Manufacturing defects in polymeric medical devices - Durability and Reliability of Medical Polymers - page 252

10.26 Brittle fracture surface of failed crutch showing rivet and flutes
on inner surface.

Moreover, there were signs of defects on the plastic inside the metal tube,
at a point below the subcritical cracks. They comprised a series of flutes
(Fig. 10.26) and a curious wavy line on the inner surface (Fig. 10.27). This
combination of features observed on the plastic insert indicated material
degradation, because this part should have had a smooth blemish-free surface.
Being an injection moulding, the surface had been in contact with the steel
core. If the polymer had flowed, then the viscosity must have dropped to
allow the material to flow when the product was removed from the tool,
showing that the molecular weight was too low.

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10.6.2 Wear pattern
The pair of crutches showed superficial scratches together with several
zones of heavier wear. Those zones were identified as representing contact
between the metal tubes at the head of the crutch (Figs 10.25 and 10.28).
The wear pattern on the aluminum tube showed that the failed crutch had
been used much more heavily, judging by the area affected, and it was
consistent with the greater support needed for the user's amputated leg. The
loads on the failed crutch would have been greater than those on the intact
crutch, simply because the user had placed more of her weight there. This
meant that the loads on the critical plastic insert were considerably greater
than its equivalent on the intact crutch. But what mechanism had caused the
polymer to become brittle and why had the viscosity of the polymer fallen
so drastically?