Chronology of Crack Initiation

For many years I ran a Fracture Mechanics testing and analysis laboratory. Circulating among the cognoscente was a tongue-in-cheek chronology of events associated with crack initiation*. This sequence of observations contains an unintended grain of truth that illustrates the random behavior of even well-behaved physical phenomena.

Stages in Fatigue Crack Detection

  1. Possibly see where a crack might possibly start.
  2. Possibly see where a crack will definitely start.
  3. Definitely see where a crack might possibly start.
  4. Definitely see where a crack will definitely start.
  5. Possibly see a possible crack.
  6. Possibly see a definite crack.
  7. Definitely see a possible crack.
  8. Definitely see a definite crack.
  9. Possibly see the crack growing.
  10. Definitely see the crack growing.
  11. Specimen fails along a crack other than the one being observed.

* The mathematics of Fracture Mechanics (F/M) requires the existence of a macrocrack (a small crack that is still large with respect to the surrounding microstructure), so it is convenient to divide the essentially continuous process of fatigue crack formation into “propagation” (described by F/M) from what precedes it, “initiation.” Unfortunately this has led to considerable semantic hair-splitting and the insistence by some that “initiation” doesn’t exist since there is no measurable signal of it. We will avoid such nonproductive debates here.