Module 5 Lecture 3 Final

7/30/2019 Module 5 Lecture 3 Final

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IIT, Bombay

Module5

Design for Reliability andQuality


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Lecture

3Design for Reliability


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Instructional Objectives

The objective of this lecture is to outline the basic concepts of risk, reliability and safety and the

methodologies to define reliability in a quantitative manner to aid to design for reliability.

Define Risk, Reliability and Safety

Riskis usually defined as the probability or the frequency of occurrence of a hazard or damage to

human, infrastructure, property or environment. Reliability refers to the ability of a product to

perform its specified function under service conditions. In other words, reliability can be

depicted as the probability that an item will perform appropriately for a specified time period

under a given service condition. For example, a reliability of 0.997 for a typical part implies that

there is aprobability of failure (an inverse of reliability) of 3 parts in every 1000 parts. Safety is

referred to the relative protection from exposure to hazards.

Risk Assessment

A product is considered safe if the risks associated with the product are assessed to be

acceptable. The risk assessment often involves the determination of the frequency of the

occurrence of a specified hazard and a conclusion on the acceptability of the hazard. Alternately,

the levels of risk can be classified as:

Tolerable RiskThis level of risk is usually accepted while a constant review of its cause and

ways to reduce the same is the must.

Acceptable Risk This level of risk is also acceptable and does not need immediate attention.

Unacceptable Risk: This level of risk is deemed to be unacceptable.

Quantify ReliabilityEngineering designs often neglect the stochastic nature of the material properties, the dimensions

of the components and the externally applied load and usually consider the same by applying a

factor of safety. However, a quantification of the uncertainty in design of parts is necessary in

critical applications like in space, aircraft and nuclear applications.


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For example, consider a structural member subjected to a static load experiences a stress and

exhibits the yield strength (of the material) as y . Figure 5.3.1 schematically depicts the nature

of variation of stress experienced and the material yield strength. As shown schematically in

Figure 5.3.1, the distribution of stress on the structural member can be envisaged as typicalGaussian in nature with a mean ( ) and standard deviation ( sd ). Similarly, the variation in the

yield strength of the material can also be envisaged as Gaussian with a presumed mean ( y ) and

standard deviation ( ysd ). If we subtract the stress distribution from the strength distribution, we

get a distribution as ( y ), which is always positive. Interesting to note, however, is the fact

that the frequency distribution of can overlap with that of y and can lead to a situation as

y

> , which will lead to failure. So, the probability of failure will be given as

)(PP yf >= (1)

and, the reliability will then be defined as

fP1R = (2)

The mean and the standard deviation of the distribution ( y ) can be easily determined

considering and y as two independent variables.

Figure 5.3.1 Schematic presentation of the distribution of stress on a typical structural member

and variation in the material yield strength

To avoid unforeseen failure as indicated above, the mechanical designs often undertake the

concept of a safety factorto reduce the probability of failure and hence, increase the reliability.

The safety factormay be estimated as the ratio of the mean capacity to the mean load or demand.

y

y


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Thus, reliability can be defined as the probability that a system, component, or device

will perform without failure for a specified period of time under a given operating conditions. If

R(t) and F(t) are respectively is the reliability and the probability of failure with respect to time t,

and are mutually exclusive, we can write

1)t(F)t(R =+ (3)

Furthermore, considering a typical test for reliability with 0N number of parts, and )t(NS and

)t(NF are respectively the number of parts surviving and failing till time t, we can write that

0FS N)t(N)t(N =+ (4)

We can write further that,

0

F

0

S

N

)t(N1)t(F1

N

)t(N)t(R === (5)

Hence, the instantaneous failure rate can be envisaged as the number of failures per unit time for

the total number of parts exposed during time t and given as,

)t(N

1

dt

)t(dN)t(h

0

F= (6)

In a statistical form, the instantaneous failure rate, h(t), can be envisaged as the probability that a

given test item has survived till time 1t and would fail between time interval 1t and 11 dtt + , and

can be expressed as

)tt|dtttt(P)t(R

)t(f

)t(F1

)t(f)t(h 1111 +==

= (7)

where f(t) refers to the typical nature of the statistical frequency distribution of time to failure.

Hence, f(t) can be written as

dt

)t(dR

dt

)]t(R1[d

dt

)t(dF)t(f

=

== (8)

Substituting equation (8) in (7), we can write,

=== t

0

dt)t(hexp)t(Rdt)t(h)t(R

)t(dR

)t(R

1

dt

)t(dR)t(h (9)

For a given constant value of failure rate, equation (9) can thus be used to estimate the reliability

of a test component. Figure 5.3.2 schematically shows the typical nature of the failure curves

realized for general components.


