Introduction to forensic engineering
Introduction to forensic engineering

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Introduction to forensic engineering

5.8 Afterword

The results of the investigation were clear-cut. The problem was caused by a combination of defects in the material of the wall of the reservoir. They, in turn, had been mainly caused by faulty moulding. One scenario by which they were produced has already been suggested: the failed reservoir was made during startup of the injection moulding machine, when moulding conditions had not been established. The failed reservoir was therefore a single maverick. Several conclusions could be drawn.

5.8.1 Quality improvements

In the first place, the radiator manufacturer could be reassured that the basic design was not at fault, and they could not expect to see widespread failures on new cars. Secondly, they could ask the supplier to re-examine the quality policy. It is normal and good practice for moulders to supply each operator on the machines with a diagram of likely defects to look for in mouldings as they are made. This is the first line of defence in quality control, and perhaps the most important, because every component is – or should be – examined individually.

The faulty reservoir must have been made at this stage, and had been mistakenly accepted as a good reservoir, and forwarded through to the manufacturer. The moulder should be asked to check that such quality checks were indeed in place, so that the buyer might be assured of products meeting the specification. The moulder generally has responsibility to supply quality products, but it is also true that development designs made in short runs are not always examined as closely as is necessary.

So, several useful and practical suggestions to improve the situation were thrown up by the investigation. Inspection could be improved at the audit stage in quality assurance, by lightly dusting with whiting to show up any surface defects, such as cold slugs and weld lines, which would show whether there was a problem in moulding. The design could be improved by increasing the radius of the buttress corner. Sharp radii are easy to ameliorate by simply polishing the corners of the core that produce the corner in the tool. The effects are often dramatic, especially when the material of construction is brittle, as in this case.

SAQ 19

By interpolating on the appropriate stress concentration diagram, suggest a specification to provide the toolmaker when correcting the buttress corner radius of the radiator reservoir. By examining the relevant photographs in Paper 3, suggest what other areas of the moulding should be inspected by the toolmaker for possible improvements. Could any further improvements in the material be recommended, either as a development project or for routine design development?

Answer

One appropriate stress concentration diagram to use is Figure 40, Box 14. The wall thickness of the reservoir is 2.5 mm, and the radius of curvature at the buttress root is about 0.1 mm (Paper 3, page 193). The object of the exercise is to reduce the stress concentration at the butress root.

To reduce the stress concentration to unity is impossible within the limits of the diagram. Moreover, the diagram suggests the goal would be unrealistic because the curve falls only very slowly to unity as r/d increases. If the far right-hand edge with a value of r/d=0.4 is used, the value of r is

r = 0.4 × 3.5 = 1.4 mm

which gives a stress concentration of about 1.6. Such a value of Kt is substantially below the values of 4 or 5 assumed above, and could be acceptable to the design team – provided that good mouldings are available. The change to the tool would simply involve polishing the sharp edge to the core that creates the feature on the final product.

There are several other similarly sharp corners on the mouldings shown in Figures 1–3, Paper 3. They include the buttress at the centre of the reservoir (Figure 2, Paper 3), the projecting bosses shown at either end of the reservoir in Figure 1, a corner bracket (also in Figure 1 left) and the inlet and outlet pipes (Figure 1).

The material choice could also be reviewed with the material supplier to find a grade of improved strength without sacrificing stiffness or any other desirable properties. There is wide experience of the material type in under-the-bonnet applications such as inlet manifolds, so it is unlikely a shift to an entirely new material would be feasible. This in itself would require new trials, and it is by no means certain the original tool would be capable of moulding in a new material with different flow properties.

5.8.2 General conclusions

The case study has been considered here in some depth because it provides some useful lessons in forensic methods. Composite materials of the kind used in the reservoir are of relatively recent widespread application, so ways of studying them are still being developed. This is why the author of the paper included many methods that were not especially critical in the final analysis of this particular problem. However, the point is that they could find application in other failure cases of this kind of material. Other investigators can follow up further details of the methods using the references supplied at the end of the paper.

One surprising outcome of the investigation was the utility of the gold coating method for increasing the resolution of optical microscopy, rather than the original intention of aiding SEM. Nowadays, gold coating would not be used because non-conducting samples can be examined directly with ESEM (environmental SEM). This is a new development that will be examined later in this block, but samples do not need coating, so the method is completely non-destructive in nature.

Other developments with SEM, especially in image recording and sample stages, mean that very large samples can be examined directly in the microscope. Large samples need not be cut down simply to fit into the microscope, a common problem with older SEM machines. It was also surprising how the simple and relatively cheap method of dusting with whiting, revealed surface detail not normally visible.

Another interesting result came from tensile testing: the moulded material was substantially weaker than the supplier's specification. It is true with many other polymers, perhaps because suppliers frequently either choose best samples, or choose the best results – subconsciously or not – to promote their product. Designers should be aware of the problems of taking and using supplied data without allowing for discrepancies.

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