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R-curve behaviour of Al2O3, Al2O3+Molybdenum composite and Si3N4 were studied through fracture
mechanics experiments and in situ piezo-spectroscopic measurements of bridging stresses. The bridging
stress distribution, the maximum and the average value of these stresses were recorded and used to
predict the R-curve of the materials. The microstructure plays an important role on the bridging stresses:
for Al2O they are very low being the frictional stresses the only source of toughening in this material.
Bridging stresses are higher in Al2O3+Mo due to plastically deformed Mo particles acting as bridging
ligaments in the crack wake and they are even higher in Si3N4 were elongated grains act as elastic
bridging. The Raman piezo-spectroscopy is therefore a very useful tool for assessing in situ the bridging
stresses and for determining the toughening mechanisms in ceramics