‘’OTHER MECHANICAL PROPERTIES OF

MATERIALS’’

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LectureLectureLectureLecture----7777

Outline

1) DUCTILITY

2) RESILIENCE

3) TOUGHNESS

4) HARDNESS

1-Ductility

� It is a measure of the total plastic strain that accompanies

fracture.

� % elongation (EL) or % area reduction (AR) is used for

measurement of ductility.

LAo

Initial Final

A material that experiences verylittle or no plastic deformationupon fracture is termed as

Lo LfAo

Af

lf and Af are length and area at the fracture.

%EL =

L f − LoLo

x100 %AR =

Ao − A fAo

x100or

� Ductility may be expressed quantitavely as either percent elongation or

percent area reduction.

BRITTLE

%EL =

L f − LoLo

x100 %AR =

Ao − A fAo

x100

Both Lf and Af are measured subsequent to fracture; so how you measure them ?

By repositioning thetwo broken ends

Percent elongation and percent areareduction are two indicators of ductility..HOWEVER;Elongation is not uniform over the entiregage length and is greatest at the

http://www.youtube.com/watch?v=D8U4G5kcpcM

User: MaterialsScience2000, Title: Tensile Test

Therefore percent elongation is not ABSOLUTE MEASURE of DUCTILITY and GAGE LENGTH must be stated when %EL is reported, generally it is 50 mm..

The shorter Lo, the greater fraction of total elongation

from NECK, more %EL

%AR is better indication of DuctilityDiameter is measured at the NECK, WHY?

MINIMUM

DUCTILITY is IMPORTANT for TWO REASONS:

� Indicates the designer the degree of plastic deformation before FRACTURE

� Specifies the degree of ALLOWABLE DEFORMATION during FABRICATION OPERATIONS

With Increase of TEMPERATURE,

Ductility

INCREASES due to

WEAKER BONDS at

higher temperatures

2-Resilience

� The capacity of a material to absorb energy when it is deformed

elastically and then upon unloading to have this energy recovered.

Modulus of resilience (UModulus of resilience (UModulus of resilience (UModulus of resilience (Urrrr)))) = strain energy

per unit volume required to stress a

material from an unloaded state up to the

point of yielding.

Assuming linear elastic region;

Units: Joules/m3 (or, Pa)

HOW?

How do you determine the modulus of resilience?

Area under stress-strain

curve taken to yielding

During computing Modulus of Resilience be

aware that there occurs NO

permanent distortion

in the material, AS we are in the

ELASTIC REGION

Resilient materials are those used in SPRING APPLICATIONS

3-Toughness

� It is a measure of the ability of a material to absorb energy up to

fracture.

� Approximate by the area under the stress-strain curve.

Units: Joules/m3 (or, Pa) Same as modulus of resilience

For a material to be tough, it must display both ductility and strength therefore

DUCTILE MATERIALS are often TOUGHER than BRITTLE ONES

Toughness vs. Strength

� The chart shows yield strength in tension for all materials, except for ceramics for which compressive strength is shown (their tensile strength being much lower)

� Toughness measures the energy required to crack a material; it is important for things which suffer important for things which suffer impact

� There are many cases where strength is no good without toughness, e.g. a car engine, a hammer

� Increasing strength usually leads to decreased toughness

Which one is the most brittle?“E” as it fractures

before yielding

Which one is the most ductile?“B” as it experience

the greatest %AR

Example: Stress-Strain Curves

TRY TO MAKE CALCULATIONS by

SCALING Instead of Deciding By EYE

4-HARDNESS

� Measure of a material’s resistance to

localized plastic deformation.

A small indenter is forced into the surface of a material to be

tested, under controlled conditions of load and rate of application.

The depth or size of the resulting indentation is measured, which

� Hardness measurement:Hardness measurement:Hardness measurement:Hardness measurement:

The depth or size of the resulting indentation is measured, which

in turn related to a hardness number.

e.g., 10mm sphere

apply known force (1 to 1000g)

measure size of indent after removing load

dDSmaller indents mean larger hardness.

