Introduction to forensic engineering
Introduction to forensic engineering

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Introduction to forensic engineering

4.3 Material evidence

The key evidence was the broken tip (Figures 44(a) and 44(b)). It showed a brittle fracture in an unknown, relatively rigid plastic. The fracture surface was fresh, and there was no sign of old or subcritical cracks. FTIR analysis showed the material was in fact a copolymer of polypropylene, which should normally be tough and ductile. But what did the fracture reveal, and were there any obvious features that could represent defects exposed on the surface?

SAQ 12

Sketch the fracture surface from the photograph (Figure 44(b)) and isolate the main features on the fracture surface. (Tracing paper can help with this.) Include any features adjacent to the surface that could be defects involved in the fracture event. Attempt to interpret the features on the fracture surface itself, given that a fracture origin is often surrounded by a half-moon shape that shows where an initiation crack starts to accelerate during its growth. Relate your origin to the external features using Figure 44(c), and indicate whether the external features could cause a concentration of stress. Indicate the severity of the stress concentration, and therefore account for the fracture.

Answer

Figure 45 shows the fracture surface of the broken ladder tip with main features indicated.

The void at the centre of the surface is a serious stress raiser, and probably makes the Kt value about 2. Because it is symmetrically positioned in the section, stress from any direction is likely to have the same effect.

There is a line nearby on the outer surface, but it would have a small effect. More serious is the sharp external corner.

Figure 45: Sketch of the fracture surface based on the photograph in Figure 44(b)

The central void is clearly a stress concentration. However, analysis of the crack surface itself showed the origin to lie elsewhere. It was a moulding parting line at an external corner produced by the tool used to injection mould the component. It is where two mating parts of the steel tool meet, and it often shows a mismatch owing to tool wear (Figure 44b). The conjunction of this small corner and the larger external corner thus produced a larger stress concentration than elsewhere on the corner. Although only one of the two tips had broken, both showed severe abrasive damage to the parts in direct contact with the wall just before the accident, as just seen in Figure 44c.

SAQ 13

Draw an FTA diagram for the possible causes of the ladder accident – including the broken tip and any other possible causes. Sketch a simple diagram of the situation to help find the causes of the accident. Consider the condition of the ladder, the state of the ground on which it was leant, the angle of repose and degree of extension of the ladder. With your collection of data, indicate the most likely causes of the accident.

Answer

Figure 46 shows the simple situation of the ladder leaning against a wall, with points of contact with the ground and wall.

Figure 46: Side view showing points of contact

Figure 47 indicates some of the main possible causes of the accident.

The hypothesis most favoured was that the tip broke and allowed the ladder to slip off the sill, unbalancing the user, who fell off. In turn, the tip may have broken because of faulty design – and perhaps faulty moulding – or by an impact blow caused by movement of the ladder by the user. The scenario is shown by the causation path in bold. However, other possibilities include ladder instability from wet ground or from too low an angle of repose. The failure of metal parts was not supported by examination of the ladder, so was excluded.

Figure 47: Fault-tree analysis with a possible cause chain picked out

At this stage of the investigation the causes of the broken tip could include:

  • poor material;

  • faulty manufacture;

  • poor design.

There was little support for poor material from either DSC or spectroscopic analysis (Figures 48 and 49). In Figure 49 is the result obtained from the feet of the ladder, which appeared rather more flexible. They were probably made from a polypropylene with more ethylene repeat units present. This would make the material more elastomeric, with a lower level of crystallinity. No carbonyl groups were present at the characteristic wavelength (Figure 48); so the material could not have been degraded in processing, for example. Although the void represented poor design and moulding practice, it did not initiate the crack that broke the tip. The sharp corner in the moulding had initiated the crack and it might have been a design defect if it had failed and caused the accident. Sharp internal corners are common in many moulded products, but are not necessarily defects unless it can be shown they cause product failure.

Figure 48: Infrared spectrum of the material of the ladder tips
Figure 49: DSC melting curve of the tip material; the melting point shows the material to be semi-crystalline
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