Materials Quality Control, Assurance and Selection · quality control and assurance. There are several objectives for this course: 1.) To introduce the parameters that are important

Materials Quality Control, Assurance and Selection

Dr. Emmanuel Kwesi Arthur

Email: [email protected]

Phone #: +233541710532

Department of Materials Engineering,

Kwame Nkrumah University of Science and Technology, Kumasi, Ghana

©2019

Course Code: MSE 456

1

Goal and Objectives

Goals: This course is a required course in Metallurgical and Materials Engineering. The major goal is to provide an introduction to materials selection in relation to the design process. It also focuses on materials quality control and assurance.

There are several objectives for this course:

1.) To introduce the parameters that are important to design, to understand how they are interrelated, to understand how they relate to the materials selection process, and to use these concepts in engineering design.

2.) To develop the ability to use modern software (CES EduPack) in the materials selection and design process.

3.) To develop the ability to obtain materials property and processing data needed in the materials selection and design process from both handbooks and electronic sources.

4.) To provide an introduction to team-oriented projects that introduce basic approaches to product design and materials selection

5.) To introduce the common material quality control and assurance methods used in materials manufacturing industries.

Resources:

Text :

―Materials: engineering, science, processing and design‖ by M.F. Ashby, H.R. Shercliff and D. Cebon, Butterworth Heinemann, Oxford 2007, Chapters 1 and 2

―Materials Selection in Mechanical Design‖, 4th edition by M.F. Ashby, Butterworth Heinemann, Oxford, 2006, Chapters 1 - 3.

Computer Software:

CES EduPack 2013 Design Software (grantadesign.com). We will be using this software throughout the semester. It can be used as a materials database, a processing database, and a materials selection tool. It will be installed on students‘ computers for practice.

Syllabus: Attendance is your job – come to class!

Or our regularly scheduled time (Tues. 4:00-6:00 pm & Thurs. 8:00 – 9:00 am)

Homeworks

There will be homework in the form of problem sets and projects. The projects will focus on materials selection and design and will frequently include using the materials selection software, or library and web research.

The homework will typically be shorter assignments related to the material being covered in lecture.

Don‘t copy from others; don‘t plagiarize – its just the right thing to do!!

Tutorials – by Fuseini Abdullah (TA)

Grading

Class Attendance, Pop Quizzes and Assignments – (5% of your grade!)

Mid Semester Exams – (15%) Group Project and Presentation (10%) End of Semester Exams (70%)

Homework: Homework problems will be uploaded on my website. Each student will turn in homework to the TA one week after it is assigned. On the day homework is due, students will be randomly selected to solve selected homework problems, explaining to the class how each problem is worked. Students are encouraged to work together on homework. Students will be evaluated on both the quality of their written answers and board presentations.

Design Presentation: The class will be divided evenly into groups for a materials selection in design projects. Projects for each group will be assigned by the lecturer. Each group will write a report on their respective project, as well as make an oral presentation to the class.

Exams: Exams will be based on homework and information provided in lecture, tutorials and assigned reading. All exams will be closed book. The final will be cumulative. Relevant materials selection charts, etc. will be provided.

1. Read the relevant material in the Ashby book (preferably before the lecture topic)

2. Review and understand the examples given in the book.

3. Do the assigned homework. If you are having difficulty with a particular concept, work additional problems given in the book on that topic that have the answers given in the back of the book.

4. Seek help: tutors, etc.

Academic success is directly proportional to the amount of time devoted to study.

Suggestions for success in this class:

Credits: 3 Credit Hours –Lecture Prerequisite: No prerequisite, however, knowledge in strength of materials and core materials courses is plus. Office Hours: I have an open door policy. If I am in my office, feel free to stop in and ask questions about the class or any other materials questions you may have. If you would like to meet at another time, please send me an email with several available times. Academic Dishonesty: In general, academic dishonesty will not be tolerated. You will be practicing engineers in a few months. Integrity and competence are critical to your professional success. Developing bad habits in university will hinder your professional development and will weaken the prestige of your degree.

Design Stage

Unit Objective...

Introduce fundamental design concepts in

Materials Selection

You will learn about:

• design

• how structure dictates properties

• how processing can change structure

This unit will help you to: • use materials properly

• realize new design opportunities with materials

8

MATERIALS SELECTION

Introduction

Materials selection is an important part of a larger process of creating new solutions to problems. This larger process is called ――Engineering Design‖‖

Design of engineering components is limited by the available materials, and new designs are made possible by new materials

To see how important is the material selection in the design, consider the definition of ――engineering‖‖ used by ABET in the U.S.A

According to Accreditation Board for Engineering and Technology (ABET), Engineering is the profession in which a knowledge of the mathematical and natural sciences gained by study, experience, and practice is applied with judgement to develop ways to utilise

economically the materials and forces of nature for the benefit of mankind

Materials Selection

Materials Selection

Why Materials Selection

An incorrectly chosen material may lead not

only to part failure, but also unnecessary

life-cycle cost.

Selection of material is also related with

processing of material.

Hence, the designer must seek for the best

combination of design-material-process.

Functional Requirements

For satisfying the need, designer must determine essential and desirable features of the design.

They are expressed in the form of ―functional requirements‖ concerning performance characteristics of materials (i.e. material properties).

As it is impossible to satisfy all requirements to the same degree, they are arranged in the order of importance to identify the areas of compromise.

Design Limitations Furthermore, a design must be in compliance with inevitable ―design

limitations‖.

Such requirements are expressed by means of 3M rule:

Manufacturing, Money, Maintenance

Manufacturing requirements are logically the first to be considered.

Hence, the designer must consider functional merits of the material as well as its , ability to be machined, shaped, formed, cast, welded, and so on.

Money (economic) requirements are based on the final product cost, which is composed of raw material cost and production costs with overheads. The cost of any product should be as high as the customers can pay for it.

Finally, maintenance (service life) requirements will define whether replacement or repair is required. They depend upon size of the part, extent of possible damage, facilities of the customers, and the acceptable level of costs.

Failure of Materials

Failure happens when a design is no longer able to satisfy any of functional requirements.

Failures not only cause costly damage, but also may lead to loss of lives as in airplane crashes.

In most design problems, primary concern is to minimize the possibility of a premature failure in service. The service life can be in seconds (in case of space applications) or many years (in case of bridges).

Possible failure modes during service are as follows:

Excessive deformation: yielding, buckling, stress rupture (creep)

Fracture: sudden brittle, fatigue (progressive), time dependent (creep)

Inordinate wear: abrasion

Deterioration: chemical (corrosion or oxidation), embrittlement (ductile to brittle transition), irradiation, natural (fungus, other growths)

In practice, it is impossible to predict failure mode of a part under severe service conditions.

Some failures happen soon after the part is in service, which are covered by a factor of safety.

However, time dependent failures are difficult or even impossible to avoid by applying factor of safety. In such cases, parts are withdrawn from service and tested for reliability. Such specific data are not found in general reference books.

Materials Selection

Classification of Engineering Materials

Machine Elements

18

Materials Selection

MATERIALS SELECTION

MATERIALS SELECTION IN DESIGN-

BASICS

Intro Lecture. Design Stage:

the first steps of optimised

selection

The design process and material search space

Product specification

Concept

Embodiment

Detail

Market need

Problem statement

Final choice

Material search space

Screen

Screen

Rank

All materials

Increasing

constraints

Material & process needs

Choice of material family

(metals, ceramics, polymers..)

Choice of material class

(Steel, Al-alloy, Ni-alloy…..)

Choice of single material

(Al-2040, Al-6061, Al-7075…..)

Need – Concept -- Embodiment

Concepts Need

Embodiments

Direct pull Levered pull Spring assisted pull Geared pull

Embodiment -- Detail

Methods of Material Selection

The common methods of material selection are as follows:

1. Performance indices (including the use of Ashby charts)

2. Decision matrices

– Pugh selection method

– Weighted property index

3. Selection with Artificial Intelligence tools (i.e. Expert Systems)

4. Selection with Computer-Aided Databases

5. Value analysis

6. Failure analysis

7. Benefit-cost analysis

We will be focusing on computer-Aided Databases, performance indices and weighted property index.

The decision-making strategy

Design requirements

expressed as

Constraints and

Objectives

Normative information

Material attributes

Process attributes

including prompts for

Intuitive estimation

Factual information

Final selection

Comparison engine

Screening

Ranking

Documentation

Methodic information

Translation to create Normative information

Translation: “express design requirements as constraints”

Constraints What essential conditions must it meet ?

Free variables Which design variables are free ?

Design requirements

Objectives What measure of performance is to

be maximized or minimized ?

Choice of

material

Be strong enough

Conduct electricity

Tolerate 250 oC

Be able to be cast

Cost

Weight

Volume

Eco-impact

Function What does the component do ?

A label

QUESTIONS

Translation: a heat sink for power electronics

Power micro-chips get hot.

They have to be cooled to

prevent damage.

