MECHANICAL TESTING OF METALLIC

MATERIALS

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Outline

• Hardness Testing

• Tensile Testing

• Impact Testing

• Fatigue Testing

• Creep Testing

• Wear Testing

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MATERIALS PROPERTIES

Design Engineer - Materials properties

Materials Properties

CHEMICALCompositionCrystal structureBonding

MECHANICALHardnessStrength-ductility

ImpactFatigue Creepwear

PHYSICALDensityThermal

ElectricalMagnetic

DIMENSIONALSizeShapeMicrotography

SERVICE LIFE

LT/HT/RT

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HARDNESS TESING

• Hardness is resistance of material to plastic

deformation caused by indentation

Softer materials

Deeper or larger

impression

increasing hardness

most plastics

brasses Al alloys

easy to machine steels file hard

cutting tools

nitrided steels diamond

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HARDNESS

Tribologist : – high hardness desirable to reduce plastic

deformation and wear in bearings

Machinist : - low hardness preferable for easy and

faster machining

Fracture mechanics : – hard material – brittle –

engineer undesirable under impact loads

Metal forming : – low hardness preferable for

engineer cold rolling

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• Principle of hardness test method :

- forcing an indenter into the sample surface

followed by

measuring dimensions of the indentation

(depth or actual surface area of the indentation)

BASIC PRINCIPLE

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TYPES OF HARDNESS MEASUREMENTS

There are three general types of hardness measurements

1) Scratch hardness

• The ability of material to scratch on one another

• Important to mineralogists, using Mohs’scale 1= talc, 10 = diamond

• Not suited for metal annealed copper = 3, martensite = 7.

2) Indentation hardness

• Major important engineering interest for metals.

• Different types : Brinell, Meyer, Vickers, Rockwell

hardness tests.

3) Rebound or dynamic hardness

• The indentor is dropped onto the metal surface and the

hardness is expressed as the energy of impact

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CLASSIFICATION HARDNESS TESING

-Depending on the loading force value & the indentation dimensions

• Macro-hardness tests

• Micro-hardness tests

• Nano-hardness test

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HARDNESS TESING

Macro-hardness tests :

- the most widely used methods for rapid routine

hardness measurements

- indenting forces in macro-hardness tests are in the

range of 50N to 30000N

• Brinell hardness test

• Rockwell hardness test

• Vickers hardness test

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BRINELL HARDNESS TEST

- One of the oldest tests

Static test that involves pressing a hardened steel

ball (10mm) into a test specimen while under a load of

3000 kg load for hard metals,

1500 kg load for intermediate hardness metals

500 kg load for soft materials

INDENTATION

PRODUCED IN

SURFACE

BALL PRESSED

INTO THE

SURFACE

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BRINELL HARDNESS TEST

where

BHN = the Brinell hardness number

P = the imposed load in kg

D = the diameter of the spherical indenter in mm

d = diameter of the resulting indenter impression in mm

If the BHN value is

higher, then the

material is said to be

harder

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BRINELL HARDNESS TEST

• Rules of thumb

– 3000 kg load should be used for a BHN of 150 and above

– 1500 kg load should be used for a BHN between 75 and 300

– 500 kg load should be used for a BHN less than 100

– The material’s thickness should not be less than 10 times the

depth of the indentation

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BRINELL HARDNESS TEST

• Brinell Test can not be used :

very soft or very hard materials

(Indentation becomes equal to the diameter of the

ball or

No or little Indentation)

Accurate measurement :

-impression should be a diameter

between 0.25D and 0.50D

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BRINELL HARDNESS TEST

• Limitation of Brinell Hardness Test :

Materials with hardness upto 450HBN with steel ball

Materials with hardness upto 600 HBN with tungsten

carbide ball

– Brinell numbers greater than 650 should not be trusted because the

diameter of the indentation is too small to be measured accurately

and the ball penetrator may flatten out.

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Brinell Test Method

• Advantages

– Well known throughout industry with well accepted results

– Tests are run quickly (within 2 minutes)

– Test inexpensive to run once the machine is purchased

– Insensitive to imperfections (hard spot or crater) in the material

• Limitations

– Not well adapted for very hard materials, wherein the ball deforms

excessively

– Not well adapted for thin pieces

– Not well adapted for case-hardened materials

– Heavy and more expensive than other tests

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ROCKWELL HARDNESS TESTING

Rockwell Hardness Test : - widely used method

(accuracy, simplicity and rapidity)

-there are two types of Rockwell tests

(i) Rockwell: - the minor load is 10 kgf,

the major load is 60, 100, or 150 kgf.

(ii) Superficial Rockwell: -the minor load is 3 kgf and

major loads are 15, 30, or 45 kgf.

In both tests, the indenter may be either a diamond cone or

steel ball, depending upon the characteristics of the

material being tested.

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ROCKWELL HARDNESS TESTING (ASTM18)

Principal of the Rockwell Test

1. The indenter moves down into

position on the part surface

2. A minor load is applied and a zero reference position is established (10kg)

3. The major load is applied for a specified time period (dwell time)

beyond zero

4. The major load is released leaving the minor load applied

- The Rockwell number represents the difference in depth from the

zero reference position as a result of the applied major load.

Deeper indentation Softer material

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ROCKWELL HARDNESS TESTING

Rockwell hardness numbers are always quoted with a scale symbol

representing the indenter and forces used.

The hardness number is followed by the symbol HR and

the scale designation.

Ex; 64 HRC= Rockwell hardness number of 64 on Rockwell C scale.

81 HR30N = Rockwell superficial hardness number of 81 on

Rockwell 30N scale.

Rockwell Indenters:

hardened steel ball

Diamond cone or

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Superficial hardness test : Initial force : 29.4 N

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Superficial hardness test : Initial force : 10kg

ROCKWELL HARDNESS SCALES

Note: The diameter of the balls arise from standard sizes in inches, 1.588mm being

1/16in, 3.175mm being 1/8in, 6.350mm being 1/4in, and 12.70mm being 1/2in

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Superficial hardness test : Initial force : 29.4 N

ROCKWELL SUPERFICIAL HARDNESS SCALES

Note: N scales are used for materials that if thick enough would have been tested on the C scale, the T scales for those on the B scale