EMA 3012C lab guide - EMA 3 012C: Experimental Techniques...

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Unformatted text preview: EMA 3 012C: Experimental Techniques in Mechanics and Meterials Laboratory Report # 2 HARDNESS MEASUREMENT Submitted by W Other Members of the Group: W W XXXXXDCCXX Date Experiment Performed: Date Report Submitted: HARDNESS NIEASURElWENT Objective of the Experiment: State clearly the objective of your experiment such as measuring the hardness of different metallic materials and correlating them with the results obtained by other techniques. Materials Used: Aluminum, COpper, Brass, and Steel (Mention the composition and heat treatment, if you know about them, since the hardness depends on the composition and mechanical and thermal history of the specimen) Brief Theoretical Background: 0 Briefly explain the background. :- Mention that there are different methods of measuring the hardness, their relative advantages and disadvantages and why you chose the particular method you are planning to USB. Experimental Procedure: - Explain clearly the procedure you have followed and how you have performed the experiment. Mention the precautions to be taken, steps to be taken to avoid mishaps, injuries, and accidents, etc. :- Clearly mention the dimensions of the specimen (along with the units), if you are using them for calculations, speed of loading used, etc. - Use a passive Voice. Write the report as though you have done it. For example, say that the specimen was ground and polished, and not like the instructor has told you to do it such as grind it, polish it, put the specimen on the stage, etc. Results: 0 Give a_ll the details (00mpletely and clearly) of your results such as different values you have measured in the form of a Table. Always include the units for the measurements. a Write a legend to eVery table or figure and graph and explain or at least mention every one of them in the text of your Results section. I l Mention the scatter or standard deviation, if you have measured a number of values. Explain if there is significant variati en in the values. Discussion: 0 This is the place where you compare your observations and results with those, which were expected. Consider what steps might have improved the success of the experiment. Where errors or mistakes came in, what you have learned, etc. Conclusions and Comments: 0 Summarize the results in the form of a Table. For example, give only the average values here. a Try to draw conclusions such as aluminum is very soft, brass is harder than copper because brass is an alloy of c0pper and zinc, steel is very hard, etc. (Depending on the heat treatment, the steel can have difi‘erent hardness values. You will see this in the next experiment). a Try to relate your conclusions to the objectives of the experiment. Explain if your conclusions are according to the expectation or not. I Mention if you have encountered any difficulties in performing the experiment. Appendix: 0 Include charts or any other information that was not included in the earlier Sectiorrs, but - you would consider that as important. ******** ALWAYS PAY ATTENTION T0 DETAILS *********** ."/‘ lNTRODUCTION Classification of Materials -— Metallic, Polymeric, Ceramic and Composites Materials Science and Engineering - Structure, Processing, Properties, and Performance - Importance of Materials Characterization Structure _ 0 Crystal Structure - Size and Shape of the Unit Cell and Location of the Corrstituent Atoms. X—Ray diffraction, Electron Diffraction, Neutron Diffraction o Microstructure —— Grain Size; Size, Shape and Distribution of Secondary Phases and Non-metallic Inclusions, Segregation, etc. Optical Microscopy (OM), Seanning Electron Microscopy (SEM), Transmission Electron Micros00py (IBM), and Field-Ion Microscopy (FEM). Micro- “llluruination” Maximum Limit of Special Features“ Cost scope Source Magnification Resolution (run) an. 1.ooo,ooox 0.2—0.3 Micro, cs,CA >1,000,000X Micro, (33(1)), CA 150K * Micro = Microstructure, CA: Chemical Analysis, CS = Crystal Structure Importance of_ Microstructure -— Strength and Hardness, Corrosion, Electrical and Magnetic Properties 0 Modified by Composition, Phase Transformations, and ProcessingON/Iechanical and Thermal Treatments) - ' Properties - 0 Mechanical — Hardness, Tensile, Shear, Impact, Fatigue, Wear, . . . 