Free variable Choice of material

Constraints

1. Max service temp > 200 C

2. “Good electrical insulator”

3. “Good thermal conductor”

(or T-conduction > 25 W/m.K)

Translation

Function Heat sink Keep chips below 200 C

without any electrical

coupling.

Design requirements

A Limit stage

Thermal properties Min. Max

Mechanical properties

Maximum service temperature C

Thermal conductivity W/m.K

Specific heat J/kg.K

Electrical properties

Electrical conductor

or insulator?

Good conductor

Poor conductor

Semiconductor

Poor insulator

Good insulator

Thermal properties Min. Max


Maximum service temperature C

Thermal conductivity W/m.K

Specific heat J/kg.K


Electrical conductor

or insulator?

Good conductor

Poor conductor

Semiconductor

Poor insulator

Good insulator

Screening using a LIMIT STAGE

Browse Select Search Print Search web

Screening: “Eliminate materials that can’t do the job”

2. Selection Stages

Graph Limit Tree

1. Selection data

Edu Level 2: Materials

Results X out of 95 pass

Material 1 2230 113

Material 2 2100 300

Material 3 1950 5.6

Material 4 1876 47

etc...

Ranking Prop 1 Prop 2

200

25

2000C

Screening using a GRAPH STAGE


File Edit View Select Tools

Don’t need

numbers!

1. Selection data



Material 1 2230 113

Material 2 2100 300

Material 3 1950 5.6

Material 4 1876 47

etc...


1000

0.1

Metals

Polymers &

elastomers Composites

Foams

1030 1 1010 1020

Ceramics

10

1

100

0.01

Electrical resistivity (.cm)

T-c

on

du

cti

vit

y (

W/m

.s)

PEEK

PP

PTFE

PEEK

PP

PTFE

WC

Alumina

Glass

WC

Alumina

Glass

CFRP

GFRP

Fibreboard

CFRP

GFRP

Fibreboard

Steel

Copper

Lead

Zinc

Aluminum

Steel

Copper

Lead

Steel

Copper

Lead

Zinc

Aluminum

Metals Polymers Ceramics Composites

2. Selection Stages

Graph Limit Tree

Screening using a TREE STAGE

Tree stage for material

Material

Ceramics Steels

Hybrids Al alloys

Metals Cu alloys

Polymers Ni alloys...

2. Selection Stages

Graph Limit Tree


1. Selection data


Process

Join

Shape

Surface

Cast

Deform

Mold

Composite

Powder

Prototype

Tree stage for process Results X out of 95 pass

Material 1

Material 2

Material 3

Material 4

etc...

Stacking selection stages

Pro

pert

y

Stacked stages


1. Selection data


Density

Modulus

Strength

T-conduction

2

100

10

200

Min Max

Process

Join

Shape

Surface

Cast

Deform

Mold

Composite

Powder

Prototype

2. Selection Stages

Graph Limit Tree


Material 1 2230 113

Material 2 2100 300

Material 3 1950 5.6

Material 4 1876 47

etc...


Translation: a CD case, an example of redesign


CD cases are made of polystyrene

(PS). They crack and scratch the

disks. Find a better material.

Injection-moldable

Contain and protect CD

better than the PS case.

As transparent as PS

Recylable

Design requirements

Function CD enclosure

Translation

Constraints

1. Can be injection molded

2. Toughness K1c > that of PS

3. Optically clear

4. Can be recycled

Optical properties

Transparency

Eco properties

Recycle

Optical quality

Transparent

Translucent

Opaque

3

Tree stage: injection mold 1

Fra

ctu

re t

oughness

Polystyrene

Keep these!

2

The CD case: the whole story

Select Level 2: Materials

Free variable Material


Translation

Constraints



3. Optically clear

4. Can be recycled

Documentation: the pedigree

Granta’s Web Portal (http://matdata.net) gives

indexed access to information providers’ web sites.

Documentation: “now that the number of candidates is small, explore their

character in depth”

Suppliers’

data sheets Handbooks Material

portals

Trade

associations

Documentation: the “pedigree” of surviving candidates

Quiz 1

1) The cases in which most CDs are sold have an irritating way of cracking and breaking. Which design-limiting property has been neglected in selecting the material of which they are made?

2) State two reasons why proper materials selection procedure should be used in choosing suitable material for a given application

Documentation with CES

Browse Select Search Print


Material 1

Material 2

Material 3

Material 4

Material 5

………..

Search web

Matdata.net Searches information sources

for selected record

1. Selection data


2. Selection Stages

Graph Limit Tree

Open the record

Age hardening ALUMINUM ALLOYS The material The high-strength aluminum alloys rely on age-hardening: a sequence of heat treatment steps that causes the precipitation of a nano-scale dispersion of intermetallics that impede dislocation motion and impart strength.

General properties Density 2500 - 2900 kg/m^3 Price 1.423 - 2.305 USD/kg

Mechanical properties Young's modulus 68 - 80 GPa Elastic limit 95 - 610 MPa Tensile strength 180 - 620 MPa Elongation 1 - 20 % Hardness - Vickers 60 - 160 HV Fatigue strength at 10

7 cycles 57 - 210 MPa

Fracture toughness 21 - 35 MPa.m^1/2

Thermal properties Thermal conductor or insulator? Good conductor Thermal conductivity 118 - 174 W/m.K

Age hardening ALUMINUM ALLOYS The material The high-strength aluminum alloys rely on age-hardening: a sequence of heat treatment steps that causes the precipitation of a nano-scale dispersion of intermetallics that impede dislocation motion and impart strength.

General properties Density 2500 - 2900 kg/m^3 Price 1.423 - 2.305 USD/kg

Mechanical properties Young's modulus 68 - 80 GPa Elastic limit 95 - 610 MPa Tensile strength 180 - 620 MPa Elongation 1 - 20 % Hardness - Vickers 60 - 160 HV Fatigue strength at 10

7 cycles 57 - 210 MPa

Fracture toughness 21 - 35 MPa.m^1/2

Thermal properties Thermal conductor or insulator? Good conductor Thermal conductivity 118 - 174 W/m.K

These are

often enough !

The four steps of selection:

1. Translation, giving constraints and objectives

2. Screening , using constraints

3. Ranking, using objectives

4. Documentation in CES, and http://matdata.net

The main points

CES allows Screening using

• Limit stages,

• Graph stages

• Tree stages and

• All three in any number and sequence

Pause for demo

Exercise: Stage 1, a tree stage

3.1 A material is required for a molded electrical

enclosure that may be used outdoors. There are

requirements on

Processing (this Stage)

Properties (Stage 2)

Price (Stage 3)

Apply Stage 1 – a Tree Stage Tree stage

ProcessUniverse

Shaping

Molding -- Insert

OK

Now add Stage 2 – next page

Browse Select SearchBrowse Select Search

Select from

materials or

process tree

1. Selection data

Edu Level 2: MaterialsEdu Level 2: MaterialsEdu Level 2: MaterialsEdu Level 2: Materials

2. Selection Stages

Graph Limit Tree

2. Selection Stages

Graph Limit Tree

Exercise: Stage 2, a limit stage

3.2 The material of the enclosure must have Hardness - Vickers > 8 HV

Be a good electrical insulator

Have dielectric strength > 10 MV/m

Be able to be recycled



Eco properties

Recycle

Good conductor

Poor conductor

Poor insulator

Good insulator

Hardness - Vickers 8 HV

Conductor or insulator?

Dielectric strength 10 MV/m



Eco properties

Recycle

Good conductor

Poor conductor

Poor insulator

Good insulator

Good conductor

Poor conductor

Poor insulator

Good insulator

Hardness - Vickers 8 HV

Conductor or insulator?

Dielectric strength 10 MV/m

Now add Stage 3 – next page


1. Selection data


2. Selection Stages

Graph Limit Tree

2. Selection Stages

Graph Limit Tree

Enter

limits

Exercise: Stage 3, a graph stage

3.3 The material of the enclosure should be as cheap

as possible. Find the four materials meeting all the

previous constraints that have the lowest price per kg.

Graph stage – Y-axis – Price

Hide all materials failing previous stages

Rank the final Results list by Price


1. Selection data


2. Selection Stages

Graph Limit Tree

2. Selection Stages

Graph Limit Tree

Choose

Y-axis 3. Results: 15 of 95 pass

Name Price (USD/kg) Polypropylene (PP) 1.41 - 1.62

Soda-lime glass 1.41 - 1.659

Polystyrene (PS) 1.476 - 1.574

Polyvinylchloride (tpPVC) 1.6 - 2.2

Polyethylene terephthalate (PET) 1.608 - 1.769

Polyethylene (PE) 1.718 - 1.89

Polyoxymethylene (Acetal, POM) 2.203 - 2.732

Polymethyl methacrylate 2.335 - 2.569

Acrylonitrile butadiene styrene (ABS) 2.511 - 2.952

Polyamides (Nylons, PA) 3.194 - 3.569

Polycarbonate (PC) 3.6 - 4.47

Polylactide (PLA) 3.667 - 4.584

Polyurethane (tpPUR) 3.723 - 4.45

Cellulose polymers (CA) 3.921 - 4.313

Polyetheretherketone (PEEK) 99.14 - 109

Assignment 1

1) What is meant by the design-limiting properties of a material in a given application?

