A “Toolbox” for Forensic Engineers

Failure of Medical Implements 313
Surface indications suggest that EDM machining parameters in operation at
the time of profiling were different between the two units. The broken unit
had experienced more abusive (roughening) conditions than its intact counterpart,
producing areas that were brittle and prone to cracking. As discussed
in Chapter 2, Section 2.4, once initiated, a crack would fatigue to failure from
minor defects resulting from surface grinding. However, the root cause of
failure was that the tweezers contained a crack in the first instance.
The simple remedy for this manufacturing problem lay in a return to
initial traveling-wire EDM parameters that were in operation at the time of
production of the uncracked batch of titanium tweezers. In an attempt to
explain why EDM parameters were altered, a final discussion with the fabricator
revealed that the workforce had only recently transferred to a
piece/part working practice. A probable scenario then became clear: in an
attempt to increase individual throughput, fabrication line operatives had
taken it upon themselves to increase EDM cutting speed in order to reduce
machining time.
9.9 A Failed Crutch
An elderly woman suffered trauma and injury when one of the pair of
crutches she was using to support herself suddenly broke at the junction of
the aluminum handle and the main shaft. The woman was recovering from
an operation to remove the lower part of one of her legs, and fell onto her
stump. Each crutch consisted of an aluminum tube into which was fitted an
aluminum telescopic arm, the telescopic action being utilized for height
adjustment to suit the individual user’s needs. The crutch in question had
fractured near the junction of the two tubes, inside a plastic insert
(Figure 9.21) that connected them. The fracture surface itself was not very
revealing, but did show brittle behavior. This feature was considered somewhat
unusual for the plastic, a polypropylene copolymer.
Examination of the interior of the polymer tube under magnification
revealed several subcritical cracks or crazes close to the edge of the fracture
surface. These were a strong indication that service loading had exceeded the
tensile strength of the plastic. There was no other visual evidence of excessive
loading or misuse on the exterior of the crutch tubes. However, there were
signs of defects on the plastic inside the metal tube, at a point below the
subcritical cracks. This combination of features observed on the plastic insert
was indicative of material degradation, and that the part had become defective.
The wear pattern on the aluminum tube showed that the failed crutch
had been used more heavily, consistent with the greater support needed for
the user’s amputated leg (Figure 9.22).