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ENGINEERING MATERIALS

ENGINEERING MATERIALS. testing, examination, observation and analysis of Materials PART - II. HARDNESS IS THE RESISTANCE OF A MATERIAL TO LOCALIZED PLASTIC DEFORMATION, USUALLY BY SCRATCH AND IDENTATION.

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ENGINEERING MATERIALS

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  1. ENGINEERING MATERIALS

  2. testing, examination, observation and analysis of Materials PART - II

  3. HARDNESS IS THE RESISTANCE OF A MATERIAL TO LOCALIZED PLASTIC DEFORMATION, USUALLY BY SCRATCH AND IDENTATION. THE HARDNESS OF A METAL DEPENDS ON THE EASE WITH WHICH IT CAN BE PLASTICALLY DEFORMED, AND THUS THERE IS A RELATIONSHIP BETWEEN HARDNESS AND STRENGTHS OF MATERIALS. HARD MATERIALS ARE VERY FREQUENTLY REQUIRED FOR MACHINING METALS AND CERAMIC AND ALSO NEEDED FOR INTRODUCING PERMANENT DEFORMATION TO OTHER MATERIALS. PROPERTIES OF MATERIALS - HARDNESS

  4. DIFFERENT MATERIALS TESTS HARDNESS TESTING HARDNESS IS THE MEASURE OF HOW RESISTANT SOLID MATTER IS TO VARIOUS KINDS OF PERMANENT SHAPE CHANGE WHEN A FORCE IS APPLIED. MACROSCOPIC HARDNESS IS GENERALLY CHARACTERIZED BY STRONG INTERMOLECULER BONDS. HARDNESS IS ALSO DEPENDENT ON DUCTILITY, ELASTICITY, PLASTICITY, STRAIN , STRENGTH, TOUGHNESS AND VISCOSITY. COMMON EXAMPLES OF HARD MATTER ARE CERAMICS, CONCRETE, CERTAIN METALS, AND SUPERHARD MATERIALS, WHICH CAN BE CONTRASTED WITH SOFT MATTER.

  5. HARDNESS TESTING HOWEVER, THE BEHAVIOR OF SOLID MATERIALS UNDER FORCE IS COMPLEX, THEREFORE THERE ARE DIFFERENT MEASUREMENTS OF HARDNESS SUCH AS SCRATCH HARDNESS, INDENTATION HARDNESS, AND REBOUND HARDNESS. GENERALLY, THE HARDNESS OF MATERIALS CAN BE DETERMINED BY USING EITHER A SCRATCH TEST OR BY MAKING THE SURFACE INDENTATION. SURFACE INDENTATION TEST MAY BE PERFORMED WITH THOSE MATERIALS WHICH CAN UNDERGO TO PLASTIC DEFORMATION. THE RESISTANCE OF A MATERIAL TO INDENTATION IS NOT NECESSARILY THE SAME AS ITS RESISTANCE TO ABRASION, BUT A HARDNESS MEASUREMENT OBTAINED FROM AN INDENTATION TEST CAN BE USED AS AN EMPIRICAL CHECK FOR ABRASION RESISTANCE.

  6. IN GENERAL, METAL HAVING HIGH HARDNESS WILL HAVE A HIGH RESISTANCE TO ABRAIVE WEAR. THERE ARE ALSO EMPIRICAL RELATIONSHIPS BETWEEN THE HARDNESS OF MATERIALS AND THEIR TENSILE STRENGTHS. AS A GENERAL RULE FOR METALS AS THE HARDNESS INCREASES SO ALSO THE TENSILE, COMPRESSIVE AND SHEAR STRENGTH INCREASES. ALSO DUCTILITY OF METALS DEREASES AS THE HARDNESS INCREASES. IN A SIMILAR WAY MORE HARDER MATERIALS WOULD BE MORE BRITTLE.

  7. AS THERE ARE THREE MAIN TYPES OF HARDNESS MEASUREMENTS: SCRATCH, INDENTATION, AND REBOUND, AND FOREACH OF THESE CLASSES OF MEASUREMENT THERE ARE INDIVIDUAL MEASUREMENT SCALES. FOR PRACTICAL REASONS CONVERSION TABLES ARE USED TO CONVERT BETWEEN ONE SCALE AND ANOTHER SCALE. SCRATCH HARDNESS SCRATCH HARDNESS IS THE MEASURE OF HOW RESISTANT A SAMPLE IS TO FRACTURE OR PLASTIC (PERMANENT) DEFORMATION DUE TO FRICTION FROM A SHARP OBJECT.

