14 Risk as a basis for design

The science of risk, and the assessment and management of risk, is a very complex subject and one that covers a wide diversity of disciplines. Society is becoming more aware of the risks related to increased technological innovation and industrialization. Recent reports in the media about environment (global warming), health (BSE) and technological (nuclear waste processing) risks have played their part in focusing attention on the problem of how to assess risk and what makes an acceptable risk level. As a result, risk and risk related matters are becoming as important as economic issues on the political agenda. There is, therefore, an increasing need for a better understanding of the topic and tools and techniques that can be used to help assess product safety and support the development of products and processes that are of essentially low risk (EPSRC, 1999). To meet these needs, a new British Standard, BS 6079 (1999), has been published to give guidance to businesses on the management of risks throughout the life of a project.

The term 'risk' is often used to embrace two assessments:

• The frequency (or probability) of an event occurring

• The severity or consequences of the event on the user/environment.

The product of these two conditions equals the risk:

Risk = Occurrence x Severity

We can demonstrate the notions of risk and risk assessment using Figure 1.18. For a given probability of failure occurrence and severity of consequence, it is possible to map the general relationship of risk and what this means in terms of the action required to eliminate the risk.

For example, if both occurrence and severity are low, the risk is low, and little or no action in eliminating or accommodating the risk is recommended. However, for the same level of occurrence but a high severity, a medium level of risk can be associated with concern in some situations. The level of occurrence, for some unknown reason, changes from low to medium and suddenly we are in a situation where the risk requires priority action to be eliminated or accommodated in the product.

The aim of a risk assessment is to develop a product which is 'safe' for the proposed market. A safe product is any product which, under normal or reasonably foreseeable conditions of use, including duration, presents no risk or only the minimum risk compatible with the product's use and which is consistent with a high level of protection for consumers (DTI, 1994). In attempting to protect products against failure in service and, therefore, the user or environment, difficulty exists in ascertaining the

RISK = OCCURRENCE x SEVERITY

D O 111

MEDIUM

HIGH

SEVERITY OF CONSEQUENCES

-PRIORITY ACTION

- CONCERN IN SOME SITUATIONS

- NO ACTION

Figure 1.18 Risk and the identification of priority action degree of protection most suitable for a given application. First, the choice should be expressed in terms of risk and probability of failure as shown above, but the determination of an acceptable level for a product depends on many factors, such as (Bracha, 1964; Karmiol, 1965; Welling and Lynch, 1985):

• The costs of failure

• Criticality of function that it supports

• Complexity - number of component parts, subsystems

• Operational profile - duty cycle or time it operates

• Environmental conditions - exposure to various environmental conditions

• Number of units to be produced

• Ease and cost of replacement

• 'State of the art' or present state of engineering progress

• Market sector/consumer category.

Suggestions have also been made as to the number of people that are affected by the risk at any one time, and, in principle, it should be possible to link the acceptable level of individual risk to the number of people exposed to that risk (Niehaus, 1987). For example Versteeg (1987) provides risk levels associated with three areas: acceptable risk, reduction desired and unacceptable risk, and the number of people exposed to the risk. This further compounds the problem of assigning acceptable risk targets, but implies that safety is of paramount importance.

Businesses make decisions that affect safety issues, but which are only considered implicitly. In an increasingly complex world, the resulting decisions are not always appropriate because the limits of the human mind do not allow for an implicit consideration of a large number of factors. Formal analyses are needed to aid the decision-making process in these complex situations. However, the application of a formal analysis to safety issues raises new questions. The risks perceived by society and by individuals cannot be captured by a simple technical analysis. There are many reasons for this; however, it is clear that decision making needs to account for both technical and public values (Bohnenblust and Slovic, 1998). The decision to accept risk is not based on the absolute notion of one acceptable risk level, but has some flexibility as the judgement depends on the cost/benefit and the degree of voluntariness (Vrijling et al., 1998). The notion of safety is often used in a subjective way, but it is essential to develop quantitative approaches before it can be used as a functional tool for decision making (Villemeur, 1992). A technique which 'quantifies' safety is FMEA.

1.4.1 The role of FMEA in designing capable and reliable products

In light of the above arguments, it has been found that there are two key techniques for delivering quality and reliability in new products: process capability analysis and FMEA (Cullen, 1994). FMEA is now considered to be a natural tool to be used in quality and reliability improvement and it has been suggested that between 70 and 80% of potential failures could be identified at the design stage by its effective use (Carter, 1986). For example, performing a comprehensive FMEA well will alleviate late design changes (Chrysler Corporation et al., 1995).

FMEA was first mentioned at the start of this chapter. It is recommended that the reader unfamiliar with FMEA refer to Appendix III and several other references provided to gain a firm understanding of its application in product design. In general, an FMEA does the following (Leitch, 1995):

• Provides the designer with an understanding of the structure of the system, and the factors which influence quality and reliability

• Helps to identify items that are of high risk through the calculation of the Risk Priority Number (RPN), and so gives a means of deciding priorities for corrective action

• Identifies where special effort is needed during manufacture, assembly or maintenance

• Establishes if there are any operational constraints resulting from the design

• It gives assurance to management and/or customers that quality and reliability are being or have been properly addressed early in the project.

Of the many characteristics of a product defined by the dimensions and specifications on a drawing, only a few are critical to fulfilling the product's intended function.

DESIGN FMEA

Failure motias, saverity arta critical characteristics

CAPABLE DESIGN

Failure modes, Severity, critical characteristics and capability estimates

RELIABLE DESIGN

Fie I ¡ability

95timales

CAPABLE AND RELIABLE DESIGN

Figure 1.19 The FMEA input into designing capable and reliable products

Hence, a critical characteristic is defined as one in which high variation could significantly affect product safety, function or performance (Liggett, 1993). In order to assess the level of importance of the characteristics in a design, a process of identifying the critical characteristics and then using special symbols on the detailed drawing is commonly used. For example, the symbol '▼' is used by some companies to indicate that a particular characteristic should be controlled during manufacture using SPC. The identification process is facilitated by the use of a design FMEA using multi-disciplined teams.

As seen in Figure 1.19, the important results from an FMEA in terms of designing capable and reliable products are the potential failure modes, severity rating and critical characteristics for the design. By identifying the capability of the critical characteristics, and the potential failure mode, a statistical analysis can then be performed to determine its reliability. The FMEA Severity Rating (S) is crucial for setting capability and reliability targets because it is a useful indication of the level of the safety required for the application. Although subjective in nature, the effective use of an FMEA in the design process is advocated as it brings significant benefits.

In summary, to reduce risk at the product design stage requires that we do not speculate without supporting evidence on the causes, consequences and solutions for an actual or potential design problem. This means making predictions, where appropriate, based on evidence from testing, experience or other hard facts using statistical probabilities, not vague guesses. The use of FMEA to evaluate all the potential risks of failure and their consequences, both from normal use and foreseeable misuse, is a key element in designing capable and reliable products (Wright, 1989).

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