2) There have been many attempts to manufacture and market plastic bicycles. All have been too flexible. Which design-limiting property is insufficiently large?

3) What, in your judgement, are the design-limiting properties for the material for the blade of a knife that will be used to cut fish?

4) What, in your judgement, are the design-limiting properties for the material of an oven glove?

5) What, in your judgement, are the design-limiting properties for the material of an electric lamp filament?

6) A material is needed for a tube to carry fuel from the fuel tank to the carburetor of a motor mower. The design requires that the tube can bend and that the fuel be visible. List what you would think to be the design-limiting properties.

Assignment 1

7) A material is required as the magnet for a magnetic soap holder. Soap is mildly alkaline. List what you would judge to be the design-limiting properties.

8) List three applications that, in your judgement, need high stiffness and low weight.

10) List three applications that, in your judgement, need optical quality glass.

Exercise 1

1) Designers need to be able to find data quickly and reliably. That is where the classifications come in. The CES system uses the classification scheme described in this unit. Before trying these exercises, open the Materials Universe in CES and explore it. The opening screen offers options—take the Edu Level 2: Materials.

2) Use the ‗Browse‘ facility in Level 2 of the CES Software to find the record for Copper. What is its thermal conductivity? What is its price?

3) Use the ‗Browse‘ facility in Level 2 of the CES Software to find the record for the thermosetting polymer Phenolic. Are they cheaper or more expensive than Epoxies?

4) Use the ‗Browse‘ facility to find records for the polymer-shaping processes Rotational molding. What, typically, is it used to make?

5) Use the ‗Search‘ facility to find out what Plexiglas is. Do the same for Pyroceram.

6) Use the ‗Search‘ facility to find out about the process Pultrusion. Do the same for TIG welding. Remember that you need to search the Process Universe, not the Material Universe.

Exploring design using CES

Quiz 2

1) Compare Young‘s modulus E (the stiffness property) and thermal conductivity λ (the heat transmission property) of aluminum alloys (a non-ferrous metal), alumina (a technical ceramic), polyethylene (a thermoplastic polymer) and neoprene (an elastomer) by retrieving values from CES Level 2. Which has the highest modulus? Which has the lowest thermal conductivity?

End of Unit 3

Lecture 4. Ranking:

refining the choice

Unit 3

Unit 3

This

Unit

Outline

Step 2 Screening: eliminate materials that cannot do the job

Step 3 Ranking: find the materials that do the job best

Step 4 Documentation: explore pedigrees of top-ranked

candidates

Step 1 Translation: express design requirements as constraints

and objectives

Selection has 4 basic steps

Exercises

More info:

• “Materials: engineering, science, processing and design”, Chapter 3, 4 and 6

• “Materials Selection in Mechanical Design”, Chapters 5 and 6

Analysis of design requirements

Express design requirements as constraints and objectives

Must be

Stiff enough

Strong enough

Tough enough

Able to be welded

A label Bike frame Design requirements

Constraints What essential

conditions must be met ?

Objectives What is the criterion

of excellence ?

Function What does the

component do ?

Free variable What can be

varied ?

Choice of

material

Minimize

Cost

Weight

Volume

Eco-impact

Common constraints and objectives

Case Study – Material Selection

Problem: Select suitable material for

bicycle frame and fork.

Steel and

alloys Wood

Carbon fiber

Reinforced

plastic

Aluminum

alloys

Ti and Mg

alloys

Low cost but

Heavy. Less

Corrosion

resistance

Light and

strong. But

Cannot be

shaped

Very light and

strong. No

corrosion.

Very expensive

Light, moderately

Strong. Corrosion

Resistance.

expensive

Slightly better

Than Al

alloys. But much

expensive

Cost important? Select steel

Properties important? Select CFRP

The CD case, with an objective



Translation

Constraints


2. Optically clear


4. Can be recycled

Injection-moldable

Contain and protect CD

better than the PS case.

As transparent as PS

Eco-friendly

As cheap as possible

Design requirements

OBJECTIVE Minimise material cost

Screening and ranking: the CD case

Volume of material in case, V, fixed

Density , cost per unit mass Cm

Material cost/case C = V Cm

OBJECTIVE Minimise material cost C

Rank on this index

Select Level 2: Materials

Fra

ctu

re t

oughness

Polystyrene

Keep these!

2

Ranking

2

1

3

Cost

metr

ic

Cm

Polycarbonate

Cellulose acetate

PMMA

Polystyrene

Surviving materials

Tree stage: injection mold 1

Optical properties

Transparency

Eco properties

Recycle

Optical quality

Transparent

Translucent

Opaque

3

Advanced ranking: modelling performance

3. Read off the combination of material properties that

maximises performance -- the material index

If the performance equation involves a free variable (other than

the material):

Identify the constraint that limits it.

Use this to eliminate the free variable in performance equation.

1. Identify function, constraints, objective and free variables

(list simple constraints for screening).

2. Write down equation for objective -- the “performance equation”.

4. Use this for ranking

The method:

Selection Procedure

Example 1: strong, light tie-rod

Minimize mass m:

m = A L (2) Objective

• Length L is specified

• Must not fail under load F Constraints

• Material choice

• Section area A. Free variables

Equation for constraint on A:

F/A < y (1)

Strong tie of length L and minimum mass

L

F F

Area A

Tie-rod Function

m = mass

A = area

L = length

= density

= yield strength

y

(or maximize ) ρσy /

Chose materials with smallest

yσρ

Eliminate A in (2) using (1):

y

LFmPerformance

metric m

Demo

The chart-management tool bar

Box selection

tool

Cancel

selection

Add text

Zoom

Add

envelopes

Un-zoom

Black and white

chart

Hide failed

materials

Grey failed

materials

Line selection

tool

Exercise: selecting light, strong materials (1)

4.1 The material index for selecting

light strong materials is

M =

where is the yield strength and

the density.

Make a Graph stage with these

two properties as axes

Impose a selection line (slope 1)

to find materials with the highest

values of M.

Add a Limit stage to impose the

additional constraint:

Elongation > 10%

/y

y

Results: Age-hardening wrought Al-alloys

Nickel-based superalloys

Titanium alloys

Wrought magnesium alloys


1. Selection data


2. Selection Stages

Graph Limit Tree

2. Selection Stages

Graph Limit Tree

Density

Str

en

gth

y

1

yHigh

Density

Str

en

gth

y

1

yHigh

Density

Modulus

Strength

Elongation

etc

10

Density

Modulus

Strength

Elongation

etc

10

Min Max

Exercise: selecting light, strong materials (2)

4.2 Repeat the selection of 4.1, but use the

Advanced facility to make a bar-chart with

the index

M =

on the Y-axis.

Impose a Box selection to find materials

with the highest values of M.

Add a Limit stage to impose the additional

constraint:

Elongation > 10%

/y

Ind

ex

y /

yHigh


1. Selection data


2. Selection Stages

Graph Limit Tree

2. Selection Stages

Graph Limit Tree

Density

Modulus

Strength

Elongation

etc

10

Density

Modulus

Strength

Elongation

etc

10

Min Max

List of properties Density

Modulus

Yield strength

etc

+ - */ ^ ( )

Yield strength /

Density


Modulus

Yield strength

etc

+ - */ ^ ( )+ - */ ^ ( )

Yield strength /

Density

Exercise: selecting materials for springs (1)

4.3 A material is required for a spring that may be

exposed to shock loading, and must operate in

fresh and salt water.

Constraints:

Fracture toughness > 15 MPa.m1/2

Very good durability in fresh and salt water

Objective:

Maximise stored elastic energy

y

Strain

Str

es

s

E2

1

2

12y

yy

Elastic energy

The best materials for

springs are those with the

greatest value of

the index

E

2y

Make a graph with

Young’s modulus E on the X-axis

Yield strength on the Y-axis

Put on a line of slope 0.5 (corresponding to power 2)

Select materials above the line

Add the other constraints using a limit stage

y


1. Selection data


2. Selection Stages

Graph Limit Tree

2. Selection Stages

Graph Limit Tree

Modulus ES

tre

ng

th

y

0.5

E

2y

High

Modulus ES

tre

ng

th

y

0.5

E

2y

HighDensity

Fr. toughness

etc

Fresh water

Salt water

Min Max

15

v. good

v. good

Density

Fr. toughness

etc

Fresh water

Salt water

Min Max

15

v. goodv. good

v. goodv. good

Exercise: selecting materials for springs (2)

4.4 Repeat the selection of 4.3, but use the

Advanced facility to make a bar-chart with

the index

on the Y-axis.

.