  8. THE PRINCIPLE IS THAT AN OBJECT MADE OF A HARD MATERIAL WILL SCRATCH AN OBJECT MADE OF A SOFTER MATERIAL. THE MOST COMMON TEST IS MOHS SCALE, WHICH IS USED IN MINERALOGY. ONE TOOL TO MAKE THIS MEASUREMENT IS THE SCLEROMETER. REBOUND HARDNESS REBOUND HARDNESS, ALSO KNOWN AS DYNAMIC HARDNESS, MEASURES THE HEIGHT OF THE "BOUNCE" OF A DIAMOND-TIPPED HAMMER DROPPED FROM A FIXED HEIGHT ONTO A MATERIAL. THIS TYPE OF HARDNESS IS RELATED TO ELASTICITY. THE DEVICE USED TO TAKE THIS MEASUREMENT IS KNOWN AS A SCLEROSCOPE. TWO SCALES THAT MEASURES REBOUND HARDNESS ARE THE LEEB REOUND HARDNESS TEST AND BENNETT HARDNESS SCALE.

  9. INDENTATION HARDNESS INDENTATION HARDNESS MEASURES THE RESISTANCE OF A SAMPLE TO PERMANENT PLASTIC DEFORMATION DUE TO A CONSTANT COMPRESSION LOAD FROM A SHARP OBJECT, THESE ARE PRIMARILY USED IN ENGINEERING AND METALLURY FIELDS. THE TESTS WORK ON THE BASIC PREMISE OF MEASURING THE CRITICAL DIMENSIONS OF AN INDENTATION LEFT BY A SPECIFICALLY DIMENSIONED AND LOADED INDENTER.

  10. FOLLOWING TYPES OF INDENTATION TESTS ARE AVAILABLE FOR THE HARDNESS TESTING OF METALS BRINELL TEST, VICKERS DIAMOND TEST ROCKWELL TEST MICROHARDNESS TESTING OF MATERIALS IS ALSO PERFORMED. TYPES OF INDENTATION HARDNESS TESTS

  11. THE BRINELL TEST THE BRINELL TEST WAS THE FIRST STATIC INDENTATION TEST TO COME INTO GENERAL USE. IN ITS ORGINAL FORM, IT UTILISES A HARDENED STEEL BALL INDENTOR TO 10 mm DIAMETER FORCED INTO THE SURFACE OF THE METAL BEING TESTED UNDER A STATIC LOAD OF 3000 KG AND THE LOAD IS MAINTAINED FOR 10 TO 15 SECONDS. A LOAD OF 3000 KG. IN CONJUNCTION WITH A BALL INDENTOR OF 10 mm DIAMETER IS SUITABLE FOR USE WITH STEELS AND CAST IRONS. FOR SOFTER NON-FERROUS METALS AND ALLOYS LOWEVER VALUES OF STATIC LOAD ARE USED. FOR EXAMPLE RECOMMENDED VALUES OF F/D² RATIO FOR STEELS AND CAST IRONS ARE 30, FOR COPPER AND ALUMINIUM ALLOYS IS 10, FOR PURE COPPER AND ALUMINIUM 5 AND FOR LEAD, TIN AND TIN ALLOYS 01 RESPECTIVELY.

  12. DIAMETER OF THE RESULTING IMPRESSION IS MEASURED WITH THE AID OF CALIBRATED MICROSCOPE. THE BRINELL HARDNESS NUMBER “HB” IS CALCULATED BY THE FOLLOWING RELATIONSHIP: HB = APPLIED LOAD (KG) / SURFACE AREA OF IMPRESSION (mm) IT IS TO BE NOTED THAT THE HARDNESS NUMBER OBTAINED FROM A BRINELL TEST IS NOT INDEPENDENT OF THE LOAD USED AND IF TWO TESTS ARE CONDUCTED ON THE SAME MATERIAL WITH DIFFERENT STATIC LOAD, THE HARDNESS VALUES OBTAINED MAY DIFFER. HENCE THE RECOMMENDED VALUES OF STATIC LOAD (F) AND DIAMETER OF THE BALL INDENTOR (D) AS GIVEN IN BS 240 (1986) AND ASTM E 10-84 MUST BE USED. AFTER HAVING BRINELL HARDNESS IMPRESSION, DIAMETER OF IMPRESSION (d) HAS TO BE MEASURED. ACCURATE BRINELL HARDNESS VALUES WILL DEPEND IF THE VALUE OF d IS BETWEEN THE RANGE OF 0.25D TO 0.5D

