SELF STUDY REPORT

RELIABILITY ENGINEERING

KARTIK GUPTA

2K13/PE/016

ACKNOWLEDGEMENT I would like to express my gratitude to Prof. V. Jeganathan Arulmoni for his support and guidance towards the completion of this self study project on the topic’ Reliability Engineering’ . I would also like to thank him for giving me the opportunity to embark on this project as it has enhanced my knowledge and has helped me learn a lot.

TABLE OF CONTENTS

INTRODUCTION

OBECTIVES OF RELIABILITY ENGINEERING

WHY IS RELIABILITY IMPORTANT

REASONS FOR FAILURE

MEASURING RELIABILITY

RELIABILITY PREDICTION

DESIGN FOR RELIABILITY

RELIABILITY TESTING

WHAT IS THE DIFFERENCE BETWEEN RELIABILITY AND QUALITY?

WHAT IS SIX SIGMA

CONCLUSION

REFERENCES

INTRODUCTION Reliability engineering is engineering that emphasizes dependability in the lifecycle management of a product. Dependability, or reliability, describes the ability of a system or component to function under stated conditions for a specified period of time. Reliability is theoretically defined as the probability of success (Reliability=1-Probability of Failure), as the frequency of failures, or in terms of availability, as a probability derived from reliability and maintainability. Maintainability and maintenance is often defined as a part of "reliability engineering" in Reliability Programs. Reliability engineering deals with the estimation and management of high levels of "lifetime" engineering uncertainty and risks of failure. Reliability engineering consists of the systematic application of time-honored

engineering principles and techniques throughout a product lifecycle and is thus

an essential component of a good Product Lifecycle Management (PLM) program.

The goal of reliability engineering is to evaluate the inherent reliability of a

product or process and pinpoint potential areas for reliability improvement.

Realistically, all failures cannot be eliminated from a design, so another goal of

reliability engineering is to identify the most likely failures and then identify

appropriate actions to mitigate the effects of those failures.

"Reliability is, after all, engineering in its most practical form."

OBJECTIVES OF RELIABILITY ENGINEERING

The objectives of reliability engineering, in the order of priority, are:

1. To apply engineering knowledge and specialist techniques to prevent or to

reduce the likelihood or frequency of failures.

2. To identify and correct the causes of failures that do occur, despite the

efforts to prevent them.

3. To determine ways of coping with failures that do occur, if their causes

have not been corrected.

4. To apply methods for estimating the likely reliability of new designs, and for

analysing reliability data.

Fig A Basic project life cycle

WHY IS RELIABILITY IMPORTANT?

There are a number of reasons why reliability is an important product attribute,

including:

Reputation. A company's reputation is very closely related to the reliability

of its products. The more reliable a product is, the more likely the company is

to have a favorable reputation.

Customer Satisfaction. While a reliable product may not dramatically affect

customer satisfaction in a positive manner, an unreliable product will

negatively affect customer satisfaction severely. Thus high reliability is a

mandatory requirement for customer satisfaction.

Warranty Costs. If a product fails to perform its function within the warranty

period, the replacement and repair costs will negatively affect profits, as well

as gain unwanted negative attention. Introducing reliability analysis is an

important step in taking corrective action, ultimately leading to a product

that is more reliable.

Repeat Business. A concentrated effort towards improved reliability shows

existing customers that a manufacturer is serious about its product, and

committed to customer satisfaction. This type of attitude has a positive

impact on future business.

Cost Analysis. Manufacturers may take reliability data and combine it with

other cost information to illustrate the cost-effectiveness of their products.

This life cycle cost analysis can prove that although the initial cost of a

product might be higher, the overall lifetime cost is lower than that of a

competitor's because their product requires fewer repairs or less

maintenance.

Customer Requirements. Many customers in today's market demand that

their suppliers have an effective reliability program. These customers have

learned the benefits of reliability analysis from experience.

Competitive Advantage. Many companies will publish their predicted

reliability numbers to help gain an advantage over their competitors who

either do not publish their numbers or have lower numbers.

REASONS FOR FAILURE

The load and strength of an item may be generally known, however there will always be an element of uncertainty. The actual strength values of any population of components will vary; there will be some that are relatively strong, others that are relatively weak, but most will be of nearly average strength. Similarly there will be some loads greater than others but mostly they will be average. Figure 1, below shows the load strength relationship with no overlaps.

However if, as shown in figure 2, there is an overlap of the two distributions then failures will occur. There therefore needs to be a safety margin to ensure that there is no overlap of these distributions.

