Example 2.2 shows that two different functions of a single system give rise to two different RBD. Observe also that the blocks in the two RBDs represent different component functions in (b) and (c).
Remark 2.3 (Terminology problem)
Many authors use the term “component” instead of block. There is nothing wrong with this terminology–and we also use it later in this book–but we have to be very careful when, for example, saying that “component
2.8.9 Practical Construction of RBDs
A specific system function, SF, usually requires a long range of subfunctions. For the essential function of a car, for example, we need the functions of the engine, the brakes, the steering, the ventilation, and many more. The RBD for the SF is then a long series structure of the required subsystem functions, as shown in Figure 2.18. Each of the required subfunctions may again need sub‐subfunctions.
Figure 2.18 Construction of the RBD in levels.
How many levels are required to depend on how complicated the system function is and the objectives of the analysis. RBDs are further discussed in Chapter 4. Chapter 6 deals with quantitative reliability analysis based on RBDs.
2.9 Problems
1 2.1 Identify and describe briefly the main subsystems of a family car and establish a system breakdown structure for the car.
2 2.2 Establish a function tree for a (domestic) refrigerator.
3 2.3 List the environmental, operating, and maintenance factors that should be considered when defining the operating context of a family car.
4 2.4 List some information functions that are available in a modern car.
5 2.5 Identify the main functions of a family car and establish a function tree for the car.
6 2.6 List some safety functions of a modern car. Are the identified functions online or offline functions?
7 2.7 Identify and describe the functions of the front door of a house.
8 2.8 Describe the functions of a vacuum flask (thermos) and suggest relevant performance criteria.
9 2.9 Describe briefly a system you consider to be complex.
10 2.10 Refer to the SADT functional block (see Figure 2.3) and list all the inputs, controls, and resources you need to bake a pizza. The output from the function is the new‐baked pizza. How would you set up the performance criteria for your pizza?
11 2.11 Based on an Internet search, explain what is meant by a CONOPS and list its main elements.
12 2.12 Based on an Internet search, list the main elements that are typically included in a system requirements document (or a system requirements specification).
13 2.13 Establish an RBD of the braking system of a family car.3
14 2.14 Consider a voted oo structure. The voting can be specified in two different ways:– As the number out of the components that need to function for the system to function.– As the number of the components that need to fail to cause system failure.In the first case, we often write oo:G (for “good”) and in the second case, we write oo:F (for failed).Determine the number such that a 2oo4:G structure corresponds to a oo4:F structure.Determine the number such that a oo:G structure corresponds to a oo:F structure.
15 2.15 Are there any examples of standby redundancy in a family car? Justify your answer.
References
1 Aslaksen, E.W. (2013). The System Concept and Its Application to Engineering. Heidelberg: Springer‐Verlag.
2 Blanchard, B.S. and Fabrycky, W.J. (2011). Systems Engineering and Analysis, 5e. Boston, MA: Pearson.
3 Einstein, A. and Infeld, L. (1938). The Evolution of Physics. Cambridge University Press.
4 Lambert, M., Riera, B., and Martel, G. (1999). Application of functional analysis techniques to supervisory systems. Reliability Engineering & System Safety 64 (2): 209–224.
5 Marca, D.A. and McGowan, C.L. (2006). IDEF0 and SADT: A Modeler's Guide. Auburndale, MA: OpenProcess.
6 Ødegaard, S. (2002). Reliability assessment of a subsea production tree. Project thesis. Trondheim, Norway: Norwegian University of Science and Technology.
7 U.S. Air Force (1981). Integrated Computer Aided Manufacturing (ICAM) Architecture. Part II. Volume IV, Functional Modeling Manual (IDEF 0). Technical Report AFB AFWAL‐TR‐81‐4023. Wright Patterson Air Force Base, OH: Air Force Materials Laboratory.
8 U.S. DoD (2001). Systems Engineering Fundamentals. Fort Belvoir, VA: Defense Acquisition University Press.
Notes
1 1 IEV defines indenture level as the “level of subdivision within a system hierarchy” (IEV 192‐01‐05).
2 3 You may need to search the Internet to find technical information on the braking system.
Chapter 3 Failures and Faults
3.1 Introduction
Failure is the most important concept in any reliability study, where typical questions addressed include:
How long time will the item, on the average, be able to operate until the first failure occurs?
What will the frequency of failures be? How many failures per year should we expect?
What is the probability that the item will operate without failure during a specified time interval?
If an item is demanded, what is the probability that it will fail to perform as required?
If we do not have a clear understanding of what a failure is, the reliability study may be of limited value. The term failure is used frequently in our daily language with many different interpretations and we also use a plethora of terms with similar meaning. Among these terms are blunder, breakdown, bug, collapse, defect, deficiency, error, fault, flaw, impairment, malfunction, mishap, mistake, and nonconformance.
How the term failure is interpreted varies between professional disciplines. Engineers working with quality, maintenance, warranty, safety, and reliability may have quite different opinions about whether or not a particular event constitutes a failure.
To perform a reliability study, it is important to understand thoroughly what is meant by the term failure in the context of reliability. Several definitions of failure