Reliability engineering:

• Reliability is the probability that a system or component will perform

without failure for a specified period of time under specified operating

conditions.

• Reliability engineering is the study of the causes, distribution and

prediction of failure.

• Assessing the reliability of a design (product or process) is an essential

step in modern quality-engineering products.

Causes of Unreliability

These causes fall into five categories:

Design mistakes: failure to include important operating factors,

poor materials selection, incomplete information on loads and

environmental conditions.

Manufacturing defects

Maintenance

Exceeding design limits

Environmental factors: subjecting equipment or products to

environmental conditions for which it was not designed.

Failure

• Failure is the inability of a process or product to function as desired.

That is the performance will drop to a level below predetermined

acceptance standards.

• Minor failures are not necessarily bad, they give us the method to

control the process or improve the product.

• If the minor failures occur very frequently, however, there is a chance

(risk) that some of them may escalate to a higher level.

Event escalation model:

• Before this can occur, three barriers, PEOPLE,PLANT and PROCEDURES must be overcome.

PEOPLE:

• Whose competence, training and monitoringenable them to put and correct the conditionsthat cause minor failure and therefore reducetheir impact.

PLANT:

• Consisting of the hardware (or software). Variousprotective systems are provided to prevent theescalation of minor failures. First aid boxes, fireextinguishers, furnace protection systems,emergency shut down or release systems.

People

Plant

Procedure

Minor failures

Severe failures

PROCEDURES:

• Are the means of transferring other people’s knowledge and experience to

those operating the process. Manufacturers will tell you how to operate their

equipment and software vendors will give you navigation guides and help

screen. Some of the knowledge may have been gained as a result of earlier

failures.

• The three barriers act in combination with one another. Well-trained and

motivated people can compensate for the lack of procedures.

• A well documented and organised plant can be managed with relatively few

trained people.

Few (random) failuresInfant

mortality

Failures due

to wear-out

Time

Failure Rate Curve

Reliability Engineering Measurement:

• Many techniques have been established for measuring and improving

the reliability of a product design.

• Generally, reliability engineering is identified based on risk

assessment methods.

Hazards Analysis

Failure Mode and Effect Analysis (FMEA)

Failure Mode Effect and Criticality Analysis (FMECA)

Fault Tree Analysis

1. Failure Mode and Effect Analysis (FMEA)

• FMEA (and FMECA) is a group of activities designed to:

Recognize and evaluate the potential failure of a product/process and its

effects

Identify actions which could eliminate or reduce the chance of potential

failure occurring

Document the process

• Product/ Design FMEA is a tool used to assure that potential product

failure modes and

• Their associated causes have been considered and addressed in the

design or manufacturing process

• The FMEA approach is to:

1. Identify known or potential failure modes, which may affect a product

2. Identify those design or process elements which may cause a product to fail

3. Assess and prioritise potential failure modes for corrective action

4. Assess the effectiveness of correction action and provide follow up

• Key resources necessary to conduct successful FMEA programs:

Commitment of top management

Knowledge in: design, manufacturing, assembly, service, quality, reliability

Advantages of FMEA:

Enhance design and manufacturing efficiencies

Reduce late change crises

Minimise exposure to product failures

Increase business records

Add to customer satisfaction

Limitations of FMEA

Training of employees

Initial impact on product & manufacturing schedules

Financial impact required to upgrade design, manufacturing, process

equipment and tools

FMEA Procedure

There are two phases in FMEA

To identify the potential failure modes and their effects.

To perform criticality analysis to determine the severity of the failure modes.

• We need to construct a table with columns for:

Component

Failure mode

Effect of failure

Cause of failure

• Add columns for estimating “value”

• Determine value (scale of 1 – 10) for:

Occurrence 1 – low, 10 - high

Severity 1 – minor, 10 - serious

Problem of detection 1 – certainty, 10 – nearly impossible

• Calculate Risk Priority Number

Risk priority number (RPN) = occurrence x severity x problem of detection

• For each, give corrective action

FAILURE MODE AND EFFECTS ANALYSIS (FMEA)

Subsystem/Name: DC motor P = Probabilities (chance) of Occurrences Final Design: d/m/y

Model Year/Vehicle(s): 2000/DC motor S = Seriousness of Failure to the Vehicle Prepared by:

D = Likelihood that the Defect will Reach the customer

R = Risk Priority Measure (P x S x D) Reviewed by: Name

FMEA Date (Org.): d/m/y (Rev.) d/m/y

1 = very low or none 2 = low or minor 3 = moderate or significant 4 = high 5 = very high or catastrophic

No

.Part

Name

Part

No.

