Finite Element Simulation And Experimental Study Of The Toughening Effect And LTD Of Zirconia Ceramics

Zirconia ceramics have received wide applications in various engineering fields such as thermal barrier coatings,biomedical implants,etc.On the one hand,the superior fracture toughness of Zirconia ceramics can originate from the stress-induced tetragonal to monoclinic phase transformation.The tensile stress field around a growing crack front promotes the t-m phase transformation.A permanent phase transformation zone is thus left behind in the wake of the crack.The transformation strains change the stress state around the crack tip and bring about toughening effect.In early studies,the small-scale transformation assumption was usually adopted,where the finite element model was usually 2D.On the other hand,when exposed long in vaper environment,transformation of zirconia ceramics will take place,which will rather decrease its fracture toughness,and this phenomenon is called as low-temperature degradation.The transformation toughening behavior of a edge through crack and a interface crack in zirconia ceramics is studied with the finite element method.The continuum based constitutive law is implemented into a commercial finite element code.The FE model is fully 3D and the small transformation assumption is given up.The crack propagation behavior is simulated at the structural level.The propagation behavior of a finite edge through crack and a interface crack in a zirconia plate is then studied respectively with the cohesive zone model and compared with the case where the material is purely elastic.The influences of the stress state,the cohesive strength,the shear transformation strain,the activation energy threshold and the elastic modulus of the substrate on the size of the transformation zone and the crack propagation behavior are systematically discussed.The influence of the low temperature degradation on the zirconia plate is also discussed.The numerical results indicate that the transformation zone is larger when the crack front is in the plane stress state.Higher cohesive strength is beneficial to the toughening effect.The shear transformation strain increases the toughening effect of the phase transformation.The increase of the activation energy threshold will retard the phase transformation and as a result,the toughening effect is decreased.For the interface crack,the increase of the elastic modulus of the substrate will increase the size of the phase transformation,at the same time,the crack growth become easier.Through LTD test we find that the component of monoclinic phase will increase and the fracture toughness will decrease when the zirconia plate exposed longer to LTD environment.