Relationships between multiaxial stress states and internal fracture patterns in sphere-impacted silicon carbide

Internal fracture patterns developed in silicon carbide cylindrical targets as a result of dynamic indentation (63–500 m/s) by tungsten carbide spheres are defined. Microscopy of recovered and sectioned targets delineate into three regions, each associated with distinct cracking modes, i.e., shallow cone macrocracking at and near the impact surface, steep interior cone macrocracks that radiate into the target from the impact region and local grain-scale microcracking directly underneath the impact region. The observed fracture patterns are found to maintain a noticeable degree of self-similarity upto the impact velocity of 500 m/s. Linear elastic analysis of the full (surface and interior) stress field developed under static (Hertz) contact loading delineate the target into four regions, based on the number of principal stresses that are tensile (none, 1, 2 or all 3). A strong correlation is found between the principal stress conditions within each region and the forms of cracking, their locations and orientations present therein. This correlation has a number of implications, including non-interaction of crack systems, which are discussed. Illustrative linear elastic fracture mechanics analyses are performed for three regions, and calculated and observed macrocrack lengths are found to be in reasonable agreement.