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 knifeedges. 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 Diagonals length (mm) Specimen
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
Brinell Hardness Number
Tensile Strength, Σ t
Number (HV)
(BHN)
(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
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