0 Physical — Density, Electrical Magnetic, ' 0 Chemical — Corrosion, Oxidation, Stress Corrosion Cracking, . . .. In this course, we will concentrate on optical microscopic examination (we will do one experiment using the Scanning Electron Microscope) and mechanical characterization of materials and try to understand the relation between structure and prOperties. DIETALLOGRAPHIC SPECIMEN PREPARATION 0 Requirements of a good specimen I Flat, free from scratches, stains, and other imperfections I Contain all non-metallic inclusions intact ' I Show no chipping or gelling of hard and brittle intermetallic compounds I Be free from all traces of disturbed metal 0 Specimen Preparation Stages 0 Sectioning — a representative sample from the parent piece which is characteristic of the material that should serve the objective of examination. Typical size 3%: to l" at V2" 0 Coarse Grinding — To remove deformatiou produced during Sectioning and produce an initial flat surface by a file or a motor—driven emery belt 0 Mounting — for ease in manipulation and edge preservation, etc. EsPecially useful fer small and awkwardly shaped specimens, e.g., chips, wires, small rods, tubing, sheet metal specimens, thin sections, etc. Done by embedding the sample in a plastic medium (epoxy, bakelite, etc.) 0 Fine Grinding — Remove the zone of deformation caused by sectioning and Coarse Grinding and limit the depth of defamation. The deformed layer (which can be up tolO—SO times the depth of scratch produced) is slowly removed by grinding the specimen surface on fixed—type abrasives, e.g., SiC, emery, A1203, bonded through glue, resin, or resin over glue, to a paper or a cloth backing. Turn the specimen through 90° in moving from one grade of paper to the other and wash the specimen under running water. 0 Rough Polishing — Further limitation of the deformation zone produced by Pine Grinding. Diamond or Alumina abrasives. Oil or Water—soluble media promote superior lubrication and removal rates in comparison to slurry suSpensions. 0 Final Polishing — Removal of deformation zone produced during Rough Polishing. Any deformation zone produced during this stage, which is minimal, will be remOVed during etching. Alumina, Ceria, etc. on polishing wheels Electrolytic Polishing is an alternative to manual polishing. Etching — to make visible the many structural characteristics of the material. But, one can see surface defects (scratches), non-metallic inclusions, graphite flakes in cast iron, etc. without etching. Etching reagents are composed of organic or inorganic acids, alkalies of various kinds, and other complex substances in solution with an appr0priate solvent such as water, alcohol, etc. Nital (nitric acid in alcohol) or Picral (picn'c acid in alcohol) are common etchants for plain carbon steels. A mixture of ammonium hydroxide and hydrogen peroxide is suited for etching c0pper and a-brass. Look for the standard etchants in reference books (ASM Handbooks) or vendors manuals. Temperature and Time of Etching — Etch pits, coloration, etc. Etching Mechanism — Differences in rate of attack due to grain orientation differences in single—phase materials. Preferential attack or staining of one or more of the phases by the reagent, in addition to orientation differences, in multi—phase materials. . o Cleanliness is very important at all stages of Specimen Preparation. 00 OPTICAL WTALLOGRAPHY Biological vs. Metallurgical Microscope Principles of the Microsc0pe Important Components of the Microsc0pe O O O Illummalion System Objective Eyepiece rIllumination System 0 _/ O O Tungsten-Filament Lamps: Operate at low voltage (6V) and high current (SAmp); widely used for visual examination Quartz—Halogen Lamps: More intense steady illumination than tungsten-lament lamps; very dependable ' Xenon Lamps: Very high intensity extending well into the UV and IR regions. Very useful for color photomicrography. Its visual Spectrum of daylight quality permits use of daylight-type color film. Objective: Fonns the primary image of the specimen 0 O 0 Lens Aberrations — Spherical, Chromatic, Astigmatism, Coma