E2472 (no change)
crack extension, ?a [L], n—an increase in crack size.
Discussion—It should be noted that in thin-sheet and thick-plate materials under low constraint conditions, the crack extension observed on the surface of the specimen may be significantly less than that in the interior of the specimen due to the effects of crack tunneling. This must be considered if direct optical techniques are used to monitor and measure free-surface crack extension. Indirect crack extension measurement techniques such as unloading compliance and electric potential drop method may be used in place of (or to complement)
the direct optical techniques to provide a measure of average crack extension. (See Test Method E647 for compliance methods for C(T) and M(T) specimens; and ISO 12135 and Test Method E647 for electric potential-drop methods for C(T) specimens.)