Hardness Testing PDF

3.2. HARDNESS TESTING Early hardness tests were based on natural minerals with a scale constructed solely on the ability of one material to scratch another that was softer. A qualitative and somewhat arbitrary hardness indexing scheme was devised, termed the Mohs scale, which ranged from 1 on the...

3.2. HARDNESS TESTING Early hardness tests were based on natural minerals with a scale constructed solely on the ability of one material to scratch another that was softer. A qualitative and somewhat arbitrary hardness indexing scheme was devised, termed the Mohs scale, which ranged from 1 on the soft end for talc to 10 for diamond. Quantitative hardness techniques have been developed over the years in which a small indenter is forced into the surface of a material to be tested, under controlled conditions of load and rate of application. 3.2. HARDNESS TESTING The depth or size of the resulting indentation is measured, which in turn is related to a hardness number; the softer the material, the larger and deeper the indentation. Measured hardnesses are only relative (rather than absolute), and care should be exercised when comparing values determined by different techniques. A variety of hardness tests have been devised, but the most commonly used are the Rockwell test and the Brinell test. Different indenters used in these tests. 3.2. HARDNESS TESTING Hardness tests are performed more frequently than any other mechanical test for several reasons: 1. They are simple and inexpensive ordinarily no special specimen need be prepared, and the testing apparatus is relatively inexpensive. 2. The test can be considered nondestructive the specimen is neither fractured nor excessively deformed; a small indentation is the only deformation. 3. Other mechanical properties often may be estimated from hardness data, such as tensile strength. 3.2. HARDNESS TESTING ROCKWELL HARDNESS TEST For Rockwell, the minor load is 10 kg, whereas major loads are 60, 100, and 150 kg. Each scale is represented by a letter of the alphabet; several are listed with the corresponding indenter and load in Tables. For superficial tests, 3 kg is the minor load; 15, 30, and 45 kg are the possible major load values. These scales are identified by a 15, 30, or 45 (according to load), followed by N, T, W etc. depending on indenter. Superficial tests are frequently performed on thin specimens. 3.2. HARDNESS TESTING ROCKWELL HARDNESS TEST When specifying Rockwell and superficial hardnesses, both hardness number and scale symbol must be indicated. The scale is designated by the symbol HR followed by the appropriate scale identification. For example, 80 HRB represents a Rockwell hardness of 80 on the B scale, and 60 HR30W indicates a superficial hardness of 60 on the 30W scale. 3.2. HARDNESS TESTING ROCKWELL HARDNESS TEST For each scale, hardnesses may range up to 130; however, as hardness values rise above 100 or drop below 20 on any scale, they become inaccurate; and because the scales have some overlap, in such a situation it is best to utilize the next harder or softer scale. Inaccuracies also result if the test specimen is too thin, if an indentation is made too near a specimen edge, or if two indentations are made too close to one another. Specimen thickness should be at least ten times the indentation depth, whereas allowance should be made for at least three indentation diameters between the center of one indentation and the specimen edge, or to the center of a second indentation. 3.2. HARDNESS TESTING ROCKWELL HARDNESS TEST Furthermore, testing of specimens stacked one on top of another is not recommended. Also, accuracy is dependent on the indentation being made into a smooth flat surface. The modern apparatus for making Rockwell hardness measurements is automated and very simple to use; hardness is read directly, and each measurement requires only a few seconds. This apparatus also permits a variation in the time of load application. This variable must also be considered in interpreting hardness data.  3.2. HARDNESS TESTING ROCKWELL HARDNESS TEST Table: Rockwell Hardness Scales Table: Superficial Rockwell Hardness Scales  3.2. HARDNESS TESTING ROCKWELL HARDNESS TEST Table: Hardness Testing Techniques 3.2. HARDNESS TESTING BRINELL HARDNESS TEST In Brinell tests, as in Rockwell measurements, a hard, spherical indenter is forced into the surface of the metal to be tested. The diameter of the hardened steel (or tungsten carbide) indenter is 10.00 mm (0.394 in.). Standard loads range between 500 and 3000 kg in 500-kg increments; during a test, the load is maintained constant for a specified time (between 10 and 30s). Harder materials require greater applied loads. The Brinell hardness number, HB, is a function of both the magnitude of the load and the diameter of the resulting indentation (surface area of the spherical cup). 