Introduction to forensic engineering

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2.4 Product testing

Owing to the high cost of testing every product, it is rarely used for mass-manufactured products. Samples are usually chosen to be representative of the batch (as used, for example in statistical process control, referred to below). Tests are normally carried out under standard conditions, defined either by a national or international standard, or by an internal company standard. While they may provide a degree of quality control, the aim of standard tests is to simulate conditions of use of the product. If the test fails to detect product defects, faulty products can enter the market. Failures of the test method itself can occur in several ways.

• Failure to reproduce conditions of use.

• The test technique is not monitored correctly.

• The test may not adequately detect product defects.

There could be the complete absence of testing under realistic conditions, perhaps because wrong assumptions are made, or because of unfamiliarity with a new material or a process. Box 4 relates how failure to test adequately led to a widespread product liability problem. Other examples will be considered in later case studies, both in this and later blocks.

SAQ 5

A central heating system operates for long periods at temperatures around 70° C. Figure 15 shows the variation in the half-life of two thermoplastic elastomers with temperature and time of exposure, due to degradation by hydrolysis under test conditions. Calculate the time for the initial strength to drop by half for each elastomer. Which grade is more resistant to the effects of high temperature if used for a radiator seal? What will be the effect of:

• turning the heating thermostat down by 10° C?

• turning the system off every other day?

Assume the effects of hydrolysis on the material are cumulative.

Figure 15: Effect on the life of thermoplastic elastomers with exposure to water at different temperatures. (The half-life is the time taken for tensile strength to drop by 50 per cent.)

Figure 15 shows the inhibited 72D-with-additive grade is more resistant to the effects of degradation than the alternative. By reading the graphs, the half lives of the elastomer seals at 70° C are:

72 D with additive, approximately 1.5 years;

72 D, approximately 40 days.

The effects of lowering the water temperature by 10° C to 60° C will be to increase the half lives of the grades to:

72 D with additive, approximately 2.5 years;

72 D, approximately 80 days.

Switching the system off every other day would double the lifetime of each grade, so:

72 D with additive, approximately 5 years;

72 D, approximately 160 days.

2.4.1 Failure modes and effects analysis

An important tool for considering potential and actual failure modes in products is failure modes and effects analysis (FMEA). It is not in itself a product test method but a way of assessing product defects. The results could change product testing or even bring about new test methods.

Pareto analysis, fault-tree analysis (FTA), and statistical process control (SPC) are all techniques used by design teams both for new and existing products. Factories gather data on their product lines, and quality departments use that data. Many inspectors are not engineers, however, and may not recognise a particular problem as rating highly in importance compared with others. For instance, surface blemishes are easy to spot on the line and are clearly defects of a kind, but not usually serious enough to affect product safety. Such patent (obvious) defects are in contrast with latent (hidden) defects, which lie within products or components. The study of patent and latent defects is the responsibility of the design team, which will use FMEA to identify, to classify and to act. They may be defects in products returned from customers, or found within the factory by routine testing and inspection.

A common design defect in many products is sharp corners, which when stressed, raise the local stress above the failure stress of the material concerned. Fracture will start at such corners, and propagate into the interior of the sample. If the product shape is made by a mass-manufacturing route such as injection moulding or casting of a metal alloy, then it will occur in every product so its likelihood of occurrence is 10, on a scale of 1 to 10. If the severity of the consequences of fracture is high – say 8 out of 10 – and it is easy to detect – say 8 out of 10 – the product of all three factors is 640. This number is known as the risk priority number (RPN), so

where L is the likelihood of occurrence, S is the severity, and D is the detectability.

The design team assigned to an FMEA analysis will use experience to set a threshold value above which action must be taken. Suppose in this case, the RPN was set at 250. The team must now discuss ways of eliminating the defect, which here is easily done by modifying the tooling to increase the radius of curvature at the corner in question.

FMEA is not the only answer to improving quality. It is a method that focuses the minds of designers onto product quality in a systematic and rational way. Potential problems can thus be tackled at an early stage in design development, rather than in a panic when failures come back from customers.

A case study of the method is described in Paper 1. You should read the paper before tackling SAQ 6. In addition, the important international standard IEC 812 is presented in Paper 2. It describes the method as it would be applied to electrical equipment, but it can be adapted for other engineering products. Papers 1 and 2 are attached as a PDF documents which should be printed out (if possible) to gain the maximum benefit from SAQ 6.

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SAQ 6

Paper 1 above describes the application of FMEA to the problem of stapler and tacker design in the context of a total quality management programme. Summarise briefly the way an FMEA study is carried out for a specific product. What defects were found in the product mentioned in the paper? What factors were important in the way the FMEA committee was structured and operated?

Imagine that in a subsequent committee meeting, a study of a new design of office tacker agreed the following variables for sharp corners near the most highly strained part of the plastic cover.

• Likelihood of occurrence: moderate failure rate.

• Severity: failure by brittle cracking causes the tacker to behave intermittently.

• Detection: fault may be noticed by the customer.

Using the tables provided in Paper 1, what action should be taken by the FMEA team?

Paper 1 describes the first step to be taken by an FMEA committee as being to define the device or process under study. The committee is composed of individuals from different departments within the factory. The committee decides on the three factors of a particular product or process failure mode: the likelihood of occurrence of a particular failure mode; the severity of the failure; and the ease of detection of the particular defect involved in that failure mode. The three numbers are multiplied together to give a risk priority number (RPN). If the RPN falls above a threshold value, action is needed.

The specific example given was a new design of cover cap for an office stapler. It had been copied from an older design, but the old design gave problems on the production line, making hand assembly difficult owing to a tight interference fit between the plastic cover and the steel cover plate. Assembly workers had to wear plasters to protect their hands as a direct result. With a risk priority number of 160, the value was greater than the critical value of 120, so action was taken to eliminate the tight fit.

The committee operates at the design evaluation stage, so that paper changes can be made before production tooling commences. An important step is choosing the committee members. The membership should be four-to-six representatives plus the facilitator, with both engineers and non-engineers as members. The facilitator runs the team, organises meetings, and ensures all the vital data is assembled. The facilitator also manages the computer database; the computer itself provides a focus of attention to prevent cross-talk – talk at meetings that can deflect members from the important subject.

In the case of the sharp corner on a moulded tacker cover, the values for the factors are in the tables at the end of the paper.

• Likelihood of occurrence: 5.

• Severity: 9.

• Detection: 9.

Hence, the RPN would be 405, so action must be taken by the team to ameliorate the problem. In this case, increasing the corner radius by modifying the metal tool would be appropriate.