Back to the curvature issue...
This was a part with a stress concentration, with low stress so that the crack curvature had time to reverse, but it looks pretty straight to me. I think this goes to show that it’s pretty hard to prove what the theoretical features are that tell what the stress was, unless you already know what the stress was!
I don’t have much information or memory of what the object shown in Figure 6 is, but the crack-initiation location in the center of the smoothest part of the fracture edge is clear. It looks like the reduced section could have been a D-ring groove. There is a shear lip going from 9-12-3. This indicates final separation. It’s hard to see a specific thumbnail, and it is clear there is a stress concentration (the groove) but no noticeable reversal of curvature of the crack front.
Figure 7 is interesting. It appears to have crack initiations all around the circumference of the thread root. There is clearly a stress concentration. It’s possible that the arc marked as the smaller crack grew first because it is smoother. After some crack growth, it’s possible that the bolt rotated. It was being used in a vertical position without clamping preload as a hanger in a paint line, if my memory serves me. Then the crack grew faster, either because they switched to hanging heavier parts or simply because of the reduced load-bearing area due to the first crack. On the other hand, the small crack could have happened second, with much lighter hanging parts. The final separation never appears to have reversed itself on either side of the crack.
Figure 8 is a photo that one of my students took of parts I inherited from another analyst who had a “museum of failure.” At last, we see a bolt, probably loaded in uniaxial tension, perhaps with a bit of bending, with a significant change of diameter AND an immediately adjacent threaded section. Here at last is the long-sought-after obvious case where the curvature of the crack front reversed itself. A brand-new fracture analyst might even confuse the small, almost-circular area for the initiation. But the black arrows show multiple ratchet marks, which are very reliable indicators of multiple fatigue initiations, across the diameter from this circular feature.
The bottom line on whether the fracture is giving you independent corroboration of whether and exactly how well the part was aligned is much more difficult than many people like to think.
More next time.