  13. THE SURFACE AREA OF AN IMPRESSION OF DIAMETER “d”, MADE BY A SPHERICAL INDENTOR OF DIAMETER “D” IS CALCULATED BY THE FOLLOWING RELATIONSHIP: AREA OF IMPRESSION = ΠD/2{D - √(D² – d²)} THEREFORE HB = 2F / ΠD{D - √(D² – d²)} THE BRINELL TEST IS NOT SUITABLE FOR THE TESTING OF VERY HARD MATERIALS. AS THE HARDNESS OF THE METAL APPROACHES THAT OF THE BALL INDENTOR, THERE WILL BE TENDENCY FOR THE INDENTOR TO DEFORM.

  14. THEREFORE, THE BRINELL TEST WILL PRODUCE RELIABLE RESULTS UP TO HB VALUES OF AROUND 400 AND IT SHOULD NOT BE USED FOR MATERIALS WHICH GIVE VALUES MORE THAN 500. THE VICKERS DIAMOND TEST IN THIS TEST THE INDENTOR USED IS A PYRAMIDAL SHAPED DIAMON, AND SIMILAR TO THE BRINELL TEST, THE INDENTOR IS FORCED INTO THE SURFACE OF THE MATERIAL UNDER THE ACTION OF A STATIC LOAD FOR 10 TO 15 SECONDS. SIMILAR TO BRINELL TEST VICKERS DIAMOND HARDNESS NUMBER “HD” IS CALCULATED BY THE FOLLOWING FORMULA: HD = APPLIED LOAD / SURFACE AREA OF IMPRESSION

  15. THE VICKERS DIAMOND HARDNESS TEST IS COVERED BY BS 427 PART 1 (1981) AND ASTM E 92-82. THE STANDARD INDENTOR IS A SQUARE PYRAMID SHAPE WITH AN ANGLE OF 136 DEGREES BETWEEN OPPOSITE FACES. ONE OF THE ADVANTAGES OF THE VICKERS TEST OVER THE BRINELL TEST IS THAT THE SQUARE IMPRESSIONS MADE ARE ALWAYS GEOMETRICALLY SIMILAR, IRRESPECTIVE OF SIZE. THE PLASTIC FOW PATTERNS, THEREFORE, ARE VERY SIMILAR FOR BOTH DEEP AND SHALLOW INDENTATION AND, IN CONSEQUENCE, THE HARDNESS VALUE OBTAINED IS INDEPENDENT OF THE MAGNITUDE OF THE INDENTING FORCE USED.

  16. AFTER HAVING IMPRESSION MADE, THE SIZE OF THE IMPRESSION IS MEASURED ACCURATELY USING EITHER A CALIBRATED MICROSCOPE OR BY PROJECTING A MAGNIFIED IMAGE OF THE IMPRESSION ONTO A SCREEN AND MEASURING THE IMAGE. BOTH DIAGONALS OF THE IMPRESSION ARE MEASURED AND THE MEAN VALUE OF “D” THE DIAGONAL LENGTH, IS USED IN THE DETERMINATION OF THE HARDNESS NUMBER BY FOLLOWING RELATIONSHIP: HD = 2Fsin θ/2 / D² WHERE θ = 136º HD = 1.8544 F / D² WHERE “F” IS THE APPLIED LOAD IN KG AND “D” IS THE MEAN DIAGONAL LENGTH IN MM.

  17. THE ROCKWELL TEST THE ROCKWELL TEST MACHINE IS A RAPID ACTION DIRECT READING MACHINE. THIS PROVIDES A VERY CONVENIENT METHOD FOR SPEEDY COMPARATIVE TESTING. IN THIS TYPE OF TEST, OR RATHER A SERIES OF TESTS, IT IS THE DEPTH OF THE IMPRESSION WHICH IS MEASURED. IT IS DIRECTLY INDICATED BY A POINTER ON A DIAL CALIBRATED, INVERSELY, INTO 100 DIVISIONS SO THAT ONE SCALE DIVISION IS EQUAL TO 0.01 MM OF IMPRESSION DEPTH. A LOW SCALE NUMBER INDICATES A DEEP IMPRESSION AND HENCE IT IS A SOFT MATERIAL AND VICE VERSA. THERE ARE A SERIES OF ROCKWELL HARDNESS SCALES BECAUSE THERE ARE SEVERAL INDENTORS AND SEVERAL INDENTING LOADS AVAILABLE AND COVERED BY STANDARDS BS 891 (1989) AND ASTM E 18-89a.

