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PRIFYSGOL ABERTAWE SWANSEA UNIVERSITY College of Engineering SEMESTER 2 MAY/JUNE 2015 EG-M36 SYSTEMS MONITORING, CONTROL, RELIABILITY, SURVIVABILITY, INTEGRITY AND MAINTENANCE LEVEL M You may only use calculators provided by the University. Candidates may only refer to the English Dictionaries that ...

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PRIFYSGOL ABERTAWE SWANSEA UNIVERSITY College of Engineering SEMESTER 2 MAY/JUNE 2015

EG-M36

SYSTEMS MONITORING, CONTROL, RELIABILITY, SURVIVABILITY, INTEGRITY AND MAINTENANCE LEVEL M You may only use calculators provided by the University. Candidates may only refer to the English Dictionaries that are available at the venue. Translation dictionaries are not permitted.

Time allowed:

2 hours Answer THREE questions

TURN OVER Page 1 of 9

Q1. (a)

In the field of engineering, what is a system by definition? [2 marks]

(b)

In the field of quality and reliability engineering, ‘Engineering Integrity’ is comprised of four basic elements. What are these four elements and describe briefly what they are. [4 marks]

(c)

What is the difference between quality and reliability? [3 marks]

(d)

What affects quality of a product? [2.5 marks]

(e)

What affects reliability of a product? [2.5 marks]

(f)

‘Reliability of a product is its ability to retain its quality as time progresses’. Discuss this statement highlighting the interrelation between quality and reliability, which varies with time. [2 marks]

(g)

Table 1-1 lists a set of performance characteristics for a SUV-type car each of which can be represented by Gaussian probability density function. (i) Use the 𝑃𝑥 ,𝑥+𝑢 values given in Table 1-2 and the Equations 1-1 and 1-2 to

calculate the corresponding set of probabilities {P} that the SUV will fail to meet the specification for a given performance characteristic i) – iv). [8 marks]

(ii) Assuming that the probabilities of simultaneous failures are negligible, estimate the overall probability of failure to meet the specification. [1 marks]

Question 1. (g) continues overleaf Turn Over Page 2 of 9

Question 1. (g) (continued)

𝑃 = 1 − [𝑃𝑥,𝑥+(𝑥−𝑥𝑇 +∆) + 𝑃𝑥,𝑥+(𝑥𝑇 −𝑥+∆) ] ………. Equation 1-1 In the special case 𝑥 = 𝑥𝑇

𝑃 = 1 − 2𝑃𝑥,𝑥+∆ ………. Equation 1-2 (TOTAL 25 MARKS) Turn Over

Page 3 of 9

Q2. (a)

Figure 2-1 shows the ‘bathtub curve’. There are three distinct regions: early failure region, useful life region and wear-out failure region. Describe each of the failure regions with respect to the Equation 2-1 showing the relation between reliability and instantaneous failure rate.

𝑅(𝑡) = 𝑒𝑥𝑝

[− ∫0𝑡 𝜆(𝜉)𝑑𝜉 ]

………. Equation 2-1 [6 marks]

(b)

Figure 2-2 shows the ‘Cost of Reliability’ curve. With example, explain the importance of achieving optimum reliability for a product. Explanation should include the consideration of cost of design change.

[6 marks] Turn Over Page 4 of 9

(c)

There are four factors influencing failure rate: quality, temperature, environment and stress. In the aspect of stress, Load (stress) – Strength analysis is an effective approach in determining a product’s degree of reliability (i.e. intrinsically reliable, reliability too sensitive or reliability too low). A mobile phone cover is assumed to have the following load-strength values. The safety margin (SM) and loading roughness (LR) are defined by the Equations 2-2 and 2-3, respectively. [Assumptions] -

Load and Strength are normally distributed. Failure probability = 0.1 failures per application of stress

-

Load, mean = 30MPa; standard deviation = 8MPa Material = PC (UTS, mean = 40MPa; standard deviation = 5MPa) 𝑆𝑀 = 𝐿𝑅 =

𝑦−𝑥

………. Equation 2-2

𝜎𝑥

………. Equation 2-3

2) √(𝜎𝑥2 +𝜎𝑦

√(𝜎𝑥2 +𝜎2𝑦)

(i)

Using the assumptions and Equation 2-2 calculate the SM of a product. [2 marks]

(ii)

Using the assumptions and Equation 2-3 calculate the LR of a product. [2 marks]

(iii)

Using Figure 2-3 as a guide and together with the calculated results from (i) and (ii), comment on the product’s degree of reliability. [5 marks]

(iv)

Suggest a way forward to improve its reliability to ‘Region (a)’ as shown in Figure 2-3, known as ‘intrinsically reliable region’. [4 marks]

Question 2. (c) continues overleaf Turn Over Page 5 of 9

Question 2. (c) (continued)

(TOTAL 25 MARKS)

Q3.

