A “Toolbox” for Forensic Engineers

308 Forensic Materials Engineering: Case Studies
Figure 9.17 (A) Press formed double bend on part fabricated titanium surgical
tweezer blanks. (B) “In-plane” crack in the arm thickness of the partly fabricated
tweezers.
many occasions over the previous year, and with no unforeseen problems,
blanks from the larger titanium sheet cracked in an in-plane mode during
the pressing operation (Figure 9.17B).
A through-thickness section, taken at the region of cracking on one of
the tweezer blanks, revealed the microstructure to be typical of heat-treated
Ti-6Al-4V alloy. The location and extent of cracking can be seen in
Figure 9.17A. Cracking was limited to within the confines of a double bend
and approximately midway through the material thickness. Bands of oriented
beta (b) structure were visible over the section (Figure 9.18A), and the inplane
cracking had occurred along one such band (Figure 9.18B).
At some point in its history, the initial billet, from which the Ti-6Al-4V
sheet had been rolled, was held for too long at a temperature above its
alpha/beta (a/b) transus. This resulted in formation of large b grains. Subsequent
working (forming the sheet stock) had rolled the b grains into long
pancakes. The Widmanstatten basket weave structure of the b grains became
oriented in the rolling direction, causing planes of weakness. As such, a b
structure has low ductility/formability and the double shearing mechanism,