3.2. HARDNESS TESTING BRINELL HARDNESS TEST This diameter is measured with a special low-power microscope, utilizing a scale that is etched on the eyepiece. The measured diameter is then converted to the appropriate HB number using a chart; only one scale is employed with this technique. Maximum specimen thickness as well as indentation position (relative to specimen edges) and minimum indentation spacing requirements are the same as for Rockwell tests. In addition, a well-defined indentation is required; this necessitates a smooth flat surface in which the indentation is made.  3.2. HARDNESS TESTING BRINELL HARDNESS TEST Table: Hardness Testing Techniques 3.2. HARDNESS TESTING Knoop and Vickers Microindentation Hardness Tests Two other hardness-testing techniques are Knoop and Vickers (sometimes also called diamond pyramid). For each test a very small diamond indenter having pyramidal geometry is forced into the surface of the specimen. Applied loads are mostly much smaller than for Rockwell and Brinell. The resulting impression is observed under a microscope and measured; this measurement is then converted into a hardness number. Careful specimen surface preparation (grinding and polishing) may be necessary to ensure a well- defined indentation that may be accurately measured. 3.2. HARDNESS TESTING Knoop and Vickers Microindentation Hardness Tests The Knoop and Vickers hardness numbers are designated by HK and HV, respectively, and hardness scales for both techniques are approximately equivalent. Knoop and Vickers are referred to as microindentation-testing methods on the basis of indenter size. Both are well suited for measuring the hardness of small, selected specimen regions; furthermore, Knoop is used for testing brittle materials such as ceramics. 3.2. HARDNESS TESTING SCHORE SCLEROSCOPE TESTS Invented by A. F. Shore, it is done by dropping a diamond tipped hammer by its own weight (approx. 2.5 g) from a fixed height (standart height is about 250 mm) and reading rebound height. Can be used for plastics and rubbers. Higher the rebound, harder the material. Due to its portability, the testers can be used for various size of parts, including small parts (e.g. thin sheets) and large parts (e.g. steel rolls). Surface finish of the part is important for reliable measurement. Tests should not be made more than once on the same spot due to cold working occurring around that spot. Thus, the indentations must be at least 0.51 mm apart from each other.  3.2. HARDNESS TESTING HARDNESS CONVERSION Comparison of several hardness scales. Detailed information about conversion can be found ASTM Standard E 140, Hardness Conversion Tables for Metals 3.2. HARDNESS TESTING Correlation between Hardness and Tensile Strength Both tensile strength and hardness are indicators of a resistance to plastic deformation. Consequently, they are roughly proportional for tensile strength as a function of the HB for cast iron, steel, and brass. The same proportionality relationship does not hold for all metals. As a rule of thumb for most steels, the HB and the tensile strength are related according to; 3.2. HARDNESS TESTING 3.2. HARDNESS TESTING  References Materials Science and Engineering, William D. Callister, Jr. John Wiley Sons The Science and Engineering of Materials, D.R. Askeland, PWS Pub. Co. Introduction to Materials Science for Engineers, James F. Shackelford Metals The Structure and Properties of Materials , Volume III - Mechanical Behavior, H.W. HAYDEN, W.G. MOFFATT, J. WULFF  CHAPTER III MECHANICAL TESTING AND PROPERTIES BENDING, COMPRESSION AND TORSION TESTS Asst. Prof. Dr. Yasin