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Figure 5.3.2 Schematic presentation of the failure rate curve

The failure curve usually consists of three parts. The first part is a decreasing failure rate, known

as early failures that may occur due to design errors, manufacturing defects, or installations

errors. These early failures can be minimized by improving the production quality control, and

subjecting the parts to a proof test before their actual service with the customers. As time passes

the system stabilizes and these early failures leave the system leading to a constant failure rate in

the second part, which is also referred to as random failure zone. Here the failures occur because

of random overloads or random faults having no pattern at all. Mechanical components usually

do not exhibit a region of constant failure rate. After a stipulated long time, the mechanical

components and materials begin to age and wear out and this accelerates the failure rate leading

to an accelerated failure rate in the third (last) part. The failure curve is also referred to as

bathtub curve due to its shape. However, the usual nature of the distribution of mechanical

failures is slightly different from the electronic failures as shown in Figure 5.3.3.


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Figure 5.3.3 (a) Failure curve of a typical electronic equipment (b) Failure curve more typical

of mechanical equipment

With respect to the reliability of components, mean life of a part or component refers to

the average life a specified number of components which is measured over the entire span of

their life. The mean time to failure (MTTF)refers to the sum of survival time (up time) for all the

components put under test divided by the number of components, which are failed.

For example, assume the testing of 3 identical systems starting from time 0 until all of

them failed. The first system failed at 23 hours, the second failed at 26 hours and the third failed

at 20 hours. The MTTF is the average of the three failure times, which is 23 hours. The mean

time between failure (MTBF) refers to the mean or average time between two successive

component failures. MTBF is usually applied to a group of similar equipment, for example all

the pumps in a refinery. If there are 40 operating pumps with a total of 23 failures over one

month, the MTBF during that month is calculated as (40 / 23) or 1.74 months. For a single item,

MTBF is just the time period / number of failures. For example, if a pump fails thrice in one

month, the MTBF would be 1/3 = 0.33 months

System Reliability

The overall reliability of a typical mechanical or any system will be governed by the reliability of

the individual components. If the components are so arranged that the failure of any component

will lead to the total failure in the system, the same is envisaged to be in series. In contrast, a


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better arrangement of components would be one that would require failure of all components for

the total failure of the system. Such an arrangement exhibits parallel reliability. The system

reliability in series and parallel arrangement of components can be given as equations (10) and

(11), respectively.

nCBAsystem R........RRRR = (10)

)R1(........)R1()R1()R1(1R nCBAsystem = (11)

Requisiteness of Reliability of Product

There are a number of reasons why reliability is an essential attribute of a product.

Reputation

A companys reputation is very closely attached to the reliability of its products it produces. The

more reliable a product is, the more likely the company is to have a good reputation.

Customer satisfaction

A reliable product may not drastically affect customer satisfaction in a positive manner.

However, an unreliable product will definitely attract customer dissatisfaction Thus high

reliability is a quite essential requirement for customer satisfaction.

Warranty Costs

If a product fails to perform its desired function within the warranty period, the replacement andrepair costs will not only reduce the profits, but also gain unwanted negative attention.

Repeat business

A focused effort towards improved reliability shows existing customers that a manufacturer is

serious about its product, and committed to customer satisfaction. This type of attitude not only

has a positive impact on future business but also gives a competitive edge.

Cost Analysis

Companies 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 than those of its competitors product, the overall

lifetime cost is lower than that of a competitor's because their product requires fewer repairs or

less maintenance.


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Customer requirements

Demand from customers for an effective reliability program is ever increasing based on the

benefits of such programs.

Competitive advantage

Companies often publish the product reliability numbers to enhance competitiveness.

Difference between Quality and Reliability

Even though a product has a reliable design with all checks from the point of view of design for

quality, its reliability in service can be unsatisfactory that can be attributed to inappropriate

manufacturing process and / or the quality of the material used. 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 and/or due to poor quality of material used for the

manufacturing of the product.

Design for Reliability

Design for reliability is a process which is performed during the design of the product so as to

ensure that the product is able to perform to a required level of reliability. Traditionally, the

reliability achieved is the outcome of the reliability that naturally accompanies with the design required for the product, and the reliability that come due to standard and historic practices followed by the design and

manufacturing units. ( like using standard well established components in critical areas )

The demand to achieve desired performance level in an efficient and optimized manner has

led to a growing movement towards increasing applications of design for reliability and its

spread to industries where it had not been used in the past. Previously design practices tend to

focus on mainly on functionality androbustness or product integrity. In developing the design

for the products, the following characteristics are usually missed: - (a) key failure modes and

failure rate of the product, (b) key failure mechanisms that may be present in the service

environment, (c) usable life of the product, (d) cost of maintenance required to maintain the

inherent reliability, (e) availability, and (f) rigorous testing.


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As to address all of the above issues, the process ofdesign for reliability has been developed

to ensure a predictable and reliable life of usual components. Some of the key elements for

design for reliability is as follows.

Concurrent Engineering

Concurrent engineering is a feature that ensures the design is not completed before reliability

requirements are identified and dealt with.

Configuration Design

The physical configuration is the key important characteristic that determines the reliability of an

asset. Depending on the severity of the product service and the maximum economic reliability of

available components present in the product, it may be necessary to build redundancy into some

locations.

