DESIGN FOR RELIABILITY

1

DESIGN FOR RELIABILITY WHY NOW? WHY SO LATE IN MME2259a?

Because reliability may depend on details of design 2

DESIGN FOR RELIABILITY 3

HYATT’S SKY WALK COLLAPSE Original Design As Built

4

HYATT’S SKY WALK COLLAPSE Original Design

http://www.designcommunity.com/discussion/10175.html

As Built

5

HARTFORD CIVIC CENTRE ARENA ROOF COLLAPSE

http://www.eng.uab.edu/cee/faculty/ndelatte/case_studies_project/Hartford%20Civic%20Center/hartford.htm#Top 6

SINKING OF SLEIPNER A PLATFORM

http://www.ima.umn.edu/~arnold/disasters/sleipner.html

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TOW TRUCK TOWING HITCH FAILURE Original Design As Built

“Fundamentals of Engineering Design” Barry Hyman, Prentice Hall 8

STUDY OF FAILURE

• Failure of materials • Failure of components/devices • Failure of structures • Failure of systems • ???

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RELIABILITY AND FAILURE

Reliability

A measure of how a product maintains quality over time (Quality, in this context, is in terms of satisfactory performance under stated set of operating conditions).

Failure

Failure is a non-satifactory performance. Mechanical failure is any change or any design or manufacturing error that renders a component, assembly, or system incapable of performing its intended function.

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TYPICAL SOURCES OF FAILURE •Wear •Fatigue •Yielding •Jamming •Bonding weakness •Property change •Buckling •Imbalance

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CHECKLIST - DESIGN FOR RELIABILITY (DFR) 12

CHECKLIST - DESIGN FOR RELIABILITY (DFR) 13

CHECKLIST - DESIGN FOR RELIABILITY (DFR) 14

CHECKLIST - DESIGN FOR RELIABILITY (DFR) 15

CHECKLIST - DESIGN FOR RELIABILITY (DFR) 16

5.

EXAMPLES OF MECHANICAL FAILURE MODES 1.

Structural 2.

Thermodynamic 3.

Fluid 4.

Kinematic Hydraulic actuation fracture; excessive deflection, buckling overheating; reduction in efficiency leakage; distorted flow bearing seizure; reduction in accuracy of the relative movement the fitting leakage, static seal leak, fluid dirt contamination, actuator cylinder rupture 17

EXAMPLES OF MECHANICAL FAILURE MODES • Corrosion • Fracture • Material Yield • Electrical short • Open circuit • Buckling • Resonance • Fatigue • Deflections or deformations • Seizure • Burning • Misalignment • Wear • Binding • Overshooting (control) • Ringing • Leaking • Vibrations • Sagging • Cracking • Stall • Creep • Thermal expansion • UV deterioration • Acoustic noise • Scratching and hardness • Loose fitting • Radiation damage • Erosion • • Thermal shock 18

DESIGN CONSIDERATIONS IN DESIGN FOR RELIABILITY 1. Initial manufacturing costs may increase as the reliability is improved - however, overall life-cycle costs can decrease.

2. The ideal objective is to achieve operating reliability while limiting the impact on manufacturing costs.

3. The designer can control reliability by appropriate combination of: – a sound concept – – – – careful detailed design high-quality components redundancy of critical elements ample safety factors 19

FACTOR OF SAFETY AS A DESIGN VARIABLE Factor of Safety is a factor of ignorance. In relation to stress, it is used to: 1.

To reduce the allowable strength (eg. yield or ultimate strength of a material) to a lower level for comparison with the applied stress 2.

To increase the applied stress for comparison with the allowable strength 3.

As a comparison for the ratio of the allowable strength to the applied stress 20

FACTOR OF SAFETY AS A DESIGN VARIABLE FS = FS material x FS stress x FS geometry x FS failure analysis x FS reliability

Estimating the contribution for material:

FS material = 1.0

If the properties for the material are well known; if they have been experimentally determined from tests on a specimen known to be identical to the component being designed; and from tests representing the loading to be applied.

FS material = 1.1

If the material properties are known from a handbook or are manufacturer’s values.