E/2y

Ind

ex

y2 /E

E

2y

High

Plot the bar chart

Use a box selection to select the materials

with high values of the index

Add the other constraints using a limit stage

Results: CFRP, epoxy matrix (isotropic)

Nickel-based superalloys

Titanium alloys


1. Selection data


2. Selection Stages

Graph Limit Tree

2. Selection Stages

Graph Limit Tree

Density

Fr. toughness

etc

Fresh water

Salt water

Min Max

15

v. good

v. good

Density

Fr. toughness

etc

Fresh water

Salt water

Min Max

15

v. goodv. good

v. goodv. goodList of properties Density

Modulus

Yield strength

etc

+ - */ ^ ( )

(Yield strength^2)/

Young’s modulus


Modulus

Yield strength

etc

+ - */ ^ ( )+ - */ ^ ( )

(Yield strength^2)/

Young’s modulus

Quiz 3

1. Use the modulus–density chart to find, from among the materials that appear on it:

(a) The material with the highest density.

(b) The metal with the lowest modulus.

(c) The polymer with the highest density.

(d) The approximate ratio of the modulus of woods measured parallel to the grain and perpendicular to the grain.

(e) The approximate range of modulus of elastomers.

Quiz 3

Exercise 2

1) Make an E–ρ chart using the CES software. Use a box selection to find three materials with densities between 1000 and 3000 kg/m3 and the highest possible modulus.

2) Data estimation. The modulus E is approximately proportional to the melting point Tm in Kelvin (because strong inter-atomic bonds give both stiffness and resistance to thermal disruption). Use CES to make an E–Tm chart for metals and estimate a line of slope 1 through the data for materials. Use this line to estimate the modulus of cobalt, given that it has a melting point of 1760 K.

3) Sanity checks for data. A text reports that nickel, with a melting point of 1720 K, has a modulus of 5500 GPa. Use the E–Tm correlation of the previous question to check the sanity of this claim. What would you expect it to be?

Exercise 3

1) Explore the potential of PP–SiC (polypropylene–silicon carbide) fiber composites in the following way. Make a modulus–density (E–ρ) chart and change the axis ranges so that they span the range 1 < E <1000 GPa and 500 < ρ < 5000 kg/m3 . Find and label PP and SiC, then print it. Retrieve values for the modulus and density of PP and of SiC from the records for these materials (use the means of the ranges).

2) Use a ‗Limit‘ stage to find materials with modulus E > 180 GPa and price Cm < 3 $/kg.

3) Use a ‗Limit‘ stage to find materials with modulus E > 2 GPa, density ρ < 1000 kg/m3 and Price < 3/kg.

Exercise 4

1) Make a bar chart of modulus, E. Add a tree stage to limit the selection to polymers alone. Which three polymers have the highest modulus?

2) Make a chart showing modulus E and density ρ. Apply a selection line of slope 1, corresponding to the index E/ρ positioning the line such that six materials are left above it. Which are they and what families do they belong to?

3) A material is required for a tensile tie to link the front and back walls of a barn to stabilize both. It must meet a constraint on stiffness and be as cheap as possible. To be safe the material of the tie must have a fracture toughness K1c > 18 MPa.m1/2. The relevant index is

Assignment 2

1) Construct a chart of E plotted against Cm ρ. Add the constraint of adequate fracture toughness, meaning K1c > 18 MPa.m1/2, using a ‗Limit‘ stage. Then plot an appropriate selection line on the chart and report the three materials that are the best choices for the tie.

Example 2: Stiff & Light Tension Members

Example 2: Stiff & Light Tension Members

Assignment 3

1) List the six main classes of engineering materials. Use your own experience to rank them approximately:`

(a) By stiffness (modulus, E).

(b) By thermal conductivity (λ).

1) What are the steps in developing an original design?

2) Describe and illustrate the ‗translation‘ step of the material selection strategy.

3) What is meant by an objective and what by a constraint in the requirements for a design? How do they differ?

4) You are asked to design a fuel-saving cooking pan with the goal of wasting as little heat as possible while cooking. What objective would you choose, and what constraints would you think must be met?

Assignment 4

a) Sprint bikes.

b) Touring bikes.

c) Mountain bikes.

d) Shopping bikes.

e) Children‘s bikes.

f) Folding bikes.

Use your judgement to identify the primary objective and the constraints that must be met for each of these.

Bikes come in many forms, each aimed at a particular sector of the market:

Quiz 4

Examine the material property chart of modulus versus density. By what factor are polymers less stiff than metals? Is wood denser or less dense than polyethylene (PE)?

Example 3: Cheap Stiff Column

A column supports compressive

loads e.g. legs of a table or pillars

The goal is to identify the cheapest

materials that will support the load

without failing

77

Cheap Stiff Column

The objective function is cost

The buckling constraint is given by (safe design)

Noting that I = r4/4 = A2/4 and eliminating the

variable A gives

The material index for a low cost column that resists

buckling is

Performance of Stiff but Cost Effective Column

Slope=2

Quiz 5

1) Use the modulus–relative cost chart to find, from among the materials that appear on it:

(a) The cheapest material with a modulus greater than

1 GPa.

(b) The cheapest metal.

(c) The cheapest polymer.

(d) Whether magnesium alloys are more or less expensive than aluminum alloys.

(e) Whether PEEK (a high-performance engineering polymer) is more or less expensive than PTFE.

Quiz 5

Assignment 5

Pick any three engineering applications and answer the following:

1. Determine required properties: ex: mechanical,

electrical, thermal, magnetic, optical, deteriorative. 2. Express the design requirements into functions and

objectives. 3. Properties: identify candidate materials 4. Material: identify required Processing Processing: changes structure and overall shape ex: casting, sintering, vapor deposition, doping forming, joining, annealing.

QUESTIONS

Example 4: Selecting a Slender but strong Table Leg

I = r4/4

Anita Ama Yentumi, furniture designer, conceives of a lightweight table of simplicity, with a flat toughened glass top on slender, unbraced, cylindrical legs. For attractiveness, legs must be solid [to be thin] and light as possible [to make table easy to move]. Legs must support table top and load without buckling. What material would you recommend to Anita?

A light-weight table with slender cylindrical legs. Lightness and slenderness are independent design goals, both constrained by the requirement that the legs must not buckle when the table is loaded.

Example 4: (cont)

Polymers are out: they are not stiff enough; metals too: they are too heavy (even magnesium alloys, which

are the lightest).

The Selection

The choice is further narrowed by the requirement that, for slenderness, E must be large. A horizontal line on the diagram links materials with equal values of E; those above are stiffer. Placing this line at M1=100 GPa eliminates woods and GFRP. If the legs must be really thin, then the short-list is reduced to CFRP and ceramics: they give legs that weigh the same as the wooden ones but are barely half as thick.

Ceramics, we know, are brittle: they have low values of fracture toughness.

Table legs are exposed to abuse—they get knocked and kicked; common sense suggest that an additional constraint is needed, that of adequate toughness.

We then eliminate ceramics, leaving CFRP. The cost of CFRP may cause Anita to reconsider her design, but that is

another matter: she did not mention cost in her original specification.

Quiz 6

1) What is meant by a material index?

2) Derive a material index for a light and stiff panel with a square cross section.

3) Plot the index for a light, stiff panel on a copy of the modulus–density chart, positioning the line such that six materials are left above it. What classes do they belong to?

Assignment 6

1) The speed of longitudinal waves in a material is proportional to sqrt [E/ρ]. Plot contours of this quantity onto a copy of an E–ρ chart allowing you to read off approximate values for any material on the chart. Which metals have about the same sound velocity as steel? Does sound move faster in titanium or glass?

2) A material is required for a cheap column with a solid circular cross-section that must support a load F crit without buckling. It is to have a height L. Write down an equation for the material cost of the column in terms of its dimensions, the price per kg of the material, C m , and the material density ρ. The cross-section area A is a free variable—eliminate it by using the constraint that the buckling load must not be less than F crit (equation (5)). Hence read off the index for finding the cheapest tie. Plot the index on a copy of the appropriate chart and identify three possible candidates.

Example 5: stiff, light beam

Beam Function

Minimize mass m:

m = A L (2)

Objective

• Length L is specified

• Must have bending stiffness > S* Constraints

m = mass

A = area

L = length

= density

E = Young’s modulus

I = second moment of area

(I = b4/12 = A2/12)

C = constant (here, 48)

• Material choice

• Section area A. Free variables

Equation for constraint on A:

(1) 3

2

3 L12

AEC

L

IECS

Stiff beam of length L and minimum mass

L

Square

section,

area

A = b2

b

Chose materials

with smallest

1/2E

ρ

Eliminate A in (2) using (1):

2/1

2/1*5

EC

SL12m

Performance

metric m

0.1

10

1

100

Metals

Polymers

Elastomers

Woods

Composites

Foams 0.01

1000

100 0.1 1 10 Density (Mg/m3)

Young’s

modulu

s E

, (G

Pa)

Ceramics

Optimized selection using charts

CE 2/1

2

Contours of constant

M are lines of slope 2

on an E- chart

Index 1/2E

ρM

Light stiff beam:

22 M/ρE

Rearrange:

Take logs:

Log E = 2 log - 2 log M

Decreasing M

Slope 2

Optimized selection using charts

CE 2/1

6

• Bar must carry a moment, Mt ;

must have a length L.

