Residual stresses, strength and toughness of laminates with different layer thickness ratios

The effect of residual stresses on the strength, toughness and work of fracture of Al 2O 3–5 wt.% tZrO 2/Al 2O 3–30 wt.% mZrO 2 layered ceramics with different thickness ratios has been investigated. The laminates, as well as a monolithic Al 2O 3–5 wt.% tZrO 2 used as reference material, were fabricated by sequential slip casting. Residual stresses were estimated experimentally using indentation techniques and analytically using a three-dimensional finite element model. Flexural strength was evaluated by means of four-point bending tests on specimens with natural and artificial (indentation) flaws. Experimental findings show the existence of a threshold strength in the laminates whose value depends on the layer thickness ratio. Crack growth resistance behaviour was studied by crack opening displacement-controlled tests and by recourse to a weight function analytical approach. The high compressive stresses in the internal layers yield a pronounced R-curve behaviour in the laminates. Regarding work of fracture, it is found to be enhanced to levels up to about six times that of the reference monolith. The results are discussed in terms of the optimum layered architectural design for structural applications on the basis of the compromise between threshold strength and energy absorption capability associated with crack bifurcation mechanisms occurring at fracture.