Functi

on

Failure

Mode

Mechanism

(s) &

Causes(s)

of Failure

Effect(s)

Of

Failure

Curre

nt

Contr

ol

RPN Recommen

ded

Corrective

Action(s)

Action

(s)

Taken

P S D R

1 Position

Control

ler

Receiv

e a

deman

d

positio

n

Loose

cable

connecti

on

Incorrec

t

demand

signal

Wear and

tear

Operator

error

Motor

fails to

move

Position

controlle

r

breakdo

wn in a

long-run

2

4

4

4

1

3

8

48

Replace

faulty wire.

Q.C

checked.

Intensive

training for

operators.

Failure Mode Effect and Criticality Analysis (FMECA)

• The term criticality is important because it develops priorities where the

design team should be spending it resources.

• Criticality refers to

how often a failure will occur,

how easy it is to diagnose and

whether it can be fixed.

• The purpose of an FMECA is to :

analyse the probable causes of product failure,

determine how the problem affects the customer

Identify which process-control variable to focus on for prevention and

detection.

Quantify the effect on the customer.

• One of the key results of FMEA or FMECA process is the document it

produces, which is structured to:

facilitate the thinking process,

focus the mind on what is important and

document the thinking process in a standardised easy-to-follow manner.

Techniques of Failure Analysis

• When the problem of determining the cause of failure and proposing corrective

action must be faced, there is a definite procedure for conducting a failure

analysis.

• There are a number of reasons why a problem of failure should be

investigated.

for scientific purposes

In order to apportion blame (product liability)

To identify and eliminate the cause of failure

To improve performance

1. Visual Examination:

• Without doubt visual examination carried out by a skilled and competent investigator is by far the most important aspect of mechanical failure analysis.

• It identifies the mechanism of failure.

• Whenever possible, visual examination should be carried out on site.

• Visual examination should be documented with photographs

• The following critical pieces of information should be obtained during the on site inspection:

location of all broken pieces relative to each other

Identification of the origin of failure

Orientation and magnitude of stresses

Direction of crack propagation and sequence of failure

presence of obvious material defects, stress concentrations..

Presence of oxidation, or corrosion products

2. Background History and Information

• A complete case history on the component that failed should be developed as

soon as possible.

Name of item, identifying numbers, owner, manufacturer

Function of item

Data on service history including inspection of operating logs and records

Discussion with operating personnel

Documentation on materials used in the item

Information on manufacturing and fabrication methods used including any

codes or standards

Documentation on inspection standards and techniques used

Date and time of failure, temperature and environmental conditions

Date and time of failure, temperature and environmental conditions

Documentation on design standards

3. Macroscopic Examination

• This is part of the visual examination. Macro-examination used x5 to x25

magnifications where the object can remain in one piece.

• Often it is possible to identify the type of fracture from macroscopic examination

4. Microscopic Examination

• After visual examination, micro-examination is the most important aspect of

failure analysis.

• This is made at magnifications greater than x100. The investigator covers the

use of instruments as the optical microscope, SEM, TEM and EDAX.

• Correct micro-examination requires firstly the professional skill to identify the

areas where specimens should be cut.

• Secondly, there is the technical skill to correctly cut, mount, polish and etch the

selected specimens.

• Finally, there must be the metallurgical ability to recognise the structure and

effects produced.

5. Additional Tests

Chemical Analysis

• Chemical analysis of metals or alloys is frequently included in failure analysis

reports.

• It is part of the process of identifying the material and finding whether it meets

the specifications.

Mechanical Testing:

• It is important to measure the mechanical properties (hardness) of an unused specimen of the material that failed.

Non-Destructive Testing (NDT)

• The aim is to search for defects.

• The various NDT techniques include:

1. Liquid Penetrant Inspection (LPI)

2. Magnetic particle Inspection (MPI)

3. Radiography

4. Eddy Current

5. Ultrasonic Inspection

6. Report of Failure

• The technical report is an end product of the failure analysis and should be

written in clear language which can be understood by the client.

• Reports should have a format and should contain the following:

Introduction and background

Conclusion

Recommendations

Visual examinations

Technical investigation (NDT, macro and micro-examination, mechanical

tests, chemical analysis etc..

Discussion