Hardness Test

Hardness TestDwi Marta NurjayaDefinitions and purposesThe measure of how resistant solid matter is to various kinds of permanent shape change when a force is applied.Simply stated, hardness is the resistance of a material to permanent indentation.

Purposes:Measure the hardness value of materialsUsed to give an indication of tensile strength and the ability to resist wear

Hardness Classifications of SnowMeasuring MethodsThree main types of hardness measurements: scratch, indentation, and rebound.

Based on scale (indetation and Load):macro,micro nano.

Different testing methods ---- different scales and valuesScratch hardnessinvented by : Friedrich Mohs (1815)determined by the ability of a material to scratch another material: from 1 (softest = talc) to 10 (hardest = diamond).important to mineralogists and Not suited for metal

Mohs hardness scale

Indentation hardnessMethods:Press a hardened steel ball or diamond into the material to be tested under an applied force

Major important engineering interest for metals. Types of load and indentation: Brinell, Meyer, Vickers, Rockwell,Knoop/Tukon,Janka.

6Hardness tester - Desktop

Hardness testerIndenterAnvil

Hardness tester - Portable

Brinell hardness testJ.A. Brinell introduced the first standardised indentation-hardness test in 1900. The Brinell hardness test consists in indenting the metal surface with a 10-mm diameter steel ball at a load range of 500-3000 kg, depending of hardness of particular materials.The load is applied for a standard time (~30 s), and the diameter of the indentation is measured ---- giving an average value of two readings of the diameter of the indentation at right angle.To avoid excessive indentation, load can be reduced to 31.25 kg, 62.5kg, 187.5kg, 500kg or 1500 kg using 1,25 mm, 2.5 mm, and 5mm steel ball indenterBrinell hardness test

Where :P = applied force (kgf) D = diameter of indenter (mm) d = diameter of indentation (mm)

Units: kg/mm2, but not a pressureHBN, HB

Brinell hardness test withnonstandard load or ball diameterIn order to obtain the same BHN with a non-standard load or ball diameter, it is necessary to produce a geometrical similar indentations.From fig, d = D sin , giving the alternative expression of Brinell hardness number as:

The included angle 2 should remain constant and the load and the ball diameter must be varied in the ratio

Brinell CharacteristicsCalculate extreme cases:High hardness: d=0, BHN = Low hardness: d=10, BHN = 19.1Gives a macrohardness properties of a few mm3 of material.Large indenter and load make fairly large specimen necessaryAnvil effect bulging of back if specimen too thin, resulting in artificially low readingSideways bulging if sample too narrow, resulting in artificially low readingMounting material will crush --- must be removed from back of specimenWell-suited for forgings, castings, heat treated parts, gears, and cast ironSurface preparation not critical as-groundAdvantages and disadvantages ofBrinell hardness testLarge indentation averages out local heterogeneities of microstructure.Different loads are used to cover a wide rage of hardness of commercial metals.Brinell hardness test is less influenced by surface scratches and roughness than other hardness tests.The test has limitations on small specimens or in critically stressed parts where indentation could be a possible site of failure.Test is no good if > 650 BHN

Rockwell hardness testInvented by H.M. Rockwell and S.P. Rockwell (1919-1924)The most widely used hardness test in the US and generally accepted due to:Its speed,Freedom from personal error,Ability to distinguish small hardness difference,Small size of indentation.The hardness is measured according to the depth of indentation, under a constant load (not a load/area).Rockwell hardness consitst of 30 macrohardness tests (15 standard, 15 superficial).Rockwell hardness testLoading sequences:Minor load applied first, to seat indenter:10 kg for standard tests3 kg for superficial testsThen major load60, 100, 150 kg for standard tests15, 30, and 45 kg for superficial testIndentersSteel ball (1/16", 1/8", 1/4", or 1/2" dia.)Diamond cone (Brale indenter)ScalesAll are dimensionlessAll have maximum reading of 130Become inaccurate below 20 or above 100No relationship between scalesRockwell hardness testPrincipal of the Rockwell Test:Position the surface area to be measured close to the indenter.Applied the minor load and a zero reference position is establishedThe major load is applied for a specified time period (dwell time) beyond zeroThe major load is released leaving the minor load applied.

