Effect of Grain Size on Crack Growth in Alumina
Résumé
The Fracture behaviour of alumina ceramics with grain sizes ranging from 2 to 13 µm is studied by means of Double Torsion and Single Edge Notched Beam methods. The R-Curve behaviour is found to increase with grain size as a consequence of increasing crack bridging effects. R-Curves were obtained by the SENB method with a conventional compliance analysis and with in-situ measurement of the crack size. The compliance analysis leads to an underestimation of the real crack size, thus of the real crack resistance,.especially on coarse grain microstructure. Only the coarse alumina exhibits a significant amount of rising crack resistance. Slow Crack Growth laws were obtained for the different ceramics by the relaxation test with the Double Torsion technique. The results obtained in different environments show that Slow Crack Growth is due to stress corrosion by water molecules at the crack tip. The increase of crack resistance with grain size is indicated by a shift of the V-KI (crack velocity versus applied stress intensity factor) law towards high KI values. Moreover, the slope of the curve is higher for higher grain sizes. However, if the R-Curve effect is subtracted in the analysis, a unique V-KItip (crack velocity versus stress intensity factor at the crack tip) law is obtained, independently of the grain size.