4190365-25082500-587375-29146500Mechanical Engineering Department
Strength of materials Lab
Hardness test 2
Student’s Names ID Marks (20)
Mohammad Issam U17100279 Mohammad ElmahdiU17104844 Mohammad Osama U17103006 Harith Salah U17104748 Yazan Nabil U17105550 Lab Instructor Prof. Khalil AbdelrazekLab Engineer Eng. Abdulrahman
Experiment Date 4/10/2018
Submission Date 18/10/2018
Table of Contents
TOC o “1-3” h z u Abstract PAGEREF _Toc527669841 h 2Introduction and Theory PAGEREF _Toc527669842 h 3Brinell Test PAGEREF _Toc527669843 h 3Vickers Test PAGEREF _Toc527669844 h 3Rockwell Test PAGEREF _Toc527669845 h 4Apparatus and Procedures PAGEREF _Toc527669846 h 4Table(s) of Readings, Sample Calculations and Graph(s) PAGEREF _Toc527669847 h 5Discussion PAGEREF _Toc527669848 h 6Conclusion PAGEREF _Toc527669849 h 7References PAGEREF _Toc527669850 h 8
AbstractThe hardness is one of the mechanical properties of a material, which is defined as the property of the material which enables it to resist plastic deformation, usually by penetration or by indentation, there are three most famous hardness tests known in material science: Rockwell hardness test, Brinell hardness test and Vickers hardness test, each test has its own technique to calculate the hardness number, in addition to that, the range of the applied load on the specimen differs in each test. Rockwell and Brinell tests deal with large applied loads, but Vickers hardness test which we did in the experiment, has the less range of applied load from 1g to 1Kg, and it is usually used for small parts.
We were given five specimens, with unknown materials, and our objective was to calculate the hardness number of each specimen using Vickers hardness test, the “Tuckon 1102” is the machine that we used to do the test. The number which express the hardness of a material in Vickers hardness test is called Vickers hardness number VHN, it can be determined by microscopically measuring the diagonals of the impression resulting from the pre-determined load which makes a scratch, the larger VHN the harder the material, and the less is the indentation.
Introduction and TheoryHardness is generally considered as resistance to penetration. The harder the materials, the greater the resistance to penetration. Hardness is directly related to the mechanical properties of the material. Factors influencing hardness include microstructure, grain size, strain hardening, etc. Generally, as hardness increases so does yield strength and ultimate tensile strength (UTS), thus specifications often require the results of hardness tests rather than tensile tests. The most popular methods are Brinell, Vickers and Rockwell hardness tests for metals and alloys.
Brinell TestIn a standard Brinell test 10 mm diameter hardened steel ball is forced to penetrate the material by 3000 kgf for steels and cast irons. The load and ball diameter selection is important depending on the hardness of materials and 500 kgf is used for softer materials with the same ball diameter. Keeping the ratio of the load to the square of diameter constant (30 for steels and cast irons and 5 for soft metals and alloys), different load and ball diameter combinations can be selected and used in Brinell hardness testing. The Brinell Hardness Number (BHN) is obtained by dividing the applied force, in kgf; by the curved surface area of the indentation, which is actually a segment of sphere.
Where D is the diameter of the indenter ball and d is the average diameter of the indentation, both in mm.
Vickers TestThe Vickers hardness test is based on the same principle as the Brinell test, except the indenter is a diamond pyramid with square base. The angle between the faces of pyramid is 1360. The Vickers Hardness Number (VHN) of materials is obtained by dividing the applied force, in kgf, by the surface of the pyramidal depression yielding the relationship
where D is the average length of diagonals in mm. Due to the shape and hardness of indenter the method is applicable to metals and alloys with wide variety of hardness. Test load is selected between 1 and 120 kgf depending on the hardness of materials.
