c1 Mechanical Properties

8/8/2019 c1 Mechanical Properties

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APPLIED MATERIAL

(SDD 24202)

MECHANICAL PROPERTIES


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OBJECTIVES

After learning this topic, student will be able:

1. Name the two most common hardness-testing

techniques; note two differences between them.

2. (a) Name and briefly describe the two different

microindentation hardness testing techniques

(b) cite situations for which these techniques are

generally used.

3. Given an engineering stress-strain diagram, determine

(a) the modulus of elasticity

(b) the yield strength

(c) the tensile strength

(d) estimate the percent elongation


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MECHANICAL PROPERTIES

The mechanical properties of materials that

are of importance in structural designs are:

Elasticity

The ability of a material to absorb force and flex in

different directions, returning to its original position.

Our technology technician demonstrates the elasticity

of a material by springing up and down on a piece of

steel rod.


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Plasticity

MECHANICAL PROPERTIES (cont)

The ability of a material to be change in shape

permanently.

Our technology technician and his twin brother

demonstrate the plasticity of a molten

aluminium by pouring it into a mould. Once the

aluminium has cooled down, it can be removed

from the casting sand. It has a new shape.


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Ductility

MECHANICAL PROPERTIES (cont)

The ability of a material to change shape(deform) usually by stretching along its length.

Our technician stretches the lead above his

head. As it stretches if deforms (changes shape).


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PARAMETERS

Some of the important measurable

parameters that are associated with the

mechanical behaviour of materials are:

Hardness

Elastic modulus

Yield strength

Tensile strength

Toughness, etc


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HARDNESS

The hardness of a material is defined as its resistance to permanent

indentation or abrasion.

Large hardness means:

} resistance to plastic deformation or cracking in

}

compression.} better wear properties.

The strength for a particular material is roughly proportional to the

hardness; thus the higher the hardness of a material, the higher is

likely to be the tensile strength.

The ability of a material to resist scratching, wear and tear and indentation.

Our technology technician, dressed in a kilt, slides along the floor to see if it will scratch. It will beconsidered to hard wearing if it resists scratching.


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Hardness testing

Three common hardness measuring tests are Rockwell test

Brinell test

Microhardness (Vickers/Knoop) test


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e.g.,10mm sphere

apply known force(1 to 1000g)

measure sizeof indent afterremoving load

dDSmaller indentsmean largerhardness.

increasing hardness

most

plastics

brasses

Al alloys

easy to machine

steels file hard

cutting

tools

nitrided

steels diamond


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203 HB 5/3000/20

640 HV 30

HardnessBrinell

Diameter ofindenter

Hardnessvalue

Applied force(kgf) Duration (sec)

Hardness

value

Hardness

Vickers

Applied force

(kgf)

HardnessHardness

DesignationsDesignations


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203 HRB

640 HR C

Hardness

RockwellIndenter

type B

Hardness

value

Hardness

value

Hardness

Rockwell

Indenter type

B

HardnessHardness

DesignationsDesignations


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The Rockwelltesting machine operates somewhat like a press, using a

indenterto penetrate the surface of the test sample.

The depth of the indentation determines the materials hardness on a

scale of 0-100

There are several alternative scales, the most commonly used beingthe "B", and "C" scales. Both express hardness as an arbitrary

dimensionless number.

The B-scale is used for softer materials (such as aluminum, brass, and

softer steels). It employs a hardened steel ball as the indenter and a

100kg weight to obtain a value expressed as "HRB".

The C-scale, for harder materials, uses a diamond cone, known as a

Brale indenterand a 150kg weight to obtain a value expressed as

"HRC".

The depth of penetration is converted to a scale in which the harder the

material the higher the number.

Hardness testing

Rockwell Hardness TestRockwell Hardness Test


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Brinell Hardness Test

IntroductionIntroduction

Proposed by a Swedish engineer; Johan August

Brinell (1849 - 1925) in 1900

Brinell Testing refers to surface fatigue caused by

repeated impact or overloading.

The Brinellmethod presses the indenter into a

sample for a given period of time.

The ability for the sample to resist indentation

determines hardness.


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The typical test uses a 10 mm diameter steel ball as an indenter

with a 3,000 kgf (29 kN) force.

For softer materials, a smaller force is used; for harder materials, a

tungsten carbide ball is substituted for the steel ball. The indentation is measured and hardness calculated as:

where:

P= applied force (kgf)

D = diameter of indenter (mm)

d= diameter of indentation (mm)

Hardness Testing

Brinell Hardness TestBrinell Hardness Test


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Hardness testing

Microhardness TestMicrohardness Test

Microhardness testers allow you to measure a materials

hardness while leaving the least amount of damage possible on

the metals surface.

After the indenter is used, a powerful microscope is used to

determine the the amount of indentation into the componentssurface.

