Why: Neither loads nor strengths are unmovable point estimates, although most designers use point values. Failures occur and reliability terminates when loads exceed strengths.
When: Loads usually increase over time (e.g., airplanes like people gain weight over time from accumulation of dirt and extra equipment), strength usually decrease over time (small fatigue cracks appear with many cycles and load bearing strengths decline).
Where: Bridges have finite lives because of load-strength interactions, wings break off of airplanes from fatigue, etc. A few failures are dramatic but most failures sneak up from the unknown in a variety of ways to cause loss of reliability. To prevent loss of the system requires many physical inspections to learn what you don't know!
These definitions are written by H. Paul Barringer and are also posted on his web site at www.barringer1.com