An Experimental Characterization of Crack-Closure Stresses in Monolithic Ceramics

Dr. Jack C. Hay
Metals and Ceramics Division 
Oak Ridge National Laboratory 

Sponsored by the Dept. of Engineering Mechanics

Date:  Friday, February 14, 1997
Time:  3:30 p.m.
Place:  306 Bancroft Hall
 

A novel post-fracture tensile (PFT) technique offers an opportunity to isolate and characterize the crack-closure stresses of the fracture process zone in ceramics as a function of crack-face separation.  The technique has been used to elucidate important information regarding the crystallographic and environmental influences affecting the mechanism under monotonic loading conditions.  That work also identified a relationship between the pullout process associated with R-curve behavior and microstructural features on the order of the grain size. 

The latest work, however, uses the PFT technique to reveal the more subtle roles of the subgrain-size features responsible for the degradation of the R-curve by load-cycling in the small displacement regime.  Using novel equipment, a closed-loop controller forces the specimen to follow a prescribed load path, wherea piezoelectric actuator supplies the necessary displacements.  The actual specimen displacements at the crack faces are measured independently using a laser interferometric displacement gage.  Based upon the current results, features on the order of 0.1 micrometers, or 0.5% of the mean grain size, appear to strongly influence fatigue characteristics of the wake zone.  Applied displacements beyond the 0.1 micrometer threshold for this alumina result in nonrecoverable displacements.  More importantly, though, repeated loading beyond the critical displacement results in a rapid degradation of the bridging ligaments, as evidenced by decay of the specimen stiffness.