The term microhardness test usually refers to static indentations

made with loads not exceeding 1 kgf.

The indenter is either the Vickers diamond pyramid or the

Knoop elongated diamond pyramid. The surface being tested generally requires a metallographic

finish; the smaller the load used, the higher the surface finish

required.

Precision microscopes are used to measure the indentations


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Microhardness Test

Vickers vs KnoopVickers vs Knoop

Vickers indenter penetrates

about twice as deep as Knoop

indenter

Vickers indentation diagonal

about 1/3 of the length of

Knoop major diagonal

Vickers test is less sensitive to

surface conditions than Knoop

test

Vickers test is more sensitive

to measurement errors thanKnoop test

Vickers test best for small

rounded areas

Knoop test best for small

elongated areas

Knoop test good for very hard

brittle materials and very thin

sections


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Microhardness Test

Vickers Hardness TestVickers Hardness Test

The Vickers hardness test was developed in the early 1920s and

uses a pyramid-shaped indenter made of diamond.

It is based on the principle that impressions made by this indenter

are geometrically similar regardless of load.

Accordingly, loads of various size are applied, depending on the

hardness of the material to be measured.

The Vickers Hardness (HV) is then determined from the formula

where

F=

applied load, kg;D = the mean of the two diagonals of the impression made by the

indenter, in mm.


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STRESS-STRAIN BEHAVIOUR

Elastic behaviour

} The mechanical behaviour of a material is tested by applying an

external load on the material and studying the response of the material

to the load.

} The applied load is called STRESS, W

(stress is the force applied per unit area-Newton/m2)} The deformation of the materials is measured is call STRAIN,I, (strain

is defined as the ratio of change in dimension to the original dimension

& has no unit)

I

W Elastic material


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Plastic behaviour} plasticity is a property of a material to undergo a non-

reversible change of shape in response to an appliedforce.

} Plastic deformation occurs under shear stress, asopposed to brittle fractures which occur under normalstress.

} The transition from elastic behavior to plastic behavior iscalled yield.


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STRESS-STRAIN


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Determines the strength of the material whensubjected to a simple stretching operation.

Engineering Strain = Change in Length / Original Length

The engineering stress is defined as :

Engineering Stress = Applied Force /Original Area


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Fracture of a Flat Tensile Test

Specimen


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Stress -Strain Diagram


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STRESS-STRAIN


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STRESS-STRAIN

Ductile Materials

1 Ultimate/Tensile Strength

2 Yield Strength

3 Proportional Limit Stress

4 Rupture

5 Offset Strain (usually 0.002)


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STRESS-STRAIN

Brittle Materials


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The initial slope of the curve, related directly tothe strength of the atomic bonds.

Modulus of Elasticity = E = Change in Stress / Change in Strain

a.k.a Youngs Modulus


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STRESS-STRAIN

Tensile Stress and Strength

} Tensile stress attempts to pull the material

apart.

} Tensile strength is the materials ability to

resist this pulling


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STRESS-STRAIN

Tensile Stress and Strength


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STRESS-STRAIN

Compression Stress and Strength

} Compression stress attempts to squeeze the

material.

} Compressive strength is the ability to resistbeing squeezed.


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STRESS-STRAIN

Shear Stress and Strength

Shear stresses attempt to force the material to

slide against itself sideways


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STRESS-STRAIN

Shear Stress and Strength

Shear strength is the ability to resist internal

sliding.

Torsional stress is really a special type ofshear stress. This stress applies a rotational

motion on one end that attempts to twist the

material.


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STRESS-STRAIN

Ductility

} Ductility is a metals ability to be drawn,

stretched, and permanently deformed

without breaking.} Ductile metals can easily be drawn into long

bars or shaped by cold working.

} Ductility describes the amount of plastic

deformation a material can endure before itbreaks.


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STRESS-STRAIN

Ductility


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BRITTLENESS

If a material fractures under mild impact,

it is considered brittle

Brittleness is undesirable but can be

accepted because of some other useful

properties in brittle materials.


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TOUGHNESS

Toughness is the ability of a material to

absorb energy before it breaks.

Impact toughness is a particular category

of toughness that describes the ability of

a material to withstand a sudden sharp

blow.


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TOUGHNESS


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TOUGHNESS


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ELASTICITY

The ability of a metal to return to its

original shape after any force acting on it

has been removed.


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PLASTICITY

Plasticity is the ability of a material to

deform permanently without breaking or

rupturing.


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FATIGUE FAILURE

Fatigue failure is the result of loads cycling

and off or in opposite directions.

Fatigue failure may result even if the

tensile strength limits of the material have

not been exceeded.


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MALLEABILITY

The ability of material to be easily rolled,

formed, or shaped.


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c1 Mechanical Properties

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