Fasteners can fail for a variety of reasons, each requiring notably different solutions to prevent repeat failures. One should never guess why a bolt failed because, if they did, they would sometimes guess wrong. Identifying the failure mode is critical to the success of a root cause analysis (RCA) in identifying why bolts failed and what efforts should be applied to prevent future failures.
Diagnosing the failure mode will always involve examining the failed bolt itself. There is no other way to be sure why it failed. Although nothing will replace the amount of information which can be learned by laboratories staffed by experts, with training a large amount of information can be learned by those closer to the field. In particular, fatigue and ductile overload can be learned to be recognized with relative confidence. Indications of brittle fracture can also be picked out visually too. In some cases, visual examination can be enough to diagnose how and why a bolt failed.
Although this article has summarized common failure modes for bolts, these concepts apply to all types of components and equipment. To understand how to prevent future failures of anything, one must understand how and why a part failed. For those involved with investigating or preventing equipment failures, learning to recognize the most common failure modes is critical to equipment reliability efforts.