Join Date: Aug 2018
Location: Fort Worth, Texas, USA
Look at the failure face. If it is uniformly rough and looks like sand, then it was probably an instantaneous failure. If you see a portion that looks burnished / smooth, it is probable that it started failing there. A crack starts, then the flexure "hammers" the cracked surface so that it gets smooth. This continues as the crack slowly opens, burnishing that surface until ultimate failure occurs. You'll see a smooth / hammered surface next to the rough instantaneous failure surface.
If you learn that it cracked and then over time propagated, then look at the casting at the smooth area to see if there are any voids or other defects. I'd suspect that an instantaneous failure with no crack propagation is a design defect- it is just too thin in a critical area.
Build Date: July 18, 1966