MEASURING RELIABILITY

REQUIREMENTS

Many customers will produce a statement of the reliability requirements that is

included in the specification of the product. This statement should include the

following:

• The definition of failure related to the product’s function and should cover all

failure modes relevant to the function

• A full description of the environments in which the product will be stored,

transported, operated and maintained

• A statement of the reliability requirement

THE BATH TUB CURVE

The bath-tub curve is a representation of the reliability performance of

components or non repaired items. It observes the reliability performance of a

large sample of homogenous items entering the field at some start time (usually

zero). If we observe the items over their lifetime without replacement then we

can observe three distinct shapes or periods

The infant mortality or early failures portion shows that the population will

initially experience a high hazard function that starts to decrease. This period of

time represents the burn-in or debugging period where weak items are weeded

out. After the initial phase when the weak components have been weeded out

and mistakes corrected, the remaining population reaches a relatively constant

hazard function period, known as the useful life period. From figure 3 you can see

that the hazard function is constant, this shape can be modelled by the

exponential distribution (see section 2.3) when failure are occurring randomly

through time. The final portion of the bath-tub curve is called the wear-out phase,

this is when the hazard function increases with time.

LIFE DISTRIBUTIONS

If you take a large number of measurements you can draw a histogram to show

the how the measurements vary. A more useful diagram, for continuous data, is

the probability density function. The y axis is the percentage measured in a

range(shown on the x-axis) rather than the frequency as in a histogram. If you

reduce the ranges(or intervals) then the histogram becomes a curve which

describes the distribution of the measurements or values. This distribution is the

probability density function or PDF.

The survival function and is given by R(t)

PARTICULAR LIFE DISTRIBUTIONS

The exponential Distribution :

The Weibull Distribution :

RELIABILITY PREDICTION

Assuming all the parts in a system are independently exponentially distributed,

i.e. one part does not cause the other to fail then the overall system failure rate

can be calculated using the series system model shown above. For example, the

failure rate of a printed circuit board is the sum of the failure rates of each of the

components.

DESIGN FOR RELIABILITY

The objective of design for reliability is to design a given product that meets its

requirements under the specified environmental conditions. To achieve this good

sound engineering design rules should be followed. However there are a few

general principles that should observed, these include:

• Component selection – well-established and known components should be

used (company usually have their own approved components list). If this is not he

case then analysis must be done to check the component is fit for purpose.

• Consider the load-strength relationship and ensure there is an adequate safety

margin.

• Minimum complexity

• Diversity – avoids common mode failures

• Analyze failure modes and their effects (FMEA)

• Identify any single point failures and either mitigate or design them out.

• Use lessons learned from previous products to design out any known

weaknesses.

RELIABILITY TESTING

The purpose of reliability testing is to discover potential problems with the design

as early as possible and, ultimately, provide confidence that the system meets its

reliability requirements.

Reliability testing may be performed at several levels and there are different

types of testing. Complex systems may be tested at component, circuit board,

unit, assembly, subsystem and system levels

EXAMPLE OF RELIABILITY VARYING OVER THE LIFE CYCLE

WHAT IS THE DIFFERENCE BETWEEN RELIABILITY AND QUALITY?

Even though a product has a reliable design, when the product is manufactured and used in the field, its reliability may be unsatisfactory. The reason for this low reliability may be that the product was poorly manufactured. So, even though the product has a reliable design, it is effectively unreliable when fielded, which is actually the result of a substandard manufacturing process. As an example, cold solder joints could pass initial testing at the manufacturer, but fail in the field as the result of thermal cycling or vibration. This type of failure did not occur because of an improper design, but rather it is the result of an inferior manufacturing process. So while this product may have a reliable design, its quality is unacceptable because of the manufacturing process.

Just like a chain is only as strong as its weakest link, a highly reliable product is only as good as the inherent reliability of the product and the quality of the manufacturing process.

WHAT IS SIX SIGMA?

Six Sigma at many organizations simply means a measure of quality that strives for near perfection. Six Sigma is a disciplined, data-driven approach and methodology for eliminating defects (driving toward six standard deviations between the mean and the nearest specification limit) in any process – from manufacturing to transactional and from product to service.

The statistical representation of Six Sigma describes quantitatively how a process is performing. To achieve Six Sigma, a process must not produce more than 3.4 defects per million opportunities. A Six Sigma defect is defined as anything outside of customer specifications. A Six Sigma opportunity is then the total quantity of chances for a defect.

CONCLUSION

Ultimately the aim for reliability engineering is to maximize reliability during

service life by measurement & control of manufacturing, quality / screening,

optimized design & build process to improve intrinsic reliability, assure no

systematic faults present in product and to provide sufficient margin to meet life

requirements.

Thus all these factors lead to customer satisfaction which is of utmost importance

to every manufacturer.

REFERENCES

http://www2.warwick.ac.uk/fac/sci/wmg/ftmsc/modules/modulelist/peuss

/slides/section_7a_reliability_notes.pdf

http://en.wikipedia.org/wiki/Six_Sigma

http://www.weibull.com/basics/reliability.htm

http://www.lce.com/Whats_the_role_of_the_Reliability_Engineer_373-

item.html

https://learnable.com/books/the-principles-of-project-

management/online/ch01s02

http://www.isixsigma.com/new-to-six-sigma/getting-started/what-six-

sigma/