Rockwell hardness standard scaleScaleIndenterMinor LoadF0kgfMajor LoadF1kgfTotal LoadFkgfValue ofEADiamond cone105060100B1/16" steel ball1090100130CDiamond cone10140150100DDiamond cone1090100100E1/8" steel ball1090100130F1/16" steel ball105060130G1/16" steel ball10140150130H1/8" steel ball105060130K1/8" steel ball10140150130L1/4" steel ball105060130M1/4" steel ball1090100130P1/4" steel ball10140150130R1/2" steel ball105060130S1/2" steel ball1090100130V1/2" steel ball10140150130Rockwell hardness superficial scaleScaleIndenterMinor LoadF0kgfMajor LoadF1kgfTotal LoadFkgf15NDiamond cone3121530NDiamond cone3273045NDiamond cone3424515T1/16" steel ball3121530T1/16" steel ball3273045T1/16" steel ball3424515W1/8" steel ball3121530W1/8" steel ball3273045W1/8" steel ball3424515X1/4" steel ball3121530X1/4" steel ball3273045X1/4" steel ball3424515Y1/2" steel ball3121530Y1/2" steel ball3273045Y1/2" steel ball34245Vickers hardness test Developed in 1921 by Robert L. Smith and George E. Sandland at Vickers LtdVickers hardness test uses a square-base diamond pyramid as the indenter with the included angle between opposite faces of the pyramid of 136o.The Vickers hardness number (VHN) is defined as the load divided by the surface area of the indentation.

Vickers hardness testVickers hardness test uses the loads ranging from 1 - 120 kgf, applied for between 10 and 15 seconds.Provide a fairly wide acceptance for research work because it provides a continuous scale of hardness, for a given load.VHN = 5 - 1,500 can be obtained at the same load level --- easy for comparison).

Where,P = the applied load, kgL = the average length of diagonals, mm = the angle between opposite faces of diamond = 136o.Note: the unit can be VHN, DPH, HvVickers hardness numbers are reported as xxxHVyy, e.g. 440HV30, or xxxHVyy/zz if duration of force differs from 10 s to 15 s, e.g. 440Hv30/20Indentations made by Vickers hardnessA perfect square indentation (a) made with a perfect diamondpyramid indenter would be a square.The pincushion indentation (b) is the result of sinking in of the metal around the flat faces of the pyramid. This gives an overestimate of the diagonal length (observed in annealed metals).The barrel-shaped indentation (c) is found in cold-worked metals, resulting from ridging or piling up of the metal around the faces of the indenter. Produce a low value of contact area --- giving too high value.

Types of diamond-pyramid indentation (a) perfect indentation (b) pincushion indentation due to sinking in (c) barrelled indentation due to ridging.

Microhardness TestHardness testing of materials with low applied loads,Applied load beetwen 1 1000 gf,Microindentation tests typically have forces of 2 N (roughly 200 gf) and produce indentations of about 50 m,Using Vickers or Knoop/Tukon method,Useful fora. surfacesb. elongated microconstituentsc. anisotropic properties

Knoop hardness testUsed particularly for very brittle materials or thin sheets,The advantages of the test are that only a very small sample of material is required, and that it is valid for a wide range of test forces. The main disadvantages are the difficulty of using a microscope to measure the indentation (with an accuracy of 0.5 micrometre), and the time needed to prepare the sample and apply the indenter.

where:L = length of indentation along its long axisCp = correction factor related to the shape of the indenter, ideally 0.070279P = load

Both Vickers & KnoopSlowSensitive to surface condition:Requires polishing (through diamond or g-Al2O3 step)Best unetchedRequires load to be normal to surface -- plane parallel surfacesCan be done on mounted specimensSubject to error in diagonal measurementComparisonKnoop shallower indent -- more surface sensitivityVickers smaller L --- more prone to measurement errorsGeneral precautionsIndentations should be spaced minimum 2 dia. apart (edge-edge, not center-center),Specimen thickness should exceed 10t,Controlled application of load dashpot,Sustain load for ~30 s before taking readingInterconversions are material-dependent;,Correlations to tensile strength data are not universal,Need 3 tests (minimum) to avoid innacuracies due impurities and hard spot.Hardness conversion

Shore durometer hardness testThe durometer is a popular instrument for measuring the indentation hardness of the elastrometers and polymers.This is one of the standardised ASTM procedure for testing rubber and plastic (ASTM D2240-68)The operation is similar to rockwell test method. Durometer hardness is a measure of the depth of indentation. The indenter is pressed against the material being tested by a calibrated spring and the indentation depth is measured by an indicating device

Shore durometer hardness testTypes :Model A for testing soft materialsModel D for hard materialsBoth durometers are quite similar, differing primarily in the sharpness of indenters and the magnitude of the load applied to the indenter by the calibrated spring.The load is applied by foot and the reading is taken within seconds of contact of load with the material.HA=0-100 or HD= 0-100 are usual designations

Shore durometer conversion

Hardness test standardsTest MethodsForce RangeIndenter TypesASTMMeasure MethodRockwellRegular60, 100, 150 kgfConical Diamond & Small BallE 18DepthSuperficial15, 30, 45 kgfConical Diamond & Small BallE 18DepthMicrohardnessVickers5 to 2000 grf136o pyramid diamondE 384AreaKnoop5 to 2000 grf1300 x 1720o diamondE 384AreaBrinellOptival500 to 3000 kgf5 mm, 10 mm ballE 10AreaShoreRegular822 gr (A), 4550 gr (D)35o cone (A) 30o cone (D)D 2240DepthMicro257 gr (A), 1135 gr(D)35o cone (A) 30o cone (D)

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