Typical Objective Lenses — Achromatic (corrected for two colors — R and G), Semi-Apochromafie, Apochromatic (corrected for three principal colors — R, G, and V), Long Working Distance (for high-temperature microscopy), Oil Immersion (for improved resolution) Magnifying Power, Total Magnification, M = Mo )1 Mb I Projection, M = M0 x M, fo250 Numerical Aperture: Light gathering capacity. NA. = y, sin {1, where p. is the minimum refractive index and o: is the half-angle of the most oblique rays. The image quality improves with NA. Dry objectives, Oil Immersion Objectives; Minimum magnification = 400-500 NA. Resolving Power, Fineness of Detail = N2 NA. Limit of resolmion can be improved by using lenses with higher magnifications, higher N.A. andfor shorter wavelength Empty Magnification Eyepiece: To magnify the primary image produced by the objective 0 Measuring and Reticle Types; Comparison Eyepieces - Photomicrography Inverted Stage Mimoscopes — ConVenient for large Specimens and do not require mounting to have a flat bottom 0 Microscopic Techniques 0 O O O Bright-Field Illumination: Most common method of observing microstructures Oblique Illumination: Enhances surface relief Dark-Field Illumination: More positive identification of angled surfaces (those of pits, cracks, etched grain boundaries, etc. Reversal of contrast from BF) Polarized Light Microscopy: Revealing of grain structure and twinning in anisotropic metals and alloys, and for identifying anisotropic phases Differential Interference Microscopy: Topographic detail without loss of resolution as in Oblique illumination High-Temperature Microscopy: Grain growth, precipitation reactions, phase changes, sintering, diffusion and surface reactions Low—Temperature Microscopy: Limited use Straining Stages: Studies of deformation, twinning, slip, and strain-induced transformations Interferometry: Most sensitive and most accurate optical method for measuring the microtopography of surfaces. 0 Information Derivable from the Microstrueture 0 Grain Size and Shape I n = 21“, Where n is the number of grains per square inch observed at IOOX and N is the ASTM Grain Size Number. O 00 00 Number of Phases Size, Shape and Distribution of Second—Phase Particles Volunie Fraction of Phases Merostructure — volume percent; Lever- rule — weight percent; Take density into consideration) Chemical Composition??? (e.g., Carbon Content in Steels, only under equilibrium conditions 1) Thermal History Mechanical History W o The hardness of a metal is its ability to resist being permanently deformed. It means different things to different people. ' - o Hardness is measured in three different ways o Scratch hardness o Indentation hardness o Rebound hardness o ‘ Scratch Hardness: Primarily of interest to mineralogists. Mohs Scale 1. Tale 2. Gypsum 3. Calcite 4. Fluorite 5. Apatite 6. Orthoclase 7. Q11artz 8. Topaz 9. Corundum 10. Diamond — A fingernail has a value of about 2, Annealed copper 3, Martensite 7, and most hard metals have a value between 4 and 8. — Not suitable for metals and alloys since the intervals are not Widely spaced in the high hardness range. Therefore, indentation. hardness is most favored. ' o Rebound Hardness Test . — The indenter (diamond—tipped hammer) is usually dropped onto the metal surface, and the hardness is expressed as the energy of impact. The Shore scleroscope measures the hardness in terms of the height of rebound of the indenter. Hard metals have a larger rebound height and softer metals smaller. 0 Rockwell Hardness Test — Minor load (10 kg) and major load (60, 100 or 150 kg) — Hardness is measured by the difference in the depth of penetration the indenter makes. between the minor and major load - Use lower scale for soft metals (<RC 15-20) - Always designate the scale, e.g., RC 65. — Superficial Testing: 3 Kg minor load and 30 Kg major load s u bol load K) Fi es U16” dia steel k C0pper alloys, soft steels, aluminum alloys, ball malleable iron, etc. C Brale (sphero- 150 Black Steel, hard cast irons, Titanium alloys, deep diamond) harder than B 100 ' w Black Cemented carbides, thin steel, and shallow case—hardened steel 0 Brinell Hardness Test - 10 mm dia steel ball CD); Load: 3000 Kg for hard