A “Toolbox” for Forensic Engineers

Failure of Storage Vessels 229
Figure 7.11 Various designs of miner’s lamp battery cases, with the oldest at
the right in aluminum (now banned because of the possibility of thermite sparks).
The other lamps are made in polycarbonate, red filled at the right, and transparent
at the center and left. The polycarbonate molding is at the extreme left.
Number of Failed Batteries
100
80
60
40
20
0
Normal Life
of a Battery
1 2 3
Lifetime/Years
Figure 7.12 Failure records of first design of polycarbonate battery case in U.K.
coal mines.
The crack here had started at the sharp corner at a small lip created by the
joint in the tool surfaces. In other examples, cracks had started from no
visible external stress raiser, but stripping the affected product showed that
cracks could be initiated all too easily from very sharp internal corners (radii
of curvature approximately 0.1 mm). What could be the cause of the several
modes of failure?
Polycarbonate is extremely strong in the form of extruded sheet, finding
use in safety shields and bullet-proof glazing. However, when it is injection
molded into complex shapes, a lower molecular mass grade must be used in
order to penetrate all the intricacies of the molding tool. It is almost universally
true that polymer strength is very sensitive to molecular mass, falling