This image might not look like much, but to a materials science engineer, it tells a lot. This fracture surface indicates that the bolt failed in reverse bending fatigue. (This type of failure can occur when repeated bending of an object occurs cyclically, in opposite directions, causing stress.)
Multiple initiation points are observed, and the presence of oxides indicates which fracture surface is likely initiated first. Further analysis in a scanning electron microscope (SEM) can use fracture features to calculate the number of cycles and other attributes of the loading conditions.