For a series system, the system reliability is the product of the individual element reliabilities (i.e. the system will only survive if every element survives).

(a)

In system engineering, what is survivability by definition? [2 marks]

(b)

Table 3-1 shows the reliability of individual subsystem of a passenger vehicle. Assuming reliability of each subsystem (in series) is independent of the reliability of other subsystems, calculate overall system reliability.

[3 marks] Turn Over

Page 6 of 9

(c)

What would be the effective way to increase its reliability in a series system? [4 marks]

(d)

Figure 3-1 shows the example of electric power subsystem of a satellite represented in a block diagram. In a parallel system, it is known that the overall system failure (i.e. unreliability, F(t) = 1 – R(t)) is the probability that element/system 1 fails and the probability that 2 fails, etc.

(i)

Calculate the probability of losing the electric power subsystem after 20 years of operation in orbit (assume that the values given for  are based on failures in 109 hours of operation). [10 marks]

(ii)

A parallel system increases the reliability of the overall system, this is termed redundancy. What is the major difference between full active redundancy and a standby system? [3 marks]

(iii)

A standby system consisting of three identical subsystems A, B, C and a switching system S. There can be three situations of failures: a) All subsystem A, B and C fails (switching S survives); or b) Two subsystem fails and one switching failure; or c) One subsystem fails and two switching failures.

Question 3. (d)(iii) continues overleaf Turn Over Page 7 of 9

Question 3. (d)(iii) (continued) Based on these a), b) and c) situations, write down mathematical representation of overall system failure (FSYST), considering that the subsystem failure and switching failure are represented by F and Fs, respectively. [3 marks] (TOTAL 25 MARKS)

Q4.

Design for Reliability (DFR) begins from the first stage of product development and should be well integrated through all its phases.

(a)

What is DFR and how does it differ from design for six sigma (DFSS)? [2 marks]

(b)

DFR activities at ‘identify phase’ can include but not limited to benchmarking, environments assessment, quality function deployment (QFD) and programme risk assessment. Describe briefly each of these activities. [4 marks]

(c)

DFR activities at ‘verify phase’ can include accelerated life testing (ALT) and highly accelerated life testing (HALT). List four suitable approaches for ALT/HALT. [4 marks]

(d)

In the field of engineering, maintainability is the ease with which a product can be maintained in order to isolate defects or their cause, maximise a product’s useful life and cope with a changed environment, etc. (i)

What is corrective maintenance (CM) in comparison to preventive maintenance (PM)? [2 marks] Question 4. (d) continues overleaf Turn Over Page 8 of 9

Question 4. (d) (continued) (ii)

In CM approach, there are six factors contributing to mean down time (MDT). Write down these six factors and describe strategies to minimise MDT. [5 marks]

(iii)

In PM approach, factors contributing to mean maintenance time (MMT) are: a) access time, b) repair/replace time, and c) checkout time. Describe strategies to minimise MMT. [3 marks]

(iv)

Calculate the achieved availability (AA) for a protective system considering the following conditions: [Conditions] - Demonstrated failure rate of 1 failure per 1500 hours of operation -

PM is scheduled every 300 hours of operation and on average takes 3 hours

-

To repair a failed system takes on average 12 hours AA is defined by: 𝐴𝐴 =

-

𝑀𝑇𝐵𝑀𝐴 𝑀𝑇𝐵𝑀𝐴 + 𝑀𝑀𝑇

Assuming constant failure rate, mean time between maintenance actions (MTBMA) is defined by: 𝑀𝑇𝐵𝑀𝐴 =

-

1 𝜆 + 𝑓𝑃𝑀

Where: f

PM

= the frequency of PM (is equal to 1/T ) PM

Mean maintenance time (MMT) is defined by: 𝑀𝑀𝑇 =

𝜆 ∙ (𝑀𝑒𝑎𝑛 𝐶𝑀 𝑡𝑖𝑚𝑒 ) + 𝑓𝑃𝑀 ∙ (𝑀𝑒𝑎𝑛 𝑃𝑀 𝑡𝑖𝑚𝑒) 𝜆 + 𝑓𝑃𝑀 [5 marks] (TOTAL 25 MARKS) End of Paper Page 9 of 9...