Subcritical Crack Growth in Y‐TZP and Al ₂ O ₃ ‐Toughened Y‐TZP

Crack velocity curves for Y‐TZP and Al 2 O 3 ‐toughened Y‐TZP were determined for long cracks in compact tension specimens with an in situ fracture device on the stage of an optical microscope. Indications for a crack velocity threshold were found for both materials. Above this threshold, at 2.6 MPa·m 1/2 for Y‐TZP and 3.6 MPa·m 1/2 for Al 2 O 3 ‐toughened Y‐TZP, chemically assisted subcritical crack growth occurs over an extended regime of applied stress intensity factors of width 2.1–2.8 MPa·m 1/2 . It is recognized that the dependence of the shielding term on the crack‐tip stress field renders transformation‐toughened materials particularly susceptible to stress‐corrosion cracking. This interrelation leads to the definition of a steady‐state velocity at constant applied stress intensity factor. This velocity is obtained in the situation where the shielding term is fully defined by the present crack‐tip stress field, not depending on prior loading history.