FS material = 1.2

– 1.4

If the material properties are not well known.

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FACTOR OF SAFETY AS A DESIGN VARIABLE FS = FS material x FS stress x FS geometry x FS failure analysis x FS reliability

Estimating the contribution for the load stress:

FS stress = 1.0 - 1.1

If the load is well defined as static or fluctuating; if there are no anticipated overloads or shock loads; and if an accurate method of analyzing the stress has been used.

FS stress = 1.2 - 1.3

If the nature of the load is defined in an average manner, with overloads of 20%-50%, and stress analysis method may result in errors less than 50%.

FS stress = 1.4 - 1.7

If the load is not well known or stress analysis method is of doubtful accuracy.

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FACTOR OF SAFETY AS A DESIGN VARIABLE FS = FS material x FS stress x FS geometry x FS failure analysis x FS reliability

Estimating the contribution for geometry (unit-to-unit variations):

FS geometry = 1.0

If tolerances are tight and held well FS geometry = 1.0

If tolerances are average FS geometry = 1.1

– 1.2

If the dimensions are not closely held.

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FACTOR OF SAFETY AS A DESIGN VARIABLE FS = FS material x FS stress x FS geometry x FS failure analysis x FS reliability

Estimating the contribution for failure analysis:

FS failure theory = 1.0 - 1.1

If the failure analysis to be used is derived for the state of stress, as for uniaxial or multiaxial static stresses, or fully reversed uniaxial fatigue stresses.

FS failure theory = 1.2

If the failure analysis to be used is a simple extension of the above theories, such as for multiaxial, fully reversed fatigue stresses or uniaxial nonzero mean fatigue stresses.

FS failure theory = 1.3 - 1.5

If the failure analysis is not well developed, as with cumulative damage or multiaxial nonzero mean fatigue stresses.

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FACTOR OF SAFETY AS A DESIGN VARIABLE FS = FS material x FS stress x FS geometry x FS failure analysis x FS reliability

Estimating the contribution for reliability:

FS reliability =1.1

If the reliability for the part need not be high, for instance, less than 90 percent.

FS reliability =1.2-1.3

FS reliability =1.4-1.6

If the reliability is an average of 92% - 98%.

If the reliability must be high, eg. greater than 99% 25

FACTOR OF SAFETY AS A DESIGN VARIABLE In order to use the factor of safety, failure mode must be known! It is not always excessive stress that causes structural failure!

Forgetting about failure mode such as displacement or buckling is a very common error.

support Load Buckled shape 26

RELIABILITY AND FAILURE Reliability A measure of how a product maintains quality over time (Quality, in this context, is in terms of satisfactory performance under stated set of operating conditions).

Failure Failure is a non-satifactory performance. Mechanical failure is any change or any design or manufacturing error that renders a component, assembly, or system incapable of performing its intended function.

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Risk Hazard RISK AND HAZARD likelihood of harm (accident) potential to cause harm (accident) Risk Assessment The process and procedures of identifying, characterizing, quantifying and evaluating risks and their significance.

Risk Management Use of techniques that either minimize the probability of an accident or alleviate its consequences.

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RISK ASSESSMENT

Three types of risk in engineering systems

Risks that were acceptable at the time of design, but are

now

considered health or safety hazards Risks that come from abnormal conditions that are not part of the basic design concept Risks associated with design errors

In general, risk assessment techniques attempt to

Identify potential causes of failure Rate them in terms of criticality Establish the conditions under which the failure has greatest likelihood of occurance and/or gravest consequence 29

DESIGN FOR RELIABILITY TOOLS IN MME2259A

– The designer needs some means of determining the reliability of design alternatives and procedures to identify potential hazards and failures: – – – Preliminary Hazard Analysis (PHA) Fault Tree Analysis Failure Mode And Effect Analysis (FMEA) 30

HAZARD ANALYSIS

Hazard Analysis

is the designer’s response to ensuring product safety.

• Important to foresee every conceivable way that the product will be

used

and

misused

(including transport, storage, maintenance, repair, …) •

Careful records

must be maintained, this forces the engineer to justify design decisions + records may become legal documents.

