INTRODUCTION AND OBJECTIVE

Hardness is usually defined as the resistance of a material to plastic penetration of its

surface. There are three main types of tests used to determine hardness. They are Scratch

tests, Dynamic Hardness tests and Static Indentation tests. Vickers are of the Static

Indentation tests.

Static Indentation tests are based on the relation of indentation of the specimen by a

penetrator under a given load. The relationship of total test force to the area or depth of

indentation provides a measure of hardness. The Rockwell, Brinell, Knoop, and ultrasonic

hardness tests are the others of this type besides Vickers. For engineering purposes, mostly

the static indentation tests are used.

Our primary aim is to measure and determine the Vickers Hardness values for various

metals. In this case, aluminium and PVC are used. We will also focus on calculating the

ultimate tensile strengths (UTS) of the materials using equation.

APPARATUS AND SPECIMEN

1) Vickers Hardness Testing machine

2) Test loads : 1 to 1000 g

3) Indenter : 136 square-base Vickers diamond pyramid

4) Specimens : Aluminium and Polyvinyl Chloride (PVC)

THEORY

Vickers Hardness Test

The Vickers hardness test uses a square base diamond pyramid as the indenter. The

included angle between the opposite faces of the pyramid is l36°. The Vickers hardness tester

operates on the same basic principle as the Brinell tester, the numbers being expressed in the

terms of load and area of the impression. As a result of the indenter’s shape, the impression

on the surface of the specimen will be a square. The length of the diagonal of the square is

measured through a microscope fitted with an ocular micrometer that contains movable knife-

edges. Test loads are between 1 and 1000 g which referring to microhardness test. The

Vickers hardness values are calculated by the formula:

where P is the applied load in kg, and d is the diagonal length in mm.

Test Location

If indentation is placed too close to the edge of specimen, the workpiece edge will

bulge, and the hardness number will decrease accordingly. To ensure an accurate test, the

distance from the center of the indentation to the edge of the specimen must be at least two

and one-half diameters.

An indentation hardness test cold works the surrounding material. If another

indentation is placed within this cold worked area, the reading usually will be higher than the

real value. Generally, the softer the material, the more critical the spacing of indentations

becomes. However, a distance three diameters from the center of one indentation to another is

sufficient for most materials.

Hardness Testing Hardness Testing In Estimating Other Material Properties

Hardness testing has always appeared attractive as a means of estimating other

mechanical properties of metals. The following equation is used to predict tensile strength of

steels by means of hardness measurement.

UTS = 0.35*BHN (in kg/mm2)

EXPERIMENT PROCEDURE

Each specimen us identified and the properties differences are discussed as given in

Table 1. The measurement of density conducted is described.

The location for the indenter to be pressed is observed. The surface of the specimen is

made sure to be flat.

Different weight of test loads are used, which is between 1 to 1000g.

The indenter is pressed into the sample by an accurately controlled test force.

The force is maintained for a specific dwell time, normally 10-15 seconds.

After the dwell time is complete, the indenter is removed leaving an indent in the

sample that appears square shaped on the surface.

The size of the indent is determined optically by measuring the two diagonals of the

square indent. The diagonals length is measured and the result is recorded in Table 2.

The Vickers hardness number is a function of the test force divided by the surface

area of the indent. The average of the two diagonals is used in the formula given in to

calculate the Vickers hardness.

RESULTS AND DISCUSSION

Specimen 1 2

Type Aluminium PVC

Colour Shining silver Dark grey

Density (kg/m3) 2700 Rigid PVC Flexible PVC

1.3–1.45 1.1–1.35

Table 1

Specimen

Diagonals length (mm)

Daverage (mm)D1 D2

Aluminium 121.06 137.50 129.28

PVC 162.38 162.50 162.44

Table 2

Specimen Daverage (mm) Vickers Hardness Number (HV) Hardness Rank

Aluminium 129.28 111.00 1

PVC 162.44 14.06 2

Table 3

Specimen Vickers Hardness

Number (HV)

Brinell Hardness Number

(BHN)

Tensile Strength, Σt

(MPA)

Aluminium 111.0 95 HBS 326.18

PVC 14.06 15 HBS 51.50

Table 4

Describe the correlation between the hardness and tensile

strength.

The yield strength in tension is about 1/3 of the hardness . To find the ball park figure for the yield strength convert the hardness number to MPa (or psi ) and divide by 3. For example take the Vickers number, which has the dimension kg/mm2, and multiply by 10 to (approximately) convert it to /mm2 (=MPa) then divide by three.

For example: HV 300 corresponds to a Sigma-y of approximately 1000 MPa. An approximate relationship between the hardness and the tensile strength (of steel) is,

What is the importance of microhardness test in the mechanical

engineering field?

In many cases you may have different materials in contact with each other and carry loads

and etc.. in all these cases you need to know the hardness of them to avoid too much friction,

erosion, cracking, bending and so on. 

CALCULATION

Example of the Vickers hardness number calculation :

1.8544( P/d² ) =

Example of the tensile strength calculation :

0.35*BHN

CONCLUSION

REFERENCES

•ASM Metals’ Handbook

•Mechanical Testing of Materials, A. J. Fenner, Philosophical Library Inc., 1965

•Pollack, H. W., Materials Science and Metallurgy, Reston Pub. Co., Reston:1981

•Van Vlack, L. H., Materials Science for Engineers, Addison Wesly Pub. Co., Mass:1970