• Maximize the Performance Index:

-- Strength relation: -- Mass of bar:

• Eliminate the "free" design parameter, R:

specified by application minimize for small M

(strong, light torsion members)

f

N

2Mt

R3 M R2L

M 2 NMt 2 /3

L

f2 /3

P

f2 /3

Example 5: STRONG & LIGHT TORSION MEMBERS

Example 5: Torsionally stressed shaft



Other Material Indices: Cost factor

Material “indices”

FUNCTION

Tie

Beam

Shaft

Column

Mechanical,

Thermal,

Electrical...

Each combination of

Function

Constraint Objective Free variable

has a

characterising

material index

CONSTRAINTS

Stiffness

specified

Strength

specified

Fatigue limit

Geometry

specified

Minimum cost

Minimum

weight

Minimum

volume

Minimum

eco- impact

OBJECTIVE

INDEX

2/1EM

Minimise this!

Demystifying material indices

A material index is just the combination of material properties that

appears in the equation for performance (eg minimizing mass or cost).

Sometimes a single property

Sometimes a combination

Either is a material index

Example:

Objective --

minimise mass

Performance

metric = mass

Tension (tie)

Bending (beam)

Bending (panel)

ρ/E yρ/σ

1/2ρ/E 2/3

yρ/σ

1/3ρ/E1/2y

ρ/σ

Function Stiffness Strength

Constraints

(Or maximize

reciprocals) Minimize these!

Summary of Some Materials Indices

Assignment

A simply supported beam of rectangular cross section of length 1 meter, width 100 mm, and no restriction on the depth is subjected to a load of 20 kN in its middle. The main design requirement is that the beam should not suffer plastic deformation as a result of load application. Select the least expensive material for the beam

Selecting a beam material for minimum cost

Single Property Ranking Example

Overhead Transmission Cable

Single Property Ranking Example



End of Unit 4

WEIGHTED PROPERTY

INDEX

In most applications, the selected material should satisfy more than one functional requirement

In this method each material requirement (or property) is assigned a certain weight (which depends on its importance to the performance of the design)

This method attempts to:

1. Quantify how important each desired requirement is by determining a weighting factor (α)

2. Quantify how well a candidate material satisfies each requirement by determining a scaling factor (β)

MATERIALS

Weighted Property Method

Since different properties have widely different numerical values. Each property must be so scaled that the largest value does not exceed 100

Weighted Properties Method

Scaled Factor

For properties such that it is more desirable to have low values, e.g., density, corrosion loss, cost and electrical resistance, the scale factor is formulated as follows

For properties that should have maximum values [strength, toughness …], the scaling factor [β] for a given candidate material is

The relative importance is shown by using a point scale that does not exceed 100 points

e.g; if strength is 4 times as important as cost, it will be represented by an 80 / 20 division

For properties that are not readily expressed in numerical values, e.g., weldability and wear resistance, some kind of subjective rating is required. For example

The best material may either have the largest value of the given property or the smallest

For example- High strength is given 100 Low density or low corrosion rates are given 100

113

It is calculated by multiplying the scaling factor by the weight

factor. Then the summed for the criteria

The material performance index γ is

where i is summed over all the properties, and n is the number of properties under consideration

Weighted Property Index

There are two general schemes for working with the weighting factors

The most common one is to set

The other is to let w take on a range of values, with the largest value denoting the property of greatest importance

such that

Step 1- List all the essential and desirable properties of the material. Eg. Availability, shape and size, cost, corrosion resistance, weldability, forgibility, density, etc.

Step 2- Categorize these properties into two groups. a] Go-No-Go parameters- are constraints, b] Discriminating parameters- can be assigned values. For example, in the case of connecting rod endurance strength is given number 5 and cost is given number 1. Here in weightage point method, #1 means poor weightage and #5 means high priority.

Step 3- The quantitative values or weightage depends upon the importance of that particular property in the given application

Step 4- Calculation of weightage contribution and decision making

Weighted Point Method

Weight is 4 times as important as strength, strength is 4 times as important as cost, corrosion is 2 /3 as importance as strength, etc

Weighting of attributes

We can also use the Digital Logic Method

Weighting factors- Example

When many material properties are used to specify performance, it may be difficult to establish the weighting factors

One way to do so is to use a digital logic approach

Each property is listed and is compared in every combination, taken two at a time to make the comparison

The property that is considered to be the more important of the two is given a 1 and the less important property is given a 0

118

Digital Logic Approach

The total number of

possible combinations is

where n is the number of

properties under consideration

If the total number of possible decisions for each property is m, then:

119

The number of attributes that should be listed vary between 5 - 10 This method combine properties with different units. This

limitation is overcome by the use of a ―scaling factor‖ The relative merit of each property of the candidate material may

be incorporated by assigning the value of 100 (%) to the best material in that property category

Example Select a suitable material by weighted point method. There are four

materials selected on the basis of design requirement which are i] stainless steel 301, ii] aluminium 2014-T6, iii] Ti-6Al-4V and iv] Inconel 718. The material is to be used for a cryogenic storage tank for transporting liquid nitrogen at -196oC.

Mechanical Properties

On the basis of importance of these properties they are ranked on the scale of 1 to 5 [1 stands for the poorest and 5 for the best].

As the tank is to be used in -198oC, toughness should be at the top and to reduce the weight, density has to be assigned to the second place. Hence, toughness is assigned 5 points, density 4 points and so on. These are listed below-

The calculation of percentage contribution of each property is illustrated below.

The percentage contribution of toughness of Al-2024-T6 is obtained as

Since weight index for toughness is 5, the material performance index for Al-2024-T6 is

Similarly, material performance index for other materials are obtained and included in Table below

Assignment of Weighted Index

Summary of Calculations

Use of Digital Logic Method in the Cylinder Example

Properties of Sample Candidate Materials

Weighted Factors for the Cylinder Example

Scaled Values of Properties and Calculated Weighted Property Index

Calculation of Performance Index Property

The material selection for a cryogenic storage

vessel for liquefied propane gas is being

evaluated on the basis of

1) low-temperature fracture toughness, 2) elastic

modulus, 3) specific gravity,

4) thermal expansion and 5) yield strength.

Determine the weighting factors for these

properties with the aid of a digital logic table.

Class Test

The material selection for a cryogenic storage vessel for liquefied propane gas is being evaluated on the basis of 1) low-temperature fracture toughness, 2) elastic modulus, 3) specific gravity, 4) thermal expansion and 5) yield strength. Determine the weighting factors for these properties with the aid of a digital logic table. Select the best material from the following candidate materials

Class Test

Class Test

The material for the shaft of an automobile is being evaluated on the basis Fatigue strength, Fracture toughness, stiffness, thermal expansion and cost. Determine the weighting factors for these properties with the aid of a digital logic table. Hence or otherwise, select the best material from the following candidate materials: A. Unalloyed DI; B. Ni- alloyed DI; C. Cr-alloyed DI; D. NiCr-Alloyed DI

Property Candidate Materials A B C D

Fatigue strength 0 100 90 90

Fracture Toughness 50 100 10 30

Stiffness 45 100 45 90

Thermal Expansion 100 5 100 90

Cost 100 10 100 30

129

The material selection for the legs of a table is being evaluated on the basis of the following properties: (1) density, (2) stiffness, (3) cost, (4) production energy, and (5) CO2 production. What information do we need? Alternatives – Bamboo, Cast iron, Low carbon steel, and Oak • Property Values (see handout) • Weighting Factors (How do we determine these?) – Your/design team‘s intuition (good) – Pair-wise comparison (better)

Materials Quality Control and Assurance

131

Contents

Concepts of quality control

Objectives of quality control

Consequences of quality control

Costs associated with quality control

The economics of quality control

Control chats; types of control chats

Inspection of finished products and the economics of quality control

Materials Quality Control

Questions to answer in this module…

Why is Quality Control important in materials manufacturing?

What can go wrong in quality control?

How are materials quality controlled?

What does the word “quality” mean to you?

Think about a product you bought. How can you define its ―quality‖?

135

Terminology

Every product possesses a number of elements that jointly describe

what the user or consumer thinks of as quality

These parameters are often called quality characteristics

Sometimes these are called critical-to-quality [CTQ] characteristics

Quality characteristics may be of several types

o Physical- length, weight, viscosity

o Sensory- taste, appearance, color

o Time Orientation- reliability, durability, serviceability

Dimensions of Quality

Garvin (1987)

1. Performance:

Will the product do the intended job?

2. Reliability:

How often does the product fail?

3. Durability:

How long does the product last?

4. Serviceability:

How easy to repair the product to solve the problems in service?

Dimensions of Quality

5. Aesthetics:

What does the product look/smell/sound/ feel like?

6. Features:

What does the product do/ service give?

