A failed crutch

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. 

27 Broken crutch with external view of rivet showing misfit. 

28 View of the top of the broken crutch showing large wear zone where the pin of the adjustment device had rubbed against the surface of the tube. The upper arrow shows the inner defect on the plastic moulding. 

A thin film was made by scraping off the outer surface of the polymer insert and dissolving it in decalin solvent to create a thin (30 Dm thick) film. The IR spectrum recorded on the thin film showed the polymer to be a polypropylene copolymer with ethylene, but it also showed an anomalous peak at 1735.7 em (Fig. 10.29). This is the charaeteristic position of the earbonyl group (C=0) produced by premature oxidation of the polymer  

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