• Detailed hazard analysis establishes the

rationale

for dealing with all possible hazards (this may be needed in the future if accidents occur or litigation).

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HAZARDS CHECKLIST (AN EXAMPLE) 32

HAZARDS CHECKLIST (AN EXAMPLE) 33

PRELIMINARY HAZARD ANALYSIS (PHA)

System hazards are not (yet) failures.

Failures may contribute to hazards, but hazards are system states that, combined with certain environmental conditions, cause accidents. 34

PRELIMINARY HAZARD ANALYSIS (PHA)

PHA is a broad study made in the early stages of design. The PHA consists of breaking the engineering system down into subsystems or components, and for each item answering the following questions:

1.

2.

3.

4.

5.

6.

7.

What is the subsystem or item under investigation?

What is the mode of operation?

What is the hazardous condition and why?

What event(s) triggers the hazardous condition?

What is the hazardous condition?

What event(s) triggers the potential accident?

What is the potential accident?

8.

9.

What is the possible effect(s) of the accident?

What is the classification of the severity of the hazard?

10. What measures are taken to contain or prevent occurences?

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PRELIMINARY HAZARD ANALYSIS (PHA)

Severity level classification from an aviation standard: Class I: Catastrophic Un-survivable accident with hull loss.

Class II: Critical Survivable accident with less than full hull loss; fatalities possible Class III: Marginal Equipment loss with possible injuries and no fatalities Class IV: Negligible Some loss of efficiency Procedures able to compensate, but controller workload likely to be high until overall system demand reduced. Reportable incident events such as operational errors, pilot deviations, surface vehicle deviation.

safeware-eng.com

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FAULT TREE ANALYSIS

Fault tree analysis is a technique that provides a systematic description of possible occurrences in a system that can result in “failure” or “severe accident”.

The four basic steps involved in developing a fault tree are as follows: 1. Develop the top undesired event of the system to be studied.

2. Develop a thorough understanding of the system under consideration.

3. Determine the logical interrelationships of higher-level and lower-level fault events.

4. Construct the fault tree using logical symbols.

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FAULT TREE ANALYSIS

Basic symbols used in fault trees: Resultant fault event:

a rectangle denotes a fault event that results from a combination of failure events through the input of a logic gate, such as an AND gate or an OR gate

Basic fault event:

a circle denotes a basic fault event or the failure of an elementary component. The values of the parameters, such as failure probability, unavailability, failure rate, and repair rate, associated with the basic fault event are obtained from empirical studies or other sources

AND gate:

denotes that an output fault event occurs if

all

of the input fault events occur

OR gate:

denotes that an output fault event occurs if one or more of the input fault events occur 38

FAULT TREE ANALYSIS

AND

gate

OR

gate Resultant fault event Gate

AND

All basic fault events must occur in order for resultant event to occur

OR

At least one basic fault event must occur in order for the resultant event to occur Basic fault events 39

FAULT TREE ANALYSIS

The output fault occurrence probabilities for

AND

gate is:

F AND

 

i

1

Fi

where

F AND m F i

is the probability of occurrence of the AND gate output fault event is the number of independent input fault events is the probability of occurrence of input fault event

i

, for

i

=1, 2, …

m

OUTPUT EVENT

F AND

INPUT EVENTS

F 1

AND

F 2

Example: F 1 = 0.1

F 2 = 0.05

F AND = 0.1 * 0.05 = 0.005

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FAULT TREE ANALYSIS

The output fault occurrence probabilities for

OR

gate:

F OR

 1 

i m

 1 (1 

Fi

) where

F OR

is the probability of occurrence of the OR gate output fault event

m

is the number of independent input fault events

F i

is the probability of occurrence of input fault event

i

, for

i

=1, 2, …

m

OUTPUT EVENT

F OR

OR

Example: F 1 = 0.1

F 2 = 0.05

F OR = 1 – (1 - 0.1)*(1- 0.05) = 0.145

INPUT EVENTS

F 1 F 2

Note:

For small (i.e. less than 10 percent) occurrence probabilities of input fault events of the OR gate, the above equation reduces to:

F O

R



i m

 

F i

1 41

FAULT TREE ANALYSIS

Example:

Develop a fault tree for a system comprising of a windowless room with one switch and three light bulbs. The switch can only fail to close, and the top undesirable event is the room without light.