materials such as steels, etc. and 500 Kg for soft materials such as copper and aluminum PID?’ is always maintained constant (P: applied load in Kg; D=diarneter of indenter in mm) BHN (Kg/mmz) is given by the equation: P gig—W] BEN: where d is the diameter of the indentation in mm o Vickers Hardness Test The indenter is a square-based diamond pyramid with an included angle of 136° The lengths of the two diagonals are measured by a microscoPe provided with the hardness tester . . VHN (or DPI-I) is given by the equation: Vl—IN = 1.354 PIDZ, where P is the applied load in kg (usuallyl-lZO) applied for about 10-30 seconds‘ and D is the length of the diagonal in mm. - Received fairly wide acceptance for research Work because it provides a continuous scale of hardness, for a given load Relatively slow and so not commonly used in the industry; the measurement of the diagonal is also subjective to human error e Knoop Hardness Test (L/Iicrohardness) Diamond pyramid indenter with L’w = 7.114, (or wfl = 0.14056) This is a type of microhardness tester, and so low loads are applied (~1 kg) and the indentation size is small KHN is given by the equation: KEN = PICLE, where P is the applied load in kg, c is the Knoop indenter constant (= 0.07028), and L is the length of the longitudinal diagonal in mm. Useful to determine the hardness of individual phases in a microstructure Vickers microhardness testers are commercially available 0 Nanoindentation Test . Extremely low loads, a few grams. Berkovich indenter. 0 Minimum Thickness of Test Section: Should be such that a mark or bulge is not produced on the reverse side of the piece. Varies with the load but is usually about 10 times the indentation depth ' ' o I Minimum Spacing between two indentations: about 3-5 times the indentation size. 0 Comparison of Hardness Values Relation between strength and hardness: Generally VI-leyield strength = 3 TS =500xBHN' HEAT TREA'IWNT 0F STEELS 0 Concept of Phase Diagrams (Equilibrium or Constitution Diagrams) Temperature vs. composition plots to denote the regions where a particular phase is stable under thermodynamic equilibrium conditions Phase, Component, Degrees of Freedom Phase Rule, F = C+2—P, (= C+1—P for condensed systems) Solid Solutions (Suhstitutional and Interstitial), Terminal Solid Solution, Intermediate Phases (Compounds) Unary (1), Binary (2), Ternary (3), Quaternary (4), Mold-Component Systems Isomorphous (L —> on), Eutectic (L —> [1+B), Peritectic (L+o: —> B), and Eutectoid (y —> ot+B) Reactions Solidificafion Behavior of Isomorphous and Eutectic Alloys 0 The Iron-Carbon(1ron-Cementite) System Steel portion (up to 2 wt.% Carbon) Ferrite (0: and 5), Austenite (7), Cementite (Fe3C), Pearlite (oc+FesC) Cast Iron portion (>2 wt% Carbon) Gray Cast Iron (Graphite flakes in a ferrite matrix), White Cast Iron (Cementite, instead of graphite), Nodular or Spheroidal Graphite (S.G.) Cast Iron (Graphite in the form of nodules), and Malleable Cast Iron 0 NIicrostructure of Steels (Proeutectoid) Ferrite + Pearlite in Hypoeutectoid Steels Pearlite is a lamellar structure of alternate layers of Ferrite and Cemenn‘te (7:1) Only Pearlite in Eutectoid Steels (Proeutectoid) Cementite + Pearlite in Hyperenteetoid Steels Estimation of Carbon Content of the Steel from its Mierostructure 0 Heat Treatments (to modify the Microstructure and Mechanical Properties) Hardening by Quenching (Formation of Martensite, bet supersaturated solid - solution of carbon in ferrite) Tempering to reduce the hardness of martensite Annealing (to fully soften hardened steel) by cooling the steel in the furnace after austenitization (50F or 28C above the upper critical temperature) ‘ Normalizing by cooling the steel in still air after austenitization (IOOF or 56C above the upper critical temperature) Spheroidizing to improve maehinability of high carbon steels ‘_--. HARDENABEITY Definition: The property, in ferrous alloys, that determines the dePth and distribution of hardness produced by quenching, g The capacity of a steel to transform partially or completely from austenite to some percentage of martensite at a given depth when cooled under some given conditions Hardness distribution is a function of chemical composition