  18. THE INDENTORS USED ARE HARDENED STEEL BALLS OF VARIOUS DIAMETERS OR A DIAMOND CONE WITH AN INCLUDED ANGLE OF 120 DEGREES. THE STANDARD BALL INDENTORS ARE 1/16, 1/8, 1/4 AND 1/2 INCH DIAMETER. THE STANDARD INDENTING LOADS ARE 60, 100 AND 150 KG. EACH SCALE OF HARDNESS IS DESIGNATED BY A LETTER – A SCALE, B SCALE AND SO ON. THERE ARE OTHER ROCKWELL SCALES OF HARDNESS AVAILABLE. THESE SCALES ARE KNOWN AS “N” AND “T” SCALES AND INVOLVE SMALLER INDENTING FORCES AND ARE SUITABLE FOR HARDNESS TESTS ON THIN SAMPLES.

  19. MICROHARDNESS TESTING MICROHARDNESS TESTS ARE THOSE WHICH INVOLVES GRAMS INSTEAD OF KILOGRAMS. THIS TYPE OF TESTING IS PERFORMED ON A METALLURGICAL MICROSCOPE. THE SMALL PYRAMIDAL DIAMOND INDENTOR IS EMBEDDED IN THE SURFACE OF A SPECIAL OBJECTIVE LENS. THE SURFACE OF THE TEST SAMPLE IS PREPARED TO HIGH POLISH AND ETCHED FOR MICROEXAMINIATION. DURING TEST ANY PARTICULAR MICRO CONSTITUENT OR FEATURE CAN BE CENTRED IN THE FIELD OF VIEW AND A MICRO-SIZED DIAMOND IDENTATION IS MADE USING A SMALL INDENTING LOAD.

  20. THE TWO MOST WIDLEY USESD METHODS ARE THE VICKERS DIAMOND TEST AND THE KNOOP DIAMOND TEST. THE PRINCIPLE OF THE VICKERS DIAMOND MICROHARDNESS TIST IS THE SAME. THE KNOOP HARDNESS TEST THE KNOOP TEST WAS DEVELOPED IN USA AND IT USES A DIAMOND PYRAMID INDENTOR DESIGNED TO GIVE A LONG THIN IMPRESSION, THE LENGTH BEING SEVEN TIMES GREATER THAN THE WIDTH AND THIRTY TIMES GREATER THAN ITS DEPTH. THIS SHAPE OFFERS AN ADVANTAGE OVER THE PYRAMID OF VICKERS TEST IN THAT THE LENGTH “L” IS THREE TIMES GREATER THAN THE DIAGONAL “D” AND CAN BE MEASURED WITH A GREATER DEGREE OF ACCURACY.

  21. THE KNOOP HARDNESS NUMBER “HK” MAY BE CALCULATED BY THE FOLLOWING REAIONSHIP: HK = 10F / L2 x 7.028 THE RANGE OF LOADS USED WITH THE KNOOP INDENTOR IS SIMILAR TO THAT USED IN VICKERS MICROHARDNESS TESTS. KNOOP HARDNESS TEST RESULTS ARE VERY SIMILAR TO THOSE OBTAINED FROM THE VICKERS TEST. STANDARDS BS 5441 PART 6 (1988) AND ASTM E 384 -89 COVER MICROHARDNESS TESTING.

  22. PROBLEM CALCULATE BOTH THE BRINELL AND VICKERS HARDNESS VALUES FOR THE STEEL AND ACCOUNT FOR ANY DIFFERENCES IN THE TWO VALUES. FOLLOWING IS THE INPUT DATA: LOAD (F) 30 KG, DIAMER OF INDENTOR (D) = 1MM, DIAMETER OF BRINELL IMPRESSION (d) = 0.308 MM, MEAN DIAGONAL LENGTH (D) = 0.362, HB = 2F / ΠD{D - √(D² – d²)} HD = 1.8544 F / D²

  23. IMPACT AND TOUGHNESS TESTING TOUGHNESS IS A MEASURE OF THE AMOUNT OF ENERGY A MATERIAL CAN ABSORB BEFORE FRACTURING. IT BECOMES OF ENGINEERING IMPORTANCE WHEN THE ABILITY OF A MATERIAL TO WITHSTAND AN IMPACT LOAD WITHOUT FRACTURING IS CONSIDERED. ONE OF THE SIMPLEST METHODS OF MEASURING TOUGHNESS IS TO USE AN IMPACT-TESTING MACHINE. IN THIS TYPE OF TESTING A BAR SPECIMEN WITH A MILLED NOTCH IS STRUCK BY A FAST-MOVING HAMMER, AND THE ENERGY THAT IS ABSORBED IN FRACTURING IS MEASURED.