TOP EVENT OR FAULT EVENT OR AND BASIC FAULT EVENT

[Dhillon 1996]

BASIC FAULT EVENT

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[Dhillon 1996]

FAULT TREE ANALYSIS

Example:

(cont'd) Assume that the probabilities of occurrence of basic fault events A ,B, C, D. E, and F are 0.1, 0.12,0.15, 0.15, 0.15, and 0.08, respectively.

Calculate the probability of occurrence of the top event (T) (i.e. the room without light).

F O

1 

R

(1   1 

i m

 1 (1  0.208)(1 

Fi

)  0.003375)(1   0.27

0.08)  Probability of having a room without light is 27%

OR OR AND

F O

R

1  (1 

i m

 1 (1 0.1)(1  

Fi

) 0.12)   0.208

F AND

 1

Fi



i

0.15 * 0.15 * 0.15

 0.003375

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FAULT TREE ANALYSIS

Develop a fault tree for a climb. The top undesirable event is not making it to the summit by at least one member of your team You climb in a team which can not split. Team is lead by a guide.

Each team member on your team has 40% probability of failure Guide has 10% probability of failure Weather/terrain has 20% probability of turning bad What is the probability of the climb failure?

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TWO CLIMBERS PLUS GUIDE

1 – (1- 0.676) (1- 0.2) = 0.74

0.74

OR 1 – (1- 0.4) (1- 0.4)(1- 0.1) = 0.676

0.676

OR

0.4

0.4

0.1

0.2

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SINGLE CLIMBER PLUS GUIDE version 1

1 – (1- 0.46) (1- 0.2) = 0.568

0.568

OR 1 – (1- 0.4) (1- 0.1) = 0.460

0.460

OR

0.2

0.4

0.1

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SINGLE CLIMBER PLUS GUIDE version 2

1 – (1- 0.24) (1- 0.1) = 0.568

0.316

OR 1 – (1- 0.2) (1- 0.05) = 0.24

0.24

OR

0.1

0.2

0.05

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FAILURE MODES AND EFFECTS ANALYSIS (FMEA)

Failure modes

FMEA

Failure effects Failure effects Failure modes

FMECA

Failure criticality 48

FAILURE MODES AND EFFECTS ANALYSIS (FMEA)

Failure Mode and Effect Analysis (FMEA) is a very common analysis method used to improve product reliability and safety. It is used to identify: how a product can fail (its failure modes) the causes of those failures the effects of the failures on system/product performance 49

FAILURE MODES AND EFFECTS ANALYSIS (FMEA)

Failure Modes and Effects Analysis is a detailed analysis of the malfunctions that can be produced in the components of an engineering system. Similar to the QFD approach, FMEA techniques involve charts that are developed, amended and updated over time.

Primary Goal of Failure Modes and Effects Analysis is to try to identify and list all possible ways in which product or a process could fail to conform to its specified requirements.

[McMahon and Bowne 1993] 50

FAILURE MODES AND EFFECTS ANALYSIS (FMEA)

FMEA poses the following questions: What can fail/go wrong with each component of a product?

To what extend it might fail, and what are the potential hazards produced by the failure?

What steps should be implemented to prevent the failure?

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FAILURE MODES AND EFFECTS ANALYSIS (FMEA)

Types of Failure Modes and Effects Analyses There are several types of FMEAs, some are used much more often than others. FMEAs should always be done whenever failures would mean potential harm or injury to the user of the end item being designed. The types of FMEA are: System - focuses on global system functions Design - focuses on components and subsystems Process - focuses on manufacturing and assembly processes Service - focuses on service functions http://www.npd-solutions.com/fmea.html

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FAILURE MODES AND EFFECTS ANALYSIS (FMEA)

If performed early in the design process, FMEA supports the product development in reducing the risk of failure by: Aiding in the objective evaluation of design requirements and design alternatives Aiding in the initial DFM and DFA requirements Increasing the probability that potential failure modes and their effects on system operation have been considered in the design process Providing additional information to aid in the planning of thorough and efficient design improvements and development testing Providing an open issue format for recommending and tracking risk reducing action Providing future references to aid in analyzing filed concerns, evaluating design changes and developing advanced designs 53