7. Perceived Quality:

What is the reputation of the company or its products/services?

8. Conformance to Standards:

Is the product/service made exactly as the designer/standard intended?

What is Quality?

―The degree to which a system, component, or process meets

(1) specified requirements, and

(2) customer or users needs or expectations‖ – IEEE

Degree to which a set of inherent characteristics fulfils requirements – ISO 9000:2000

The word Quality does not mean the Quality of manufactured product only. It may refer to the Quality of the process (i.e., men, material, machines) and even that of management.

What is Quality?

Quality means those features of products which meet customer needs and thereby provide customer satisfaction.

In this sense, the meaning of quality is oriented to income

The purpose of such higher quality is to provide greater customer satisfaction and one hopes to increase income

Quality means freedom from deficiencies

In this sense, the meaning of quality is oriented to costs, and higher quality usually costs less

More about Quality

Quality begins with the design of a product in accordance with the customer specification.

Further it involves the established measurement standards, the use of proper material, selection of suitable manufacturing process and the necessary tooling to manufacture the product. It also involve the performance of the necessary manufacturing operations and the inspection of the product to check the manufacturing operations and the inspection of the product to check on performance with the specifications.

Quality characteristics can be classified as follows :

(1) Quality of design

(2) Quality of conformance with specifications

(3) Quality of performance.

Modern Importance of Quality

―The first job we have is to turn out quality merchandise that consumers will buy and keep on buying. If we produce it efficiently and economically, we will earn a profit.‖

- William Cooper Procter

141

Factors Affecting Quality

(1) Men, Materials and Machines

(2) Manufacturing conditions

(3) Market research in demand of purchases

(4) Money in capability to invest

(5) Management policy for quality level

(6) Production methods and product design

(7) Packing and transportation

What is Quality Control?

Quality Control (QC) is the implementation of regular testing procedures against your definitions of quality and more specifically the refinement of these procedures

Formal use of testing

Acting on the results of your tests

Requires planning, structured tests, good documentation

Relates to output - Quality Circle

Standards - ISO 9000 & BS5750

Quality Control (QC) process evaluates actual performance, compares actual performance to goal and takes action on the difference

Objectives of Quality Control

(1) To decide about the standard of Quality of a product that is easily acceptable to the customer.

(2) To check the variation during manufacturing.

(3) To prevent the poor quality products reaching to customer.

144

Quality Control

The process through which the standards are established and met with standards is called control. This process consists of observing our activity performance, comparing the performance with some standard and then taking action if the observed performance is significantly to different from the standards.

The control process involves a universal sequence of steps as follows :

(1) Choose the control subject.

(2) Choose a unit of measure.

(3) Set a standard value i.e., specify the quality characteristics

(4) Choose a sensing device which can measure.

(5) Measure actual performance.

(6) Interpret the difference between actual and standard.

(7) Taking action, if any, on the difference.

The Feedback Loop

Quality control takes place by use of the feedback loop. A generic form of the feedback loop is shown below

ISO As A Data Quality Management System

ISO 9004-1: General quality guidelines to implement a quality system.

ISO 9004-4: Guidelines for implementing continuous quality improvement within the organisation, using tools and techniques based on data collection and analysis.

ISO 10005: Guidance on how to prepare quality plans for the control of specific projects.

ISO 10011-1: Guidelines for auditing a quality system.

ISO 10011-2: Guidance on the qualification criteria for quality systems auditors.

ISO 10011-3: Guidelines for managing quality system audit programmes.

ISO 10012: Guidelines on calibration systems and statistical controls to ensure that measurements are made with the intended accuracy.

ISO 10013: Guidelines for developing quality manuals to meet specific needs.

Source: http://www.iso.ch/

Materials Quality Control

Quality Control is conducted by a team…

Design engineer

Materials/Metallurgical engineer

Stress engineer

Raw materials producer

Production Planner

Technician

Quality Assurance Inspector

Statistical Quality Control (SQC)

Statistica1 quality control (SQC) is the term used to describe the set of statistical tools used by quality professionals. Statistical quality control can be divided into three broad categories:

1) Descriptive statistics are used to describe quality characteristics and relationships. Included are statistics such as the mean, standard deviation, the range, and a measure of the distribution of data.

2) Statistical process control (SPC) involves inspecting a random sample of the output from a process and deciding whether the process is producing products with characteristics that fall within a predetermined range. SPC answers the question of whether the process is functioning properly or not.

3) Acceptance sampling is the process of randomly inspecting a sample of goods and deciding whether to accept the entire lot based on the results. Acceptance sampling determines whether a batch of goods should be accepted or rejected

SQC

A Quality control system performs inspection, testing and analysis to conclude whether the quality of each product is as per laid quality standard or not.

It‘s called ‗‗Statistical Quality Control‘‘ when statistical techniques are employed to control quality or to solve quality control problem.

SQC makes inspection more reliable and at the same time less costly.

It controls the quality levels of the outgoing products.

SQC should be viewed as a kit of tools which may influence related to the function of specification, production or inspection.

DESCRIPTIVE STATISTICS

Descriptive statistics can be helpful in describing certain characteristics of a product and a process. The most important descriptive statistics are measures of central tendency such as the

The Mean

The Range and Standard Deviation

DESCRIPTIVE STATISTICS

When a distribution is symmetric, there are the same number of observations below and above the mean

When a disproportionate number of observations are either above or below the mean, we say that the data has a skewed distribution.

Differences between symmetric and skewed distributions

Normal distributions with varying standard deviations

Developing Control Charts

A control chart (also called process chart or quality control chart) is a graph that shows whether a sample of data falls within the common or normal range of variation.

A control chart has upper and lower control limits that separate common from assignable causes of variation.

We say that a process is out of control when a plot of data reveals that one or more samples fall outside the control limits.

Control Charts

The center line (CL) of the control chart is the mean, or average, of the quality characteristic that is being measured.

The upper control limit (UCL) is the maximum acceptable variation from the mean for a process that is in a state of control.

Similarly, the lower control limit (LCL) is the minimum acceptable variation from the mean for a process that is in a state of control.

Control Chart

We say that a process is out of control when a plot of data reveals that one or more samples fall outside the control limits.

You can see that if a sample of observations falls outside the control limits we need to look for assignable causes.

Assignable causes of variation involves variations where the causes can be precisely identified and eliminated.

Examples of this type of variation are poor quality in raw materials, an employee who needs more training, or a machine in need of repair.

CONTROL CHARTS FOR VARIABLES

Control charts for variables monitor characteristics that can be measured and have a continuous scale, such as height, weight, volume, or width

When an item is inspected, the variable being monitored is measured and recorded.

For example, if we were producing candles, height might be an important variable. We could take samples of candles and measure their heights.

Mean (x-Bar) Charts: A control chart used to monitor changes in the mean value of a process.

Range (R) Charts: A control chart that monitors changes in the dispersion or variability of process.

Constructing a Mean (x-Bar) Chart

To construct the upper and lower control limits of the chart, we use the following formulas:

The center line of the chart is then computed as the mean of all sample means, where is the number of samples:

Constructing a Mean (x-Bar) Chart from the

Sample Range

Another way to construct the control limits is to use the sample range as an estimate of the variability of the process.

The spread of the range can tell us about the variability of the data.

In this case control limits would be constructed as follows:

Notice that A2 is a factor that includes three standard deviations of ranges and is dependent on the sample size being considered.

Factors for three-sigma control limits of and R-charts

Factors for three-sigma control limits of and R-charts

EXAMPLE : Constructing a Mean (x-Bar) Chart

A quality control inspector at the Cocoa Fizz soft drink

company has taken twenty-five samples with four

observations each of the volume of bottles filled. The

data and the computed means are shown in the table. If

the standard deviation of the bottling operation is 0.14

ounces, use this information to develop control limits of

three standard deviations for the bottling operation.

Test Data

Continuation of Test Data

Solution

Resulting Control Chart

EXAMPLE 6.2 Constructing a Mean (x-Bar)

Chart from the Sample Range

Range (R) Charts

Range (R) charts are another type of control chart for variables. Whereas x-bar charts measure shift in the central tendency of the process, range charts monitor the dispersion or variability of the process.

The method for developing and using R-charts is the same as that for x-bar charts.

The center line of the control chart is the average range, and the upper and lower control limits are computed as follows:

where values for D4 and D3 are obtained from Table 6-1.

Constructing a Range (R) Chart

The quality control inspector at Cocoa Fizz would like to develop a range (R) chart in order to monitor volume dispersion in the bottling process. Use the data from Example 6.1 to develop control limits for the sample range.

The resulting control chart is:

CONTROL CHARTS FOR ATTRIBUTES

Control charts for attributes are used to measure quality characteristics that are counted rather than measured.

Attributes are discrete in nature and entail simple yes-or-no decisions.

For example, this could be the number of nonfunctioning lightbulbs, the proportion of broken eggs in a carton, the number of rotten apples, the number of scratches on a tile, or the number of complaints issued.