Rockwell TestIn the Rockwell test, a diamond cone or a hard steel ball is employed as the indenter depending on the hardness of materials. Diamond cone with angle of 1200 is used to test hard materials and the balls of sizes between 1.6 mm (1/16″) and 12.7 mm (1/2″) are used in testing softer materials. Rockwell tests differ from other indentation hardness tests in that the depth of indentation determines the hardness rather than the indentation size. Therefore, surface condition of specimens is very important in Rockwell testing because of its high dependency on the accuracy in indentation depth measurements. In order to establish a reference position a minor load of 10 kgf. is first applied, and the major load is then applied. Additional penetration due to major load is measured and readings are obtained from a calibrated scale (dial) directly, which has a maximum value of 100, depending on the depth of penetration. The hardness numbers are designated HRX, where X indicates the scale used (i.e. 50 HRC for 50 points on the C scale of dial). It should be noted that a Rockwell hardness number is meaningless unless the scale is not specified.
Apparatus and Procedures
1.Prepare the surface of the sample by polishing it.
2.Correct the elevation of the platform to have the sample at the desired position under the turret.
3.Place the sample on the platform and rotate the turret to choose a good spot on the sample surface for the test.
4.Choose the load applied and the time of the test.
5.Run the machine to start the test.
6.After the time finish look at the microscope and measure the diagonals of the diamond shape formed on the surface using the axis lines.
7.The machine calculates the HV and show it on the screen,
8.Repeat the test 10 times and record the values of HV.
Table(s) of Readings, Sample Calculations and Graph(s) Table SEQ Table * ARABIC 1 -VHN for all the tests done on the samples-
Test Sample 1 Sample 2 Sample 3 Sample 4 Sample 5
Test 1 239.4 236.8 240.3 248.7 274
Test 2 234.7 232.8 256.1 285.8 264.4
Test 3 254.4 254.6 250.8 271.5 253.8
Test 4 248.6 249.5 254.5 291.5 242.4
Test 5 282.8 246 264.8 311.5 253.8
Test 6 263.3 240.9 264.8 317.6 280.7
Test 7 277.3 232.3 263.4 294.5 272.9
Test 8 276.1 230.6 260.6 259.8 266.4
Test 9 263.8 235 232.8 249 275.2
Test 10 286.3 231.2 249.1 273.9 281.1
Figure SEQ Figure * ARABIC 1 -Average HV for the five samples-
DiscussionBy observing the figure above, sample 4 was the hardest material with an average HV of (305.93), while samples 1 and 4 showed approximately close Vickes’s hardness numbers (262.67) and (266.47) respectively and as a result they might be of the same material. The values of the HV for all samples showed high variations, ranging from ?=8.44 for sample 2 to ?=24.13 for sample 4. This impreciseness occurs due to different related factors. Starting with what is called material factors, the samples could be nonhomogeneous or deflected. Therefore, different regions on the surface may have different values of HV. Secondly, our samples weren’t prepared with the most functional way, the thing that results in every individual having his own approximation for the perfect spot on the surface to do the test (Human factor). Also, to adjust the axis of the lens perfectly every time to measure the diagonals would be impossible. Finally, when each test was performed the timing between the tests and the spacings weren’t constant. The microscopic structure of the material won’t be stable as it went through several plastic deformations without rest (instruments and measuring factors).
ConclusionThe hardness of a material is defined as the relative capacity of a material for scratching another or for being scratched or indented by another, and there are different types of tests to measure the hardness of the material, such as, Rockwell hardness test, Brinell hardness test, Knoop hardness test and Vickers hardness test, and the hardness test is very important, because we can estimate another mechanical properties such as tensile strength.
Our experiment was about Vickers hardness test, which uses much smaller applied loads than Rockwell and Brinell tests ranging between 1g to 1Kg, during the test, a square-base diamond pyramid indenter starts to scratch the specimen, then the microscope displays the indentation on the software, and there is a number shown by the software called diamond pyramid number(HV), which is determined by dividing the load over the surface area of indentation, and its unit is kgf/mm2, (kgf stands for kilogram-force which is equivalent to the constant (g)in unit of Newton’s), and the softer the material, the larger and deeper the indention, and the lower the hardness index number.
1 G. Dieter, “Mechanical Metallurgy,” 1986. Online. Available: http://stu.westga.edu/~bthibau1/MEDT%207477-Cooper/Calibre%20Library/Dieter_%20George%20Ellwood/Mechanical%20metallurgy%20(13)/Mechanical%20metallurgy%20-%20Dieter_%20George%20Ellwood.pdf.