  24. IMPACT AND TOUGHNESS TESTING THIS TYPE OF TEST HAS THE ADVANTAGE OF REVEALING A TENDENCY TO BRITTLENESS THAT IS NOT REVEALED BY THE STANDARD TENSILE TESTING OR HARDNESS TESTING. ONE MAJOR USE OF THE NOTCH IMPACT TEST SI TO DETERMINE WHETHER HEAT TREATMENT OF A MATERIAL HASS BEEN CARRIED OUT SUCCESSFULLY AS THIS TYPE OF TEST GIVE THE MOST INFORMATION ABOUT THAT. THERE ARE TWO TYPES OF IMPACT TEST KNOWN AS CHARPY TEST AND IZOD TEST. IN BOTH TYPE OF TESTS THE SPECIMEN IS IN THE SHAPE OF A BAR OF SQUARE CROSS SECTION, INTO WHICH A V-NOTCH IS MACNINED.

  25. THE LOAD IS APPLIED AS AN IMPACT BLOW FRAOM A WEIGHTED PENDULUM HAMMER THAT IS RELEASED FROM A COCKED POSITION AT A FIXED HEIGHT “h”. UPON RELEASE, A KNIFE EDGE MOUNTED ON THE PENDULUM STIRKES AND FRACTURES THE SPECIMEN AT THE NOTCH, THAT ACTS AS A POINT OF STRESS CONCENTRATION. THE PENDULUM CONTINUES ITS SWING, RISING TO A MAXIMUM HEIGHT “ĥ, WHICH IS LOWER THAN h. THE ENERGY ABSORPTION, CALCULATED FROM THE DIFFERENCE B/W h AND ĥ, IS A MEASURE OF THE IMPACT ENERGY. THE PRIMARY DIFFERENCE BETWEEN THE CHARPY AND IZOD TECHNIQUES LIES IN THE MANNER OF SPECIMEN SUPPORT.

  26. THEORY OF MPACT TESTING

  27. THIS IS TO BE NOTED THAT SPECIMEN SIZE AND SHAPE AS WELL AS NOTCH CONFIGURATION AND DEPTH INFLUENCE THE TEST RESULTS. IN THE CHARPY TEST THE TEST-PIECE IS IN THE FORM OF A SIMPLY SUPPORTED BEAM AND SHARP EDGE OF THE PENDULUM STRIKES AT THE MID-SPAN DIRECTLY BEHIND THE MILLED NOTCH. IN THE IZOD TEST, SPECIMEN IS TESTED IN A CANTILEVER MODE. THE TEST PIECE IS FIRMLY CLAMPED IN A VICE AND THE IMPACT BLOW IS DELIVERED ON THE SAME SIDE AS THE NOTCH. IMPACT STRENGTHS ARE GENERALLY QUOTED IN J/mm2 AND ARE BASED ON THE X-SECTIONAL AREA OF THE TEST-PIECE BELOW THE NOTCH.

  28. THE IMPACT STRENGTHS DETERMINED IN BOTH TESTS ARE ROUGHLY COMPARABLE PROVIDED THAT THE SAME TYPE OF NOTCH IS USED. IMPACT TESTING IS COVERED IN BS 131 PARTS 1, 2 & 3 AND ASTM E 23-88.

  29. RELATIONSHIP B/W HARDNESS & OTHER PROPERTIES AS A GENERAL RULE, AS THE VALUES OF HARDNESS OF METALS INCREASE, SO THE TENSILE, COMPRESSIVE AND SHEAR STRENGTHS ALSO INCREASE. THERE IS NO SPECIFIC RELATIONSHIP BETWEEN HARDNESS AND STRENGTH, BUT FOLLOWING EMPIRICAL EQUATION IS USED TO ESTIMATE THIS: TENSILE STRENGTH = Hd X 3.4 MOST MATERIALS OF HIGH DUCTILITY TEND TO BE SOFT AND IT DECREASES AS THE HARDNESS INCREASES. TOUGHNESS AND BRITTLENESS ALSO TENDS TO VARY WITH HARDNESS. HARD MATERIALS ARE ALWAYS BRITTLE.

  30. QUESTIONS AND QUERIES IF ANY! IF NOT THEN GOOD BYE NEXT LECTURE CONCEPTS OF TENSILE, COMPRESSIVE AND SHEAR TESTING

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