness compared to the lower portion of the crack. In this area, labeled “Beach Marks,” there are some very faint, very closely spaced fine dark lines. These are crack-arrest marks, which we call beach marks when visible to the eye.
Only when the crack has gotten over three-quarters of the way through are we able to visually see any beach marks. Why? This bolt was on a vibration machine in an air-conditioned test lab. The vibrations and the humidity and temperature were all constant. Only at the tail end of the life of this bolt, when the remaining load-bearing area was so small, are we able to see the results of the local stress effectively increasing to a point to change the crack appearance.
What’s with the “ears” that are bent upward at 90 degrees to the initiation and final separation? My speculation is that there were some minor vibrations causing bending back and forth perpendicular to the main loading, and when the completed crack caused the nut to pull away, it pulled on the flaps caught in the next thread back.
Figure 4 shows a painted-steel specialty leaf spring. Note that there are no beach marks (unless you want to count the very faint band shown by the tiny red arrows in the right-side, marked up version of the photo). This was another example of a test of uniform loading in an air-conditioned lab. The very smooth crack surface at left is another classical fatigue feature but confined to very low-stress loading conditions. The white arrows show a faint ridge or fan pattern whose “V” base points back toward the initiation. The small black arrows show a frame around the right half of the crack. This is a shear lip, and it is indicative of sudden separation. Thus, unlike all the other cracks we have looked at here, once a mere half of the cross section was gone, the part was doomed to immediate separation at the time of the next load cycle.
Finally, Figure 5 shows us another example of a ridge or fan pattern (white rectangle, tiny black arrows) whose base is the initiation. The entire part surface is rough, however, not smoothed out like that on the left side of Figure 4, the bottom of Figure 3 and almost all of Figure 2, as well as the entirety of the augur at the right of Figure 1 and the striped areas of the big screw on the left side of Figure 1 in the first part of this blog.
This part also has a protruding shear lip from 12:00 to 3:00 on the right-side fragment (with the protruding, rather than recessed, ridge pattern). The shear lip is recessed from 6:00 to 9:00. Note the wear or rubbing damage in the red rectangle. This was a U-shaped component of a hoist chain assembly. The loading pinched the (not visible) open ends toward each other, which created a tensile stress on the bottom of the U. Cracks must have tensile stresses present to grow. This image is presented for compare-and-contrast purposes, and Figure 5 is not a fatigue crack. However, it shares some features with the sudden overload (right end) side of the leaf spring of Figure 4.