A “Toolbox” for Forensic Engineers

44 Forensic Materials Engineering: Case Studies
Figure 2.9 Single bending fatigue fracture in a pneumatic tool. (A) Fracture
surface on the shank. (B) Blunted cutting edge that set up abnormally high cyclic
bending stresses in the shank.
approximately 40% of their tensile strength but for most materials there is
no safe limit below which it ceases to propagate. Eventually the effective
cross-sectional area becomes so reduced that the component ruptures under
one final cycle of a load near the top of its normal service spectrum, but
one at a level that had been satisfactorily withstood on many previous
occasions before the crack propagated. The final fracture occurs suddenly
in a ductile or a brittle mode, depending on the characteristics of the
material.
Fatigue fractures exhibit a characteristic appearance, which reflects the
initiation site and the progressive development of the crack front, culminating
in an area of final overload fracture. Figure 2.9A shows a typical single
initiation fatigue fracture, in this instance a percussion tool used for road
breaking. This particular tool was a tarmac cutter having a wide, chiselshaped
blade that had been allowed to become blunt (Figure 2.9B). As quite
usual for such a tool, the shank was subjected to a levering action that applied
bending cycles just above where the blade flared out, predominantly in the