SARIKAVAK 3.3. BENDING TEST A member subjected to bending moment is called as beam. Both tensile and compressive stresses will develop in a beam under bending moment. Simple bending and either 3-point or 4-point bending tests are used. 3.3. BENDING TEST Figure A and B illustrates simple tests requiring little equipment other than a vice, whilst C represents a more widely accepted test in which the wire is bent through 90° over a cylinder of specified radius R , then back through 90° in the opposite direction. This is continued until the test-piece breaks, the number of bending cycles being counted. The surface affected by the bending process is also examined for cracks, and, if necessary, for coarse grain. 3.3. BENDING TEST Simple Bending Test Simple bend tests. (A) The material is bent back upon itself. (B) It is doubled over its own thickness, the second bend being the test bend. (C) A specific radius R is used. 3.3. BENDING TEST 3 Point / 4 Point Bending Test Stress due to bending moment: = Mc/I M:Bending moment (Nm) c: Distance from neutral axis (m) I: Moment of inertia Three and four point bending tests. 3.3. BENDING TEST Bending Test Unless stated by the standard, bending test pieces have a (length / thickness) ratio more than 10. Bending tests are very commonly used for concrete, cast irons, ceramics, welded structures and some plastics. Various definitions of bending strength; Flextural strength Cross-bending strength Modulus of rapture Transverse strength (commonly used) 3.4. COMPRESSION TEST The behavior of brittle and ductile materials during a compression test. 3.5. SHEAR (TORSION)TEST Direct Shear All metal cutting actions fall into this type of loading. Press cutting with punches and dies are good examples. 3.5. SHEAR (TORSION)TEST TORSION TEST Shear stress G Shear modulus T Applied torque Angle of twist R Distance from the axis (gauge radius) L Gauge length J Polar moment of inertia = ( R)/L FAILED SPECIMENS  References Materials Science and Engineering, William D. Callister, Jr. John Wiley Sons The Science and Engineering of Materials, D.R. Askeland, PWS Pub. Co. Introduction to Materials Science for Engineers, James F. Shackelford The Structure and Properties of Materials , Volume III - Mechanical Behavior, H.W. HAYDEN, W.G. MOFFATT, J. WULFF  CHAPTER III MECHANICAL TESTING AND PROPERTIES IMPACT TEST AND CREEP TEST Asst. Prof. Dr. Yasin SARIKAVAK 3.6. IMPACT TEST Details of standard test-pieces used in both the Izod and Charpy tests. 3.6. IMPACT TEST The nature of the fractured surface in the Impact test. 3.6. IMPACT TEST Factors effecting impact energy: a)Temperature Gradually transition Typical ductile-brittle transition from ductile to brittle curve for annealed low carbon behaviour. Gradual steel. Sudden change (Material B) change (Material A) 3.6. IMPACT TEST Factors effecting impact energy: b) Composition Influence of carbon content on the Charpy V-notch energy versus temperature behaviour for steel. 3.6. IMPACT TEST Factors effecting impact energy: c) Microstructure Effects of specimen orientation on energy absoption 3.7. CREEP TEST Creep is a time- dependent and permanent deformation of materials when subjected to a constant load (stresses lower than yield strength) at a high temperature (> 0.4 Tm). 3.7. CREEP TEST Stages of Creep Test 1. Instantaneous deformation, mainly elastic. 2. Primary/transient creep. Slope of strain vs. time decreases with time: work- hardening 3. Secondary/steady-state creep. Rate of straining is constant: balance of work- hardening and recovery. 4. Tertiary. Rapidly accelerating strain rate up to failure: (local deformation necking took place) formation of internal cracks, voids, grain boundary separation, necking, etc. 3.7. CREEP TEST Stages of Creep Test The stage of secondary/steady-state creep is of longest duration and the steady- state creep rate is the most important parameter of the creep behavior in long-life applications. Another parameter, especially important in short-life creep situations, is time to rupture, or the rupture lifetime tr. 3.7. CREEP TEST Stress and Temperature Effect With increasing stress or temperature: The instantaneous strain increases The steady-state creep rate increases The time to rupture decreases