and rate of quenching o The maximum hardness can be achieved only on the surface of bars and I The hardness in the interior of the sample dr0ps significantly. I The surface hardness drops significantly with increasing bar diameter 0 The hardening response is lower with lower quenching rates. The surface hardness is well below the hardness expected from a fully martensitic structure 0 The alloy steels are much more hardenable than the plain carbon steels, i.e., the alloy steels have a higher hardenability than plain carbon steels. Alloying elements increase the time required for the formation of ferrite (cementite) and pearlite from austenite and so martensite can form at lower cooling rates. Severity of Quench is the effectiveness of a giVen quenching medium and is determined by quenching a series of round bars of a given steel. The larger the bar diameter, the greater the unhardened diameter (Du). Unhardencd means <50% martensite. o Severity of quench altered by the quenchant and amount of circulation 0 Air 002, Water 1.0, Brine 2.0, Violently circulated brine 5.0 Grossman and Bain method of determining the hardenability of steels 0 Critical size: largest size of a bar quenched in a given medium which contains no unhardened core after quenching — related to a given quenching medium; the higher the quench severity the greater the critical size 0 Ideal size: size of the bar hardened to 50% martensite by a theoretically perfect quench in which it is assumed that the surface of the bar cools instantly to the temperature of the quenching medium — a true measure of the hardenability Determination of Ideal Size 0 Ideal size is affected by I Austenitic Grain Size I Carbon Content I Alloy Content 0 Base hardenability is determined by the carbon content and grain size. The base hardenability is then multiplied by factors for the various concentrations of alloying elements. o Plots are available relating the critical size, ideal size, and severity of quench J ominy End Quench Test has the great advantage of characterizing the hardenabiiity of a given steel from a single specimen rather than from a series of round bars. J ominy curves: Higher hardness persists to greater distances from the quenched end in the more hardenable steels. 0 Each position of the specimen corresponds to a well-defined cooling rate. Therefore, if cooling rates as a function of position in parts of various geometries are known, it is possible to use Jorniny curves to plot hardness profiles in the parts. - The Jominy data is o A highly accurate method of selecting steels of just the right hardenability for a given required hardness distribution 0 A steel can be selected that will not only satisfy the hardness requirements but also have just the right alloy content, Therefore permitting selecfiOn at minimum cost from the many steels that might have sufficient or even excess hardenability for the application 0 Alloy steels that can he hardened by moderate quenching may be seleeted to replace leaner steels in which the severe quenching required to obtain high hardness causes quench cracking THE TENSILE TEST The engineering tensile test is widely used to provide basic design information on the strength of materials and as an acceptance test for the Specification of materials. In the tensile test a Specimen is subjected to a continually increasing uniaxial tensile force while simultaneous observations are made of the elongation of the Specimen. Elastic (temporary and recoverable) versus plastic (permanent) deformation Stress, o = Force per unit area; kgfmmz, psi, ksi, MPa. 1 psi = 6.89 x 103 Pa 0 Engineering stress = average uniaxial tensile forceJoriginal area of cross section 0 True stress = average uniaxial tensile force/instantaneous area of cross section 0 True stress = engineering stress (1+8) Strain, e = elongation of the gage length of the specimen divided by its original length 0 Engineering strain and True strain 0 True strain = 111(1+£) Strain hardening - increasing strength with increasing deformation The key? mechanical properties obtained from the tensile test include: o Modulus of elasticity, also known as Young’s modulus, E = ole and is related to the bonding strengdi between atoms. (5 cc 6 in