Two of the most common types of control charts for attributes are p-charts and c-charts. P-charts are used to measure the proportion of items

in a sample that are defective.. C-charts count the actual number of defects.

Problem-Solving Tip:

The primary difference between using a p-chart and a c-chart is as follows. A p-chart is used when both the total sample size

and the number of defects can be computed. A c-chart is used when we can compute only the

number of defects but cannot compute the proportion that is defective.

P-Charts

P-charts are used to measure the proportion that is defective in a sample.

The center line is computed as the average proportion defective in the population, 𝑃 .This is obtained by taking a number of samples of observations at random and computing the average value of p across all samples.

To construct the upper and lower control limits for a p-chart, we use the following formulas:

z is selected to be either 2 or 3 standard deviations, depending on the amount of data we wish to capture in our control limits. Usually, however, they are set at 3.

The sample standard deviation is computed as follows:

where n is the sample size.

Constructing a p-Chart

A production manager at a tire manufacturing plant has inspected the number of defective tires in twenty random samples with twenty observations each. Following are the number of defective tires found in each sample:

Constructing a p-Chart

Construct a three-sigma control chart

Construct a three-sigma control chart (z = 3) with this information.

Solution The center line of the chart is

In this example the lower control limit is negative, which sometimes occurs because the computation is an approximation of the binomial distribution. When this occurs, the LCL is rounded up to zero because we cannot have a negative control limit.


The resulting control chart is as follows:

C-CHARTS

C-charts are used to monitor the number of defects per unit. Examples are the number of returned meals in a restaurant, the

number of trucks that exceed their weight limit in a month, the number of discolorations on a square foot of carpet, and the number of bacteria in a milliliter of water.

Note that the types of units of measurement we are considering are a period of time, a surface area, or a volume of liquid.

The average number of defects, 𝐶 , is the center line of the control chart.

The upper and lower control limits are computed as follows:

Computing a C-Chart

The number of weekly customer complaints are monitored at a large hotel using a c-chart. Complaints have been recorded over the past twenty weeks. Develop three-sigma control limits using the following data:

As in the previous example, the LCL is negative and should be rounded up to zero. Following is the control chart for this example:


Before You Go On

We have discussed several types of statistical quality control (SQC) techniques.

One category of SQC techniques consists of descriptive statistics tools such as the mean, range, and standard deviation.

These tools are used to describe quality characteristics and relationships.

Another category of SQC techniques consists of statistical process control (SPC) methods that are used to monitor changes in the production process.

To understand SPC methods you must understand the differences between common and assignable causes of variation.

Common causes of variation are based on random causes that cannot be identified.

You should also understand the different types of quality control charts that are used to monitor the production process: x-bar charts, R-range charts, p-charts, and c-charts.

Statistical Quality Control (SQC)

A successful SQC programme is expected to yield the following results :

(1) Improvement of quality.

(2) Reduction of scrap and rework.

(3) Efficient use of men and machines.

(4) Economy in use of materials.

(5) Removing production bottle-necks.

(6) Decreased inspection costs.

(7) Reduction in cost/unit.

(8) Scientific evaluation of tolerance.

(9) Scientific evaluation of quality and production.

(10) Quality consciousness at all levels.

(11) Reduction in customer complaints.

Advantages and Limitations of SPC

Advantages

1) Emphasis on early detection -An advantage of SPC over other methods of quality control, such as "inspection", is that it emphasizes early detection and prevention of problems, rather than the correction of problems after they have occurred.

2) Increasing rate of production -In addition to reducing waste, SPC can lead to a reduction in the time required to produce the product. SPC makes it less likely the finished product will need to be reworked or scrapped.

Limitations

1) SPC is applied to reduce or eliminate process waste. This, in turn, eliminates the need for the process step of post-manufacture inspection. The success of SPC relies not only on the skill with which it is applied, but also on how suitable or amenable the process is to SPC. In some cases, it may be difficult to judge when the application of SPC is appropriate.

What is Quality?

Quality is the ability of your product to be able to satisfy your users

What is Quality Assurance?

Quality Assurance is the process that demonstrates your product is able to satisfy your users

What is Quality Assurance?

What is the aim of Quality Assurance?

o When good Quality Assurance is implemented there should be improvement in usability and performance and lessening rates of defects

Quality control and quality assurance have much in common. These may include

1) Each evaluate performance

2) Each compares performance to goals

3) Each acts on the difference

However they also differ from each other. Thus for quality control

1) It has its primary purpose to maintain control

2) Performance is evaluated during operations, and performance is compared to goals during operations

3) The resulting information is provided to both the operating forces and others who have a need to know

4) Others may include plant, functional, or sector management, corporate staff, regulatory bodies, customers, and the general public

The Relation to Quality Assurance

Quality Assurance vs. Quality Control

The difference is that Quality Assurance is process oriented and focuses on defect prevention, while quality control is product oriented and focuses on defect identification.

Testing, therefore is product oriented and thus is in the QC domain. Testing for quality isn't assuring quality, it's controlling it.

Quality Assurance makes sure you are doing the right things, the right way.

Quality Control makes sure the results of what you've done are what you expected.

What does QA give?

Quality’ means your product is ‗useful‘ - without ‘quality’ you may have little to offer

‘Quality’ can help to future-proof products

But ‗quality assurance’ needs documented standards and best practices to be meaningful

‘Quality’ & ‘Best Practice’ can be considered in terms of being ‘Fit for Purpose’

Inspection

Inspection is the most common method of attaining standardisation, uniformity and quality of workmanship.

It is the cost art of controlling the product quality after comparison with the established standards and specifications.

It is the function of quality control.

If the said item does not fall within the zone of acceptability it will be rejected and corrective measure will be applied to see that the items in future conform to specified standards.

It helps to control quality, reduces manufacturing costs, eliminate scrap losses and assignable causes of defective work.

Objectives of Inspection

(1) To collect information regarding the performance of the product with established standards for the use of engineering production, purchasing and quality control etc.

(2) To sort out poor quality of manufactured product and thus to maintain standards.

(3) To establish and increase the reputation by protecting customers from receiving poor quality products.

(4) Detect source of weakness and failure in the finished products and thus check the work of designer

Purpose of Inspection

(1) To distinguish good lots from bad lots

(2) To distinguish good pieces from bad pieces.

(3) To determine if the process is changing.

(4) To determine if the process is approaching the specification limits.

(5) To rate quality of product.

(6) To rate accuracy of inspectors.

(7) To measure the precision of the measuring instrument.

(8) To secure products – design information.

(9) To measure process capability.

Stages of Inspection

(1) Inspection of incoming material

o It consists of inspecting and checking of all the purchased raw materials and parts that are supplied before they are taken on to stock or used in actual manufacturing. It may take place either at supplier‘s end or at manufacturer‘s gate. If the incoming materials are large in quantity and involve huge transportation cost it is economical to inspect them at the place of vendor or supplier.

(2) Inspection of production process

o The work of inspection is done while the production process is simultaneously going on. Inspection is done at various work centres of men and machines and at the critical production points. This had the advantage of preventing wastage of time and money on defective units and preventing delays in assembly.

Stages of Inspection

(3) Inspection of finished goods.

o This is the last stage when finished goods are inspected and carried out before marketing to see that poor quality product may be either rejected or sold at reduced price.

Inspection Procedures

There are three ways of doing inspection. They are Floor inspection, Centralised inspection and Combined inspection.

Floor Inspection

o It suggests the checking of materials in process at the machine or in the production time by patrolling inspectors. These inspectors moves from machine to machine and from one to the other work centres. Inspectors have to be highly skilled. This method of inspection minimise the material handling, does not disrupt the line layout of machinery and quickly locate the defect and readily offers field and correction.

Disadvantages

(1) Difficult in inspection due to vibration.

(2) Possibility of biased inspection because of worker.

(3) Pressure on inspector.

(4) High cost of inspection because of numerous sets of inspections and skilled inspectors.

Advantages

(1) Encourage co-operation of inspector and

foreman.

(2) Random checking may be more successful

than batch checking.

(3) Does not delay in production.

(4) Saves time and expense of having to more

batches of work for inspection.

(5) Inspectors may see and be able to report

on reason of faculty work.

Centralised Inspection

Materials in process may be inspected and checked at centralised inspection centre which are located at one or more places in the manufacturing industry.

Advantages

(1) Better quality checkup.

(2) Closed supervision.

(3) Absence of workers pressure.

(4) Orderly production flow and low inspection cost.

Disadvantages

(1) More material handling.

(2) Delays of inspection room causes wastage of time.

(3) Work of production control increases.

(4) Due to non-detection of machining errors in time, there may be more spoilage of work.

Combined Inspection

Combination of two methods what ever may be the method of inspection, whether floor or central. The main objective is to locate and prevent defect which may not repeat itself in subsequent operation to see whether any corrective measure is required and finally to maintained quality economically.

Methods of Inspection

There are two methods of inspection. They are 100% inspection and Sampling inspection.

