A “Toolbox” for Forensic Engineers

156 Forensic Materials Engineering: Case Studies
Figure 5.19 Broken hook with (A) fractured ends replaced and (B) fracture face
exhibiting bright “fish eyes” characteristic of hydrogen cracking.
Both parts had been bright electroplated with zinc. Nylon webbing would
be stitched over the straight limbs and the straps wrapped over the load on
the vehicle, or sometimes attached to a large piece of machinery on a flat
bed trailer and then tensioned with a specially designed grip.
By far the most likely cause of premature service failures of spring steel
electroplated with zinc is hydrogen-induced cracking. Accordingly, a hook
and a keeper from the unused samples were mounted in a vice and the jaws
tightened so as to cause noticeable elastic deflection. The pressure was eased
to check that no permanent plastic deformation had taken place. The jaws
were then retightened to the same elastic deflection and left in that state at
ambient temperature. Only one limb of the hook was tensioned in this way,
the opposite limb being left unstressed. The triangular keeper was stressed
by compressing one point against the opposite base, which imposed tensile
bending stress on the inside face of the base. The stressed limb of the hook
broke completely after between 48 and 60 h. Figure 5.19A shows the broken
limb placed with the fracture faces almost in contact; while in Figure 5.19B
the hook end is turned back so as to reveal the characteristic fish-eye feature
on both sides of the fracture surfaces. This fracture is quite characteristic of
hydrogen-induced fracture.