the elastic portion and therefore 6 = Be. Shear modulus, I Represents the stiffness of the material, i.e., its resistance to elastic strain 0 Yield strength (Y S), also known as yield point is the stress at which plastic deformation or yielding is observed to begin (depends on the sensitivity of the strain measurements, usually 10“ infin); also referred to as 0.2% offset yield strength, proof stress (in Great Britain). Proportional limit 0 Ultimate Tensile Strength (UTS) or simply the tensile strength (TS) is the maximum strength reached in the engineering stress-strain curve; onset of necking Percent Elongation at Fracture (%EL) Percent Reduction in Area at Fractme (%RA) Resilience: Ability of a material to absorb energy when deformed elastically and to return it when unloaded o Modulus of resilience: Strain energy per unit volume required to stress the material from zero stress to the yield stress. Uo = 1/5 0x3; 0 Toughness: total area under the stress- strain curve; ability to absorb energy in the plastic range 0 Specific Strength (Strength per unit density) Engineering versus True Stress—True Strain curve The region of the true stress and true strain curve between the onset of plastic deformation and the onset of necking is approximated by 6—; = Kern where n is the strain hardening exponent. The higher values of n represent improved ability to be deformed during the shaping process without excessive thinning or fracture of the piece 0 It might appear that plastic deformation beyond TS softens the material because engineering stress falls. But, this drop in stress is the result of defining the engineering stress relative to the original area of cross section. At the T8 the sample begins to neck down, and therefore area of cross section is less and thus the true stress continues to rise to the point of fracture. 000 Ductility is frequently quantified as the percent elongation at failure (= %El 1: 100). Note that the percent elongation depends on the gage length used. Ductility indicates the general ability of the metal to be plastically deformed. Yield point - Upper yield point, Lower yield point. Serrated yielding, especially in low- carbon steels, associated with nonhornogeneous deformation that begins at a point of stress concentration. 0.001 % carbon or nitrogen. Formation of Liiders bands. The Actual Tension Test 0 Gage length 0 ASTM standards 0 Round 01' Plate Specimens, Wire Specimens The shape of the 0-8 curve depends on the alloy 0 Composition Heat treatment Prior history of plastic deformation (work hardening) Strain rate State of stress imposed, and Temperature 00000 FATIGUE 0 Fatigue is the general phenomenon of material failure after several cycles of loading to a stress level below the ultimate tensile strength. 0 Definitions 0 Sum, maximum level of stress 0 Smin, minimum level of stress 0 Range of stress AS = Smax - 8min, Stress amplitude Sa = AS/‘2, Fatigue cycle, Successive maxima (or minima) in load or stress Nf, Number of fatigue cycles to failure S-N curve, a plot of stress (S) versus number of cycles (N) on a log scale Cyclic frequency = number of fatigue cycles per second, Endurance limit (or fatigue strength), S: below which fatigue failure does not occur regardless of the number of cycles- Some materials such as nylon, Al, Cu and other FCC metals do not exhibit a well-defined Se. The S-N curve continues to slope downwards. Then S, is the stress amplitude corresponding to N=107. I Fatigue mechanism. The back—and-forth slip during cyclic causes intrusions and extrusions that result in the formation of a notch within the slip band. The notch is the nucleus of the fatigue crack, which grows during subsequent cycling. When the crack covers a sufficient area so that the remaining metal at the cross section cannot support the applied load, the sample ruptures. 0 Correlations between Fatigue Strength and other Mechanical Properties 0 Characterization of fatigue properties requires many specimens, and fatigue tests are more complicated than tensile and hardness tests. They also require more elaborate and carefiilly prepared specimens, Specialized equipment (which is expensive) and considerable time (several hours to weeks). 