100% Inspection

o This type will involve careful inspection in detail of quality at each strategic point or stage of manufacture where the test involved is non-destructive and every piece is separately inspected. It requires more number of inspectors and hence it is a costly method. There is no sampling error. This is subjected to inspection error arising out of fatigue, negligence, difficulty of supervision etc. Hence complete accuracy of influence is seldomly attained.

o It is suitable only when a small number of pieces are there or a very high degree of quality is required. Example : Jet engines, Aircraft, Medical and Scientific equipment.

Sampling Inspection

In this method randomly selected samples are inspected. Samples taken from different batches of products are representatives. If the sample prove defective. The entire concerned is to be rejected or recovered. Sampling inspection is cheaper and quicker. It requires less number of Inspectors. Its subjected to sampling errors but the magnitude of sampling error can be estimated. In the case of destructive test, random or sampling inspection is desirable. This type of inspection governs wide currency due to the introduction of automatic machines or equipment which are less susceptible to chance variable and hence require less inspection, suitable for inspection of products which have less precision importance and are less costly.

Example: Electrical bulbs, radio bulbs, washing machine etc.

o Destructive tests conducted for the products whose endurance or ultimate strength properties are required.

Example: Flexible strength, resistance capacity, compressibility etc.

Drawbacks of Inspection

(1) Inspection adds to the cost of the product but not for its value.

(2) It is partially subjective, often the inspector has to judge whether a product passes or not.

o Example : Inspector discovering a slight burnish on a surface must decide whether it is bad enough to justify rejection even with micrometers a tight or loose fit change measurement by say 0.0006 inches. The inspectors design is important as he enforces quality standards.

(3) Fatigue and Monotony may affect any inspection judgement.

(4) Inspection merely separates good and bad items. It is no way to prevent the production of bad items.

Materials Quality Control Techniques

Materials property verification

Destructive Testing

Non-destructive Testing


Destructive Testing

Corrosion Testing

Tensile Testing

Impact Testing


Non-destructive Testing

Liquid penetrant Testing

Radiograhic Testing

Impulse Excitation Testing

Ultrasonic Testing

Electromagnetic Testing

Acoustic Emission Testing

Positive Material Identification

Hardness Testing

Infrared and Thermal Testing

Laser Testing

Leak Detection

Introduction to Nondestructive Testing

The use of noninvasive techniques to determine the integrity of a material, component or structure

or quantitatively measure some

characteristic of an object.

i.e. Inspect or measure without doing harm.

Definition of NDT

Methods of NDT

Visual

What are Some Uses of NDE Methods?

Flaw Detection and Evaluation

Leak Detection

Location Determination

Dimensional Measurements

Structure and Microstructure Characterization

Estimation of Mechanical and Physical Properties

Stress (Strain) and Dynamic Response Measurements

Material Sorting and Chemical Composition Determination

When are NDE Methods Used?

To assist in product development

To screen or sort incoming materials

To monitor, improve or control manufacturing processes

To verify proper processing such as heat treating

To verify proper assembly

To inspect for in-service damage

Six Most Common NDT Methods

• Visual • Liquid Penetrant • Magnetic • Ultrasonic • Eddy Current • X-ray

Most basic and common inspection method.

Tools include

fiberscopes, borescopes, magnifying glasses and mirrors.

Robotic crawlers permit observation in hazardous or tight areas, such as air ducts, reactors, pipelines.

Portable video inspection unit with zoom allows inspection of large tanks and vessels, railroad tank cars, sewer lines.

Visual Inspection

A liquid with high surface wetting characteristics is applied to the surface of the part and allowed time to seep into surface breaking defects. The excess liquid is removed from the surface

of the part.

A developer (powder) is applied to pull the trapped penetrant out the defect and spread it on the surface where it can be seen.

Visual inspection is the final step in the process. The penetrant used is often loaded with a fluorescent dye and the inspection is done under UV light to increase test sensitivity.

Liquid Penetrant Inspection

Magnetic Particle Inspection The part is magnetized. Finely milled iron particles coated with a

dye pigment are then applied to the specimen. These particles are attracted to magnetic flux leakage fields and will cluster to form an indication directly over the discontinuity. This indication can be visually detected under proper lighting conditions.

Magnetic Particle Crack Indications

Radiography

The radiation used in radiography testing is a higher energy (shorter wavelength) version of the electromagnetic waves that we see as visible light. The radiation can come from an X-ray generator or a radioactive source.

High Electrical Potential

Electrons

- +

X-ray Generator or Radioactive Source Creates

Radiation

Exposure Recording Device

Radiation Penetrate the Sample

Film Radiography

Top view of developed film

X-ray film

The part is placed between the radiation source and a piece of film. The part will stop some of the radiation. Thicker and more dense area will stop more of the radiation.

= more exposure

= less exposure

The film darkness (density) will vary with the amount of radiation reaching the film through the test object.

Radiographic Images

Conductive material

Coil Coil's magnetic field

Eddy currents

Eddy current's magnetic field

Eddy Current Testing

Eddy Current Testing

Eddy current testing is particularly well suited for detecting surface cracks but can also be used to make electrical conductivity and coating thickness measurements. Here a small surface probe is scanned over the part surface in an attempt to detect a crack.

High frequency sound waves are introduced into a material and they are reflected back from surfaces or flaws.

Reflected sound energy is displayed versus time, and inspector can visualize a cross section of the specimen showing the depth of features that reflect sound.

f

plate

crack

0 2 4 6 8 10

initial

pulse

crack

echo

back surface

echo

Oscilloscope, or flaw

detector screen

Ultrasonic Inspection (Pulse-Echo)

Ultrasonic Imaging

Gray scale image produced using the sound reflected from the front surface of the coin

Gray scale image produced using the sound reflected from the back surface of the coin (inspected from ―heads‖ side)

High resolution images can be produced by plotting signal strength or time-of-flight using a computer-controlled scanning system.

Common Application of NDT

Inspection of Raw Products

Inspection Following Secondary Processing

In-Services Damage Inspection

Inspection of Raw Products

Forgings,

Castings,

Extrusions,

etc.

Machining

Welding

Grinding

Heat treating

Plating

etc.

Inspection Following Secondary Processing

Cracking

Corrosion

Erosion/Wear

Heat Damage

etc.

Inspection For In-Service Damage

Power Plant Inspection

Probe

Signals produced

by various

amounts of

corrosion

thinning.

Periodically, power plants are shutdown for inspection. Inspectors feed eddy current probes into heat exchanger tubes to check for corrosion damage.

Pipe with damage

Wire Rope Inspection

Electromagnetic devices and visual inspections are used to find broken wires and other damage to the wire rope that is used in chairlifts, cranes and other lifting devices.

Storage Tank Inspection

Robotic crawlers use ultrasound to inspect the walls of large above ground tanks for signs of thinning due to corrosion.

Cameras on long articulating arms are used to inspect underground storage tanks for damage.

Aircraft Inspection Nondestructive testing is used

extensively during the manufacturing of aircraft.

NDT is also used to find cracks and corrosion damage during operation of the aircraft.

A fatigue crack that started at the site of a lightning strike is shown below.

Jet Engine Inspection

Aircraft engines are overhauled after being in service for a period of time.

They are completely disassembled, cleaned, inspected and then reassembled.

Fluorescent penetrant inspection is used to check many of the parts for cracking.

Sioux City, Iowa, July 19, 1989

A defect that went undetected in an engine disk was responsible for the crash of United Flight 232.

Crash of United Flight 232

Pressure Vessel Inspection

The failure of a pressure vessel can result in the rapid release of a large amount of energy. To protect against this dangerous event, the tanks are inspected using radiography and ultrasonic testing.

Rail Inspection

Special cars are used to inspect thousands of miles of rail to find cracks that could lead to a derailment.

Bridge Inspection

The US has 578,000 highway bridges.

Corrosion, cracking and other damage can all affect a bridge‘s performance.

The collapse of the Silver Bridge in 1967 resulted in loss of 47 lives.

Bridges get a visual inspection about every 2 years.

Some bridges are fitted with acoustic emission sensors that ―listen‖ for sounds of cracks growing.

NDT is used to inspect pipelines to prevent leaks that could damage the environment. Visual inspection, radiography and electromagnetic testing are some of the NDT methods used.

Remote visual inspection using a robotic crawler.

Radiography of weld joints.

Magnetic flux leakage inspection. This device, known as a pig, is placed in the pipeline and collects data on the condition of the pipe as it is pushed along by whatever is being transported.

Pipeline Inspection

Special Measurements

Boeing employees in Philadelphia were given the privilege of evaluating the Liberty Bell for damage using NDT techniques. Eddy current methods were used to measure the electrical conductivity of the Bell's bronze casing at various points to evaluate its uniformity.

SEE YOU IN THE EXAM

233

Demo: trade off plots

Contribution

Comment

Observation

235

Documents

Materials Quality Control, Assurance and Selection · quality control and assurance. There are several objectives for this course: 1.) To introduce the parameters that are important