Component Selection

The second important characteristic that determines reliability is the choice of components that

make the product. Components with better load bearing ability rather than cheap components

should be considered as better option.

Design and Build

It is possible to create a strong configuration and select robust components, and still produce a

product that is unreliable. There are design and assembly practices like use of protective

grommets at points of wear, use of strain relief at bends, or changes in direction that ensure theconfiguration and components deliver the desired reliability.

Verification and Performance Testing

The final assembled product may not always perform as expected. Interactions between dynamic

components can produce unexpected effects. As a result, it is necessary to verify that the

assembled product functions as expected. It is also essential to simulate the wear and tear that

represents an entire life using accelerated testing.

Customer Needs

The product must be designed not only based on functionality but also considering the customer

needs.


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Steps to ensure Design for Reliability

The activities towards design for reliability for a component should be spread over from the

conceptual design, embodiment design, detail design, actual production and service. For

example, the design for reliability activities in the conceptual, embodiment and detail design

stages involves

(a)the integration of the estimation of reliability requirement and the determination of likelyservice environment in the problem definition,

(b)investigation of redundancy and ensuring provision for accessibility for maintenanceduring configuration design,

(c)selection of reliable parts and components, establishment of failure modes and effects,estimations of likely failure rate and mean time between failures,

(d)building and testing of prototypes, and so on.Similar activities in theproduction andservice stages involve environment tests and checking for

quality assurance, collecting feedbacks on service failures and customers comments, ensuring

proper replacement of parts, etc.

The overall process starts with the conceptual design to identify the problem or objective, to

estimate the required level of reliability, and to carefully understands and consider all the factors

that make up the service environment. In the configuration design step of embodiment design the

physical arrangements of the various components present in the product are to be determined as

it critically affects the reliability. Also various redundancies are tested and it should be ensured

that the physical arrangement is good enough to allow access for maintenance. In the parametric

step of embodiment design, the selected components are to be tested for their reliability. Both

physical and computer models should be tested and subjected to the widest range of service

environmental conditions and various failure modes must be established. During detail design

the final specifications are finalized for manufacturing and testing the preproduction prototype

and the final productions designs are prepared. The work of the design department is not finishedeven after the designs are released to the production department. The production models are

given further environmental tests and these help to determine the quality assurance program and

the maintenance schedule. Once the product is put to service there is a constant feedback related

to field failures and mean time between failures that help the design department to redesign

efforts and follow-on products.


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Ways to improve Reliability by design

Various techniques are used by the design team to improve reliability. The work of the design

department is not finished even after the designs are released toReducing variability

Mechanical properties of engineering material exhibit variability. Fracture and fatigue properties

show greater variability that the yield strength and the tensile strength. Conservative design

values of material properties are essential so as to obtain design of a reliable product. Variability

in the material property had a huge impact on the probability of the failure of product hence

affecting the reliability of the product too. The probability of failure can be reduced if without

changing the mean value of the material properties if we could reduce the variability in the

properties.

Derating

The reliability of the product can be increased if their maximum operating conditions

(temperature, pressure, etc) are at values lower than their nameplate values. This reduces the

probability of failure and increases the reliability of the product.

Redundancy

One of the most efficient ways to increase reliability is with redundancy. Components that are

critical are duplicated such that two or more of them may exist in parallel to perform the same

function within the product thus increasing the reliability of the product. The existence of

parallel paths results in load sharing and each duplicate component is derated and has its life

increased by a longer than the average time. Another way to increase the redundancy is by

having a standby unit that cuts in and takes over when the current operating unit fails. The unit

should be provided by sensors and switching mechanisms to sense the failure and to place the

unit in service. The sensors and the switching mechanisms are the weak links in a standby

redundant systemDurability

The material selection and the design details should be finalized with the objective of producing

a product that is resistant to degradation from factors like corrosion, erosion, fatigue, wear, etc.

This usually requires selection of high performance material which can be expensive so as to


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increase service life and reduce the maintenance cost. Such decisions can be justified by using

techniques like life cycle costing and so on.

Ease of inspection

It is very essential that cracks or any other flaws are identified when they occur in the product.

Ideally it should be possible to perform various visual methods for detections of such cracks or

other flaws, but special design features may have to be provided in order to ensure this. The

product should be designed in such a way that it is capable for ready inspection.

Simplicity

Simplification of the component and the product reduces the chances for failure and errors and

increases the reliability.

Specificity

The greater the level of specificity, the higher will be the inherent reliability of the design.

Specifying standard components for the product increases the reliability. It means the component

being used has a history and their reliability is known.

Replacement

Whenever it is required to use components with high failure rates the design should specifically

take care for the ease of replacement of such component.


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Exercise

1. If a device has a failure rate of 5x10-6 failures/h, what is the reliability for an operating period

of 1000 h?

References

[1] 1. David M. Anderson and David M. Anderson, Design for Manufacturability andConcurrent Engineering, CIM Press, 2004.

[2] G Dieter, Engineering Design - a materials and processing approach, McGraw Hill, NY,2000.

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