Propagation Behavior of Machining Cracks in Delayed Fracture

The propagation behavior of machining‐induced cracks in silicon nitride was investigated by conducting constant‐stress and constant‐stress‐rate tests. A dye‐impregnation technique that forced a palladium nitrate solution into the cracks gave clear evidence of subcritical crack growth of machining cracks in suspended constant‐stress test specimens. The fracture origin before subcritical crack growth usually consisted of a series of small cracks within an elongated semielliptical envelope. The dye‐impregnation results gave evidence that some of the elemental cracks had coalesced during subcritical crack growth. It was hypothesized that coalescence might explain an initially higher crack‐growth rate suggested by constant‐stress‐rate tests.