Fracture of a Bone Drill
A stainless tool steel bone drill broke during an operation on a patient and was examined. It showed two fatigue fractures, one of which had started from a sharp-edged, coarsely milled slot (fracture A1), and the other from a point on the outer sheath surface which was not subjected to particularly high stresses (fracture A2). Fatigue fracture A1 resulted from the stress concentration built up at this point as a result of the sharp edges and the coarse machining grooves. The remains of a number, which had been inscribed with an electrical engraving tool for identification purposes, were found at the point of origin of fracture A2. The material had been heated to the melting point during the engraving of the number, and multiple cracking occurred during cooling. One of these cracks led to the development of fatigue fracture A2.