FAILURE MODES AND EFFECTS ANALYSIS (FMEA)

Top-down, functional approach This approach is used in early design, before parts have been identified. The goal here is to look for logic errors in the expected function and operation of a product. One identifies a failure mode for the product as a whole, then traces its causes "down" into subsystems or sub-functions. Bottom-up, "hardware" approach This approach is used when specific parts or at least major assemblies have been designed. The goal here is to look for physical errors in the detailed design/manufacture of parts. One identifies a failure mode, and then follows its effects "up" to the product as a whole in order to predict how the product will respond to the failure 54

FAILURE MODES AND EFFECTS ANALYSIS (FMEA)

Basis steps: 1. A complete list of the components and their function is prepared.

2. From an analysis of the operating and environmental conditions, the failure mechanisms that could affect each component are determined.

3. The failure modes of all components are identified.

4. Each failure mode is analyzed as to whether it has an effect on the entire system or product 5. The preventative measures or corrective actions that have been taken to control or eliminate the hazard are listed.

6. The probability of failure of each component is listed, and the probabilities of failure of the subassemblies and complete system are caluclated from reliability theory.

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RANKING PROCEDURE FOR FMEA

Risk Priority Number (RPN)

and

detection

-is assigned to each failure mode based on of failure cause.

occurrence

,

severity

,

RPN

=

R occurrence

x

R severity

x

R detection

where 1 <

RPN

< 1000

R occurrence

Identify every possible cause of each failure, and rank each cause according to the likelihood of its occurrence on a scale of 1 to 10: 1 - cause will almost never arise ( ie. 1 in 10 6 ) 5 - occasional failure ( ie. 1 in 400) 10 – regular occurrence 56

RANKING PROCEDURE FOR FMEA

Risk of occurrence http://egweb.mines.edu/eggn491/lecture/FMEA/FMEA%20Homework.htm

57

RANKING PROCEDURE FOR FMEA

Risk Priority Number (RPN)

-is assigned to each failure mode based on

occurrence

,

severity

, and

detection

of failure cause.

RPN

=

R occurrence

x

R severity

x

R detection

where 1 <

RPN

< 1000

R severity

Rate the

severity

of each possible failure on a scale of 1 to 10 1 - the customer would hardly notice the failure 5 - customer would be made uncomfortable or annoyed by the failure 10 - a major failure such as a significant safety hazard or non-compliance with a government regulation 58

RANKING PROCEDURE FOR FMEA

Severity of effect of failure http://egweb.mines.edu/eggn491/lecture/FMEA/FMEA%20Homework.htm

59

RANKING PROCEDURE FOR FMEA

Risk Priority Number (RPN)

-is assigned to each failure mode based on

occurrence

,

severity

, and

detection

of failure cause.

RPN

=

R occurrence

x

R severity

x

R detection

where 1 <

RPN

< 1000

R detection

List current technologies being used to detect a failure cause and assign a likelihood of detection

prior to failure

to each failure based on a scale of 1 to 10: 1 – almost certain detection 10 – practically undetectable mode 60

RANKING PROCEDURE FOR FMEA

Probability of detection http://egweb.mines.edu/eggn491/lecture/FMEA/FMEA%20Homework.htm

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RANKING PROCEDURE FOR FMEA

Risk Priority Number RPN = R occurrence x R severity x R detection RPN = 1 failure is highly unlikely and unimportant RPN = 30 it is OK RPN = 100 RPN = 1000 failure will occur hazardous and harmful failure will occur 62

STEPS IN FMEA … AGAIN

http://www.suppliermanager-online.com/training/corporation/fmea_training.pdf

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FMEA EXAMPLE OF A CAR FRONT DOOR 7 x 6 x 7=294 Front door Corroded interior Upper edge of wax too low Durability test T-118 6 Wax layer too thin 4 7 7 7 7 7 x 4 x 7=196 Causes of failure Failure effect on product/system Failure mode (how product can fail?)

294 196

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