Example of a Vickers indentation. Only one of four corners is shown (length of diagonal 115 μ m). With the crack tip as a starting point (x = 0) the crack width 2*u(x) is measured at the distance x (COD after Irwin ; ceramic material for this example: Empress 1). The residual tensions cause crack growth over a long time interval until, at the end of the crack, K
is equal to K
. Crack tip shielding by secondary effects (micro structural elements which toughen material as the crack extends) may slightly distort results (measured K
then lower than true K
). Insert: idealized COD.