0 0.25 < (Se/UTS) < 0.5 o Fracture Mechanics Approach 0 Crack growth rate, daJdN = C OAK)“ where 2a = crack length, C and n are constants and AK is the range of the cyclic stress intensity factor. K = m/(rra). 000000 0 masts” — (n;)“"’”’l _N " (2 — newscast- I Major factors which affect the fatigue strength 0 Stress concentration. Fatigue strength is greatly reduced by the presence of stress raisers such as notches, holes, or sharp changes in cross sections. 0 Surface roughness. The smoother the surface finish, the higher the fatigue strength. Rough surfaces create stress raisers. 0 Surface condition. Decarburizing, which softens the surface lowers fatigue life. 0 Environment. Presence of corrosive environment accelerates the rate at which fatigue cracks propagate. Corrosion fatigue. Fatigue life can be increased by (a) reducing the applied stress range, (b) avoiding sharp corners and stress raisers, (c) having a smooth surface through fine grinding, ((1) increasing the surface hardness through shot peening or case carburizing. - Fatigue failure analysis FATIGUE TESTING 0 Fatigue is the general pheu0menon of material failure after several cycles of loading to a stress level below the ultimate tensile strength. - Definitions 0 Sm, maximum level of stress Sm, minimum level of stress Range of stress AS = S1m — Sm, Stress amplitude S, = AS/Z, Fatigue cycle, Successive maxima (or minima) in load or stress Nf, Number of fatigue cycles to failure S-N curve, a plot of stress (S) versus number of cycles (N) on a log scale Cyclic frequency = number of fatigue cycles per secOnd, Endurance limit (or fatigue strength), S, below which fatigue failure does not occur regardless of the number of cycles. Some materials such as nylon, Al, Cu and other FCC metals do not exhibit a well—defined 8,. The S—N curve continues to slope downwards. Then S, is the stress amplitude corresponding to N=107. - Fatigue mechanism. The back-and—forth slip during cyclic causes intrusions and extrusions that result in the formation of a notch within the slip band. The notch is the nucleus of the fatigue crack, which grows during subsequent cycling. When the crack covers a sufficient area so that the remaining metal at the cross section cannot support the applied load, the sample ruptures. 0 Correlations between Fatigue Strength and other Mechanical Pmperties 0 Characterization of fatigue pmperties requires many specimens, and fatigue tests are more complicated than tensile and hardness tests. They also require more elaborate and carefully prepared specimens, specialized equipment (which is expensive) and considerable time (several hours to weeks): 0 0.25 < (SJUTS) < 0.5 0 Fracture Mechanics Approach 0 Crack growth rate, dade = C (AK? where 2a = crack length, C and n are constants and AK is the range of the cyclic stress intensity factor. K = m/(rta). 00000000 0 - Major factors which affect the fatigue strength 0 Stress concentration. Fatigue strength is greatly reduced by the presence of stress raisers such as notches, holes, or sharp changes in cross sections. 0 Surface roughness. The smoother the surface finish, the higher the fatigue strength. Rough surfaces create stress raisers. 0 Surface condition. Decarburizing, which softens the surface lowers fatigue life. 0 Environment. Presence of corrosive environment accelerates the rate at which fatigue cracks propagate. Corrosion fatigue. Fatigue life can be increased by (a) reducing the applied stress range, (b) avoiding sharp corners and stress raisers, (c) having a smooth surface through fine grinding, (d) increasing the surface hardness through shot peening or case carburizing. - Fatigue failure analysis IMPACT TESTING 'I'hree basic factors contribute to a brittle-cleavage type of fracture: o Triaxial state of stress such as exists at a notch 0 Low temperatures 0 High strain rate or rapid rate of loading. All three of these factors need not have to be present at the same time to produce brittle fracture. Brittle failure of welded Liberty ships and T—2 tankers during World War 11 Just as hardness is an analog of strength, impact energy is a similar analog of toughness Two standardized tests — Charpy and Izod — are designed to measure the impact energy. The Ch V-notch C test is more commonly used in the US while the Izod Specimen is favored in Great Britain (rarely used today). 