Edwin K. Beauchamp

Decrease in fracture toughness of chert by heat treatment

The fracture toughness of Ocala chert, as measured with short rod specimens and with the microhardness indenter, decreases to 60% of its original value as a result of heat treatment to 500° C while the elastic modulus increases 22%. The change in fracture toughness is associated with a transition from crack propagation around particles in the porous boundaries of densely packed zones in the chert to propagation through the zones. The transition is related to an increase in particle/particle bonding within the porous boundaries. Consolidation of a silica gel in the boundary regions, which resulted in a loss of water of 1.12% by weight, is apparently responsible for the increased bonding.

Effect of inclusions on size of surface flaws in glass-crystal composites

Indentation fracture studies were conducted on three sodium borosilicate glasses containing a dispersed phase of alumina inclusions with different degrees of thermal expansion mismatch between the glass matrices and the alumina. The alumina inclusions were found to cause a significant decrease in the size of the indentation cracks compared to those in the glass. This effect was greatest at the higher values of indentation load, which resulted in cracks of dimensions of sufficient size that their propagation was impeded by the tougher alumina dispersions. The fracture toughness for the composite samples calculated from the indentation data showed a significant increase with increasing crack size. For the smallest cracks in these composites, the value for fracture toughness was well below the value obtained in an earlier study by the single-edge notch-beam technique. The fracture toughness for the larger crack sizes which interacted with the alumina dispersions showed excellent agreement with the notch-beam data. The residual stresses due to the thermal expansion mismatch appeared to lead to a slight increase in the mean crack size regardless of the direction of thermal expansion mismatch.

Dynamic compaction of SiC powder

Dynamic compaction techniques have considerable potential in the consolidation and fabrication of hard-to-densify powder materials such as non-oxide ceramic materials. Consolidation of the powder materials by this technique involves the densification of the powder at the shock wave front followed by interparticle bonding which occurs during the shock-compression process. In the dynamic compaction of a porous material, most of the energy of shock compression has been assumed to be deposited at the surfaces of the particles (1, 2) due to the heterogeneous deformation of material during shock loading (3). This preferential energy deposition, in some cases, allows the particle surfaces to melt and results in interparticle bonding.