In situ investigation of subcritical crack growth in lead zirconate titanate ceramics using atomic force microscopy

Crack-velocity-curves ((upsilon)-K-curves) as well as R-curves of unpoled and poled lead zirconate titanate are investigated by directly observing the crack extension in compact tension specimens. Depending on the velocity range, either optical microscopy or atomic force microscopy is used for this purpose. With this method a broad velocity range of eight orders of magnitude down to 10^-13 m s^-1 is covered. A threshold KI,th is proposed for both states of the material. It is found that poling of the material elevates the threshold value by 35%. This corresponds to the increase of the intrinsic fracture toughness determined from the R-curves. At higher velocities the influence of the R-curve effect on subcritical crack growth is discussed by means of time dependent ferroelastic switching processes.