Introduction to forensic engineering
2 Production design and manufacture
Manufacturing is about making products to a specification, which is often a standard, such as one published by the British Standards Institution. It is useful to consider the steps that result in a finished product (Figure 5).
The core phases are shown in Figure 5. The early stage of design will establish what environment a product will be used in, and the length of time a customer will use it – its expected lifespan. Product conception is followed by planning and modelling, leading to a product specification. Provided the model achieves the desired technical performance, detailed consideration of materials follows. Testing and evaluation lead to a prototype. Through all the design stages, the failure of models and prototypes is an essential part of the development process, leading to improved prototypes. The process is an iterative one, where failure at any stage causes reconsideration of the specification or materials. Such failures are of course a normal part of new product development.
When a successful prototype emerges, production can be planned. Production machinery is costly, so this stage demands careful planning and testing.
It is after the product is sold into the open market, that product failure looms large in the design equation (Figure 6). Product quality will be a touchstone for public perception of the product itself, so any failures need immediate attention by the design team. Failure investigation using forensic methods is the key to identifying any design flaws or production defects, and correcting them quickly.
One tool for finding and eliminating product defects of any kind – due to design, or materials, or manufacture – is known as Pareto analysis (Box 2). It groups product defects in terms of their importance, so the investigators can gain an overview. The team can then concentrate on the most serious defects, using fault-tree analysis (Box 3).
Another technique of great value in tackling product defects is known as failure modes and effects analysis (FMEA). It is in widespread use in many industries, being required by many standards and codes of practice. It is based on an assessment of the real or suspected defect using three criteria: occurrence, severity, and detection. As it is a key tool for designers working both on new products and products of long standing, it will be described in more detail below.
Box 2 Pareto distributions
Failure analysis assumes the design team will have already identified the ways a product can fail. If not, it will be first task of the team to specify the way it could fail, probably by starting with failures during testing of prototypes or models. A tool that can help concentrate minds is Pareto analysis as shown in Figures 7 and 8. It is said it can give an 80 per cent improvement from dealing with 20 per cent of the defects: the 20 per cent made up from columns 1 and 2.
Pareto analysis of defects helps to pinpoint the areas to concentrate on first. Figure 8 identifies the ‘vital few’ being the large numbers of components with defects 1 and 2, and the ‘trivial many’ as the smaller numbers having defects 3–7. It suggests that by reducing the incidence of defects 1 and 2 by 50 per cent, concentration on the vital few will yield the greatest returns for expended effort. But minor defects cannot be ignored if they are safety-critical. The philosophy taken at face value fails to grade defects into their criticality. If, for example, lack of concentricity means a machined part does not fit another, it is far from trivial because material is wasted. More seriously, if safety-critical defects are ignored – as ignoring defects 3–7 might indicate – someone could be injured or killed.
The problem is a dilemma for company managers, because it is short term and focuses on production costs, but in the long term it could lead to consumer dissatisfaction, even injury or death. A number of US car companies have been criticised by US courts in product liability suits, where company accountants have resisted recalls or remedial work on the grounds that the cost would exceed the costs of court awards to injured customers. The first and most notorious case is the Ford Pinto car, which involved petrol tanks that blew up in rear shunts.
Box 3 CEDAC analysis
Fault-tree analysis when applied in a manufacturing environment is often called cause-and-effect analysis, and results in a cause-and-effect diagram with added cards (CEDAC). It was developed by Sumitomo Corporation in Japan as part of a quality improvement programme, and involves constructing a fault tree. The diagram is like the fault-tree you saw in Figure 2 turned from 12 o'clock to 3 o'clock: the ‘effect’ (the fault) is on the right, contributing causes lie to the left in a fishbone pattern (Figure 9).
Such a diagram is exhibited in the factory and workers can add new possible causes to the diagram – the added cards. The problem is therefore under continual review.
A CEDAC team performs the review, by selecting the most likely causes and testing hypotheses against test data. The goal is to find the root cause or causes of a particular problem, as shown in Figure 10.
Corrective action can then be taken to alleviate or eliminate the problem. The method is in widespread use in manufacturing industry, often in combination with FMEA and other total-quality methods.