HARDNESS TESTS:HARDNESS TESTS:HARDNESS TESTS:HARDNESS TESTS:

1) Rockwell Hardness Testing (HR)

2) Brinell Hardness Testing (HB)Macro-hardness testing

Early hardness tests were based on natural minerals ….. The one that scratches another is

harder and the scratched one is softer…

1 for talc

10 for diamond

increasing hardness

most plastics

brasses Al alloys

easy to machine steels file hard

cutting tools

nitrided steels diamond

2) Brinell Hardness Testing (HB)

3) Knoop Testing (HK)

4) Vickers Testing (HV)

Macro-hardness testing

Micro-hardness testing

Applied loads range between 1-1000 grams

HARDNESS tests were performed more often than the other mec hanical tests because:

� Simple and Inexpensive – no special specimen is needed and testing apparatus is relatively inexpensive

� Test is nondestructive – Specimen is neither fractured n or excessively deformed; a small indentation is the only deformation

� Other mechanical properties may be estimated such as tens ile strenght

HOWEVER;

Measured hardnesses are only relative (rather than

absolute) and care should be given when comparing

values determined from different techniques

Hardness Testing Techniques

ROCKWELL HARDNESS TEST:

With this test, both hardness number and

scale symbol must be indicated..

For example;

70 HRC, Rockwell Hardness of 70 on C scale

60 HRB, Rockwell Hardness of 60 on B scale

http://www.youtube.com/watch?v=G2JGNlIvNC4

User: MaterialsScience2000, Title: Rockwell Hardness Test

Superficial tests are applied on thin specimens

60 HRB, Rockwell Hardness of 60 on B scale

Hardness values are between 20-100

The most commonly used scales are "B" and "C"

and you are responsible to know these scales

ROCKWELL HARDNESS TEST:

The chief advantage of Rockwell hardness is its abi lity to display hardness values directly, thus obviating te dious calculations involved in other hardness measurement techniques.

It is typically used in engineering and metallurgy. Its commercial

popularity arises from its;

• speed,

• reliability,

robustness,

A digital rockwell tester

• robustness,

• resolution,

• small area of indentation.

Be CAREFUL !!!!

Thickness of the specimen must be at least 10 times larger than the indentation

It is certain that if DEPTH of PENETRATION

is INCREASED

HARDNESS is DECREASED

HARDNESS TESTS are well defined by ASTM (American Society for Testing and Materials);

� ASTM Standard E 18 “Standard Test Methods for Rockwell Hardness and Rockwell

Superficial Hardness of Metallic Materials”

� ASTM Standard E 10 “Standard Test Method for Brinell Hardness of Metallic Materials”

� ASTM Standard E 92 “Standard Test Method for Vickers Hardness of Metallic Materials”

and ASTM Standard E 384 “Standard Test for Microhardness of Materials”

All are about $60 ?!?!

Short notations for the hardness tests;Short notations for the hardness tests;

HRB or RB Rockwell Hardness Scale B

HRC or RC Rockwell Hardness Scale C

HB or BHN Brinell Hardness

HK or KHN Knoop Hardness

HV or VHN Vickers Hardness

For microhardness tests; specimen

preparation is necessary.

Hardness scales for HV and HK are

approximately equivalent…

Knoop is used for testing brittle materials

such as ceramics…

For all hardness tests whether it is micro or macro, an indentor made of hardened steel or

diamond is penetrated into the material under a given load and acceleration.

After withdrawal of the indentor the diameter or the depth of the impression is measured and

reported as a relative number such as Brinnel or Rockwell B, or C, etc.

The size of this impression is quite a bit larger than any of the individual grains or hard

particles. Essentially this hardness test is measuring the average hardness of many particles.

This test is Macro Hardness test.

A Micro Hardness test measures the individual hardness of each grain or particle.

Both have macrohardness of on average 600 BHN,

but when microhardness tests are applied it is seen

that local hardness increases to 1200 BHN in the

right sample (containing very hard CrC particles in

Fe).. The matrix has hardness value of 200 BHN…

When macrohardness testing is applied 600 BHN of

hardness value is achieved…

Left sample is Tool Steel and has hardness of 600

BHN on every part…

HARDNESS CONVERSION

Relationship between hardness and strength

� The hardness test is used as indicator of strength

� The strength of cast iron, steel and brass can be

estimated

� In brittle materials, e.g. Ceramics, similar

correlation between hardness and strength does

σT = 3.5 BHN

correlation between hardness and strength does

not occur since cracks may form that serve as stress

raisers for propagation of fractures.

Important Terms:

• Ductility

• Hardness

• Hardness Tests (Microhardness and Macrohardness)

• Relation between Hardness and Yield-Tensile Strength

• Resilience• Resilience

• Toughness