material selection using ashby method

7/26/2019 Material Selection Using Ashby Method

http://slidepdf.com/reader/full/material-selection-using-ashby-method 1/6

1 Module 5 : Lecture 24 :Material Selection using Ashby Method.........2

1.1 Example o Material Attributes........................................................!

1.2 Example o Material Attributes........................................................!

1.! "erormance #ndex $ Material #ndex..............................................!1.4 Example: Material #ndex%M& or a light sti 'ie.............................4

1.5 Similar Material #ndices..................................................................4

2 Module 5 : Lecture 25 : Material Selection using Ashby Method Selection

"rocess 4

2.1 'opics to co(er in this lecture:.........................................................4

2.2 )ase Study: *esign o Scanning "robe Microscope........................4

2.! Elastic *election o a 'elescope Mirror +ac,ing -nder its /n 0eight

2.4 Mirror +ac,ing Material..................................................................

"gina 1 de 6



1 Module 5 : Lecture 24 :Material Selection using Ashby

Method

Topics to cover in this lecture:

#ntroduction to Material Selection "rocess

0hat are Material attributes3

'he Ashby process o Material Selection

0e ha(e briely discussed about the structural materials used in mechanical

design. nce you are a/are o all these materials existing around us the

uestion /ill be /hich one to choose and ho/3 'here is a systematic /ay o

selecting material popularly ,no/n as Ashby "rocess. 6ollo/ing is the

outline o Ashby "rocess. E(ery Material has an array o attributes : density7strength7

stiness7 thermal expansion coeicient7 reracti(e index7 cost etc.

*esign demands a profile - for example, lo/ density7 high

strength7 in addition to a design ob8ecti(e o minimi9ation cost7

delection etc.

'o begin /ith7 you ha(e to consider all materials.

Apply property limits that is maximum allo/ed limits o a

property and reduce the number o candidate materials: or

example7 eliminate materials that can not /or, in the ser(ice

temperature range.

-se Performance Index/ Material Index to urther reduce the set

o materials

"gina 2 de 6



1.1 Example of Material Attributes

1.2 Example of Material Attributes

1 series: "ure Al ;<<=7 *uctile Elect )onductor7 6oil

2 series: Al>)u%4=&7 Strong Aircrat S,in7 ?i(et

! series: Al>Mn%1=&7 )orrosion ?esistant ?ooing Sheet7 )oo,ing

"an

5 series: Al>Mg%!=& 7 Strong "ressure @essels

1.3 Performance Index Material Index

'he design o a structural element in(ol(es minimi9ation or maximi9ation

o an index gi(en by pB67C7MD

6 6unctional ?euirement C Ceometric ?euirement M Material

?euirement

6or certain designs (ariable separation exist :

# E p 1 B6D 2 BCD ! BMD

ere M denotes the Material #ndex

"gina 3 de 6



1.! Example" Material Index#M$ for a li%&t stiff 'ie

"erormance #ndex Minimi9e the mass o the 'ie mAl F /here A is the

area o crosssection o the 'ie7 l the length and F the density.

)onstraint 'he delection o the tie should be /ithin the permissible limit

6lGAEH I

Eliminating A 7 /e get m;%6 G I &%l 2 &% F GE&

Lightest Element that can carry 6 is the one /hich has maximum EG F .

Material #ndex M EG F

1.( )imilar Material Indices

Light Sti +eam:

6ree area % no constraint on height and /idth& M E 1G2 G F

6ree eight M E

1G!

G F 6ree /idth M EG F

Light Strong +eam: M J !G2 G F

)heap Sti )oloumn

)) m Al F 7 6Hn K 2 E#Gl 2

M E 1G2 G ) m F

2 Module 5 : Lecture 25 : Material Selection using Ashby Method

Selection rocess

2.1 'opics to co*er in t&is lecture"

)ase Study 1: *esign o Scanning "robe Microscope

)ase Study 2: *esign o Mirrorbac,ing plate

2.2 +ase )tudy" esi%n of )cannin% Probe Microscope

#n the last lecture7 /e ha(e discussed about the Ashby process or material selection. #n

this lecture7 /e /ill try to elaborate this procedure through t/o examples. 0e /ill irst

consider the design o a "robe or Scanning "robe Microscope %S"M&. 'hen /e /ill

consider the design o a 'elescope Mirror.

Scanning "robe Microscopes %S"M& ha(e contradictory speciications 6irst7 the scan

range o an S"M should exceed the si9e o the largest eatures on the sample. 'his

usually implies the use o large and thin /alled pie9o electric translators.

n the other hand7 any S"M is7 basically7 a system o mechanical resonators. ence7 it

is (ery important to optimi9e the design o the S"M or high resonance reuencies.

'his usually means to decrease the length o the probe. #n this design exercise /e /illconsider the maximisation o resonating reuency as our design ob8ecti(e.

'he eigenreuency o any single degree o reedom system is gi(en by:

"gina 4 de 6



/here , is the spring constant and meff is the eecti(e mass. 'he spring constant o a

cantile(ered beam /ith uniorm cross section is gi(en by

6or a rectangular cross section /ith a /idth b %perpendicular to the delection& and a

height h one obtains or I = bh3 /12

)ombining the expressions /e get the inal result or f as :

o/7 the eecti(e mass can be calculated using ?aleighs method. # you recall7 the

general expression or the ,inetic energy ' using ?aleighs method is

6or uniorm beam /ith a constant cross section and length7 the delection may be

expressed as:

Also7 ' can be gi(en by:

ence the eecti(e mass (9/20)m

o/7 in the inal orm7 the resonating reuency may be /ritten as:

#t is e(ident rom this euation7 that one /ay to increase the eigen reuency is to

choose a material /ith as high a ratio o E/ ρ as possible. #n act the Material "roperty

index in this case is gi(en by M%EG ρ & 1G2 . 'he table belo/ sho/s the E/ ρ (alues o a

e/ materials. As per the table both Aluminium and Steel are good candidates or this

purpose.

Material !"1#1# $%&2 ' ( "1#3 )g%&3 ' !% (

Al N 2.N 2.

+rass < O.5 1.1

"gina 5 de 6



Steel 2 N.O 2.

'ungsten !< 1<.! 2.

2.3 Elastic eflection of a 'elescope Mirror acin% nder its 0n ei%&t

Let us consider another example7 the design o a bac,ing plate or

telescopic mirror. 'he ob8ecti(e is to select a suitable material

/hich /ill gi(e minimum central delection o the circular plate

or the same mass o the plate. 'he thic,ness o the plate t is a

ree (ariable.

2.! Mirror acin% Material

'he material property index in this case is gi(en by % F! GE& .5 .

ere7 it is e(ident that the material /ith the lo/est

(alue o the index /ill be the best or the designP

accordingly7 rom the table belo/7 )6?" comes

out as the best choice.

Material ! *a ( Mg%&3 (3 %!

Steel 2 N.O 1.54

)oncrete 4N 2.5 .5

Al < 2.N .5!

Class < 2.5 .4O

C6?" 4 2. .45

0ood 12 . .1!

"- . .1 .1!

)6?" 2N 1.5 .11

"gina 6 de 6

material selection using ashby method

Documents