0 The Charpy specimen has a square cross section (10x 10 mm), 55 mm long and contains a 45° V—notch, 2 mm deep with a 0.25 mm root radius. The Specimen is supported as a beam in a horizontal position and loaded behind the notch by the impact of a heavy swinging pendulum (impact velocity is approximately 16 ft/s). The specimen fractures at a high strain rate on the order of 103 s“. The energy necessary to fracture the test piece is directly calculated from the difference in the initial and final heights of the swinging pendulum. The impact energy from the Charpy test correlates with the area under the total stress-strain curve (i.e., toughness). Ductile-to-Brittle Transition. One of the primary functions of the impact tests is to determine whether or not a material experiences a ductile—to—brittle transition with decreasing temperamre, and if so the range of temperatures over which it occurs. At higher temperatures, the CVN energy is relatively large, in correlation with a ductile mode of fracture. As the temperature is lowered, the impact energy dr0ps suddenly over a relatively narrow temperature range, below which the energy has a constant but small value; that is, the mode of fracture is brittle. 0 Not all metal alloys exhibit the DBTT. FCC metal alloys remain ductile to very low temperatures. ECG and HCP alloys experience this transition. This transition is sensitive to both alloy composition and microstructure. o Decreasing the grain size of steels results in a lowering of the transition temperature. 0 NauOCrystalline alloys _ Ductile—Brittle Transition Temperature (DBTT) is the temperature below which catastrophic failure occurs. So, DBTT is of great practical importance. Alloys that exhibit DBTT should be used only at temperatures above DB‘I'I‘. MIR—H o The temperature at which the CVN energy is 15 ft-lb (or 20 J) o The temperature at which the fracture surface becomes 100% fibrous (ductile). This is a very conservative estimate 0 The temperature at which the impact Specimen fractures with a half-ductile and half- brittle fracture surface. 0 The temperature corresponding to the average impact energy Ceramics and polymers also experience a ductile-to—brittle transiti0n. DBTT is very high for ceramic materials, ordinarily >1000 °C. Rubber immersed in liquid nitrogen is brittle! _-—- X—RAY DIFFRACTION W - Production of X-Rays o X—rays are electromagnetic radiation and have a wavelength in the range of 10 nm to 1 pm. Useful range is 0.05 to 0.25 nm. e Deceleration of fast—moving electrons by a metal target ' E=hv orl=hcfE=hcfeV 0 White radiation: useful in Lane method Characteristic radiation: Holes created by ejected electrons are filled by electrons from the neighboring shells. Ken, KB, La, . . . ' LIE—9 K: KC!) 17.48 116V; L11 fi' K! K112 17.37 keV The X—ray wavelength is target specific — K01, Kag, K13 Weighted 1 Km = (2 3L Ka1+ 7L Kd2)f3 Absorption: 1,, = 1,, exp (—;lx) or Ix = L exp (~p/p).p.x I Absorption edges - Filtering of unwanted radiation to obtain monochromatic radiation 0.175661 0.162079 0.154184 0.139222 071073 0063229 0 Bragg law. A = 2d sine (the most important equation) 0 Atomic Scattering Factor, f 0 Structure Factor, F = E 1',- exp 215i(huj + ij + leg) 0 Extinction (Reflections are absent) Conditions for Diffraction. o Primitive Lattice: None 0 Body—centered lattice: h+k+l odd 0 Face-centered lattice: h, k, and l are mixed (odd and even) 0 Hence reflections from a cubic lattice are present in the sequence: 0 000 COO il O 0 Primitive: (100), (110), (111), (200), (210), (211), (220), (300), (310), - (h2+k2+12) = 1, 2, 3, 4, s, 6, s, 9, 10, o Body-centered: (110), (200), (211), (220), (310), (222), (321), (400), _ - (h2+k2+12)=2,4,6,8,10,12,14,16,18,..... o Face—centered: (111), (200), (220), (311), (222), (400), (331), (420), 422), - 012+k2+12) = 3, 4, s, 11, 12,16, 19, 20, 24, Diffraction takes place preferentially from close-packed planes, and then from less close— packed planes, ...
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EMA 3012C lab guide - EMA 3 012C: Experimental Techniques...

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