Mode-II shielding-curve of Al2O3 from measurement of cone crack angles

Abstract

Many ceramic materials exhibit the effect of an increasing crack growth resistance during crack extension. Especially in the case of coarse-grained materials, cracks generally follow the grain boundaries, leave the original crack plane and produce a crack-face roughness. In the theoretical analysis forces transmitted by local links between the two crack faces are smoothed and replaced by so-called continuous bridging stresses acting against the crack opening. As the consequence of such bridging stresses, there exists a shielding stress intensity factor term that shields the crack tip partially from the applied loads. This effect as a reason for the occurrence of R-curves is well documented in literature for cracks under pure mode-I loading conditions [1]. In addition it has to be expected that crack-face interactions will also affect crack extension under pure or superimposed mode-II loading as for instance outlined in literature for frictional crack-face interactions [2–4]. In [5] it was outlined that the shear tractions generated under small mode-II load contributions may cause a disappearing effective crack-tip stress intensity factor KII,tip. Also the externally applied mode-II stress intensity factor KII,appl can be reduced by the mode-II shielding so that a disappearing KII,tip can occur. The distribution of the intensity of crack-face interlocking has the same consequences on the bridging tractions normal to the crack face rbr as for the tangentially transferred tractions sbr. At the same distance from the crack tip where the bridging interactions disappear, both the shear and the normal stresses must vanish. Under the assumption of the same traction versus displacement characteristics, the two stresses are proportional sðxÞ 1⁄4 k rbrðxÞ ð1Þ