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Overview of Mechanical Testing

Overview of Mechanical Testing. The Role of Testing. Material test data typically used for: Design/construction of new mechanical or structural elements Control of established processes Material development Scientific knowledge (i.e., understanding how certain factors affect materials)

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Overview of Mechanical Testing

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  1. Overview of Mechanical Testing

  2. The Role of Testing • Material test data typically used for: • Design/construction of new mechanical or structural elements • Control of established processes • Material development • Scientific knowledge (i.e., understanding how certain factors affect materials) • Others????? • Engineers must have a general understanding of: • Common test methods for material properties • What constitutes a valid test • Applicability of data / limitations of tests

  3. “Types” of material testing • Commercial testing • Concerned mostly with checking acceptability of materials under purchase specs • Standard procedures are used • Objective to determine if properties of material or part fall within required limits • Materials research testing • To obtain a new understanding of known materials • Characterize properties of new materials • Develop new or refine existing test standards or material quality standards • Scientific testing • Provide data to support development/verification of models, analyses, etc.

  4. Field tests Testing done on location (such as flight line, construction site, etc) Typically have lower precision than laboratory tests May better represent actual use environment Laboratory tests Tests done in laboratory using load frames and other specialized equipment under controlled conditions Typically more expensive and complicated than field tests Usually have greater precision than field tests. Destructive Involve breaking or damaging the sample Not applicable for finished parts Examples, tensile testing, hardness testing, fatigue testing Non destructive Do not damage the sample Often used for quality control Examples, proof testing, radiography, microhardness testing Structural – tests done on components or structures Coupon – tests done on small samples of material “Types” of Tests

  5. Significance of Tests • Concepts of properties/ testing of materials is oversimplified (many assumptions) • Properties measured by tests affected by test conditions, method of testing, quality of test samples, quality of test equipment, etc. • Uncertainties in data • Those associated with material properties due to manufacturing, processing • Those associated with level and type of loading and actual service/environmental conditions • Significance of test measured by precision • Reliability - within lab variability • Reproducibility – between lab variability • Accuracy of test = closeness to true value ** a test can be precise, but not accurate **

  6. Materials Properties • Mechanical • Microstructure Sensitive • Describe how a material responds to an applied force • Physical • Microstructure Insensitive • Describes a materials response to an applied field or chemical

  7. Stress conditions • Fundamental stress conditions describe mechanical behavior features of components and assemblies: • Axial tension or compression • Bending, shear or torsion • Internal/external pressure • Stress concentrations and localized contact loads.

  8. Axial tensile loading s= F/A Design such that sapp < sfailure Where sfailurecan be: su (UTS) if fracture is criterion for failure Ductile material: UTS = stress where necking occurs Brittle material: UTS = stress where material breaks so (yield strength) if permanent deformation is criterion for failure A Tensile Loading F

  9. l Dl F Stiffness in Tension • Elastic deformation governed by stiffness • D L = e L • e = strain • L = length of bar • n= et/el = Poisson’s ratio • In the elastic range of deformation • s = E e • E = elastic modulus • Can be considered a physical property because it is fundamentally related to bond strength, not affected much by microstructure • Can vary with direction if material has anisotropic structure • Design of stiffness critical applications • D L = FL/AE < d • d = design limit change in length

  10. Load cell • Strain gages mounted on precision machined alloy steel elements • Load cell mounted such that specimen is in direct contact with load cell or indirectly loaded through the machine crosshead, table, columns of load frame • Calibrated to provide specific voltage output signal when a certain force is detected • Can be used in tension or compression and available with variety of temperature compensation capabilities Source: www.inston.com

  11. Clip on extensometer • Attached to test sample • Measures elongation or strain as load is applied • Typically have fixed gage lengths (0. in – 2 in, etc.) • Used to measure axial strain, available to measure transverse strain to determine reduction in width or diameter Source: ASM Mechanical Testing and Eval Handbook

  12. Tensile Testing - Engineering Stress Strain Curve

  13. Yield Point yield strength: stress at which slip becomes noticeable and significant - transition between elastic and plastic deformation, sy (lb/in2); yield strain: strain at which slip becomes noticeable and significant, ey (in/in or %) Offset yield strength (0.2% or 0.002 common): stress at which material changes from elastic to plastic is not always well defined, therefore define an offset yield strength Ultimate Tensile Strength (UTS): Stress at highest applied force Breaking Strength: stress at which fracture occurs Modulus: Slope of elastic portion of stress vs strain curve, E (lb/in2) Resilience: area under stress strain curve in elastic region; indicates amount of energy a material can absorb in elastic range Toughness: area under stress strain curve usually associated with shock or impact loadings % Elongation = ((lf-lo)/lo) x 100%, lf = gage length at failure, lo = initial gage length % Reduction In Area = ((Ao-Af)/Ao) x 100%, Ao= original area; Af=final area Tensile Testing - Engineering Stress vs. Strain Curve

  14. Askeland, Phule The Science and Engineering of Materials

  15. Tensile Testing - Failure Modes • Ductile Failure • Cup cone fracture • Dimpled failure surface • Significant plastic deformation • Has “necked” or localized deformation region • Brittle Failure • Flat fracture • Cleavage(radial lines) failure surface • Little to no plastic deformation • Does not have “necked” region

  16. Compression loading • Isotropic materials • suc equal to suT • Anisotropic materials • suc not equal to suT • Buckling may preceed other forms of failure • sb = (p2 E I)/(L2 A) • I = moment of inertia of cross section of bar

  17. Dynamic Properties • Impact Loading/Impact loading occurs if time duration is less than the natural period of vibration of part or structure • Depends on material parameters and geometric factors • Design stress, s = V (Em/Al)o.5 • V = velocity of mass, m • A, l = cross-sectional area and length of bar • E = elastic modulus • Impact tests • Charpy, Izod, Hopkinson bar, Others • Factors that affect data: • loading rate: • faster => less energy, higher transition temperature • slower => more energy, lower transition temperature • specimen size and configuration • smaller energies might be required to break thicker samples • notch configuration • Impact data should be used comparatively • materials screening • not appropriate for design data • Temperature

  18. Temp at which a material changes from ductile to brittle BCC metals have distinct transition temp FCC metals do not have distinct transition temp. ductile Absorbed Energy, ft-lb brittle Test Temp, F Transition temperature

  19. Impact Tests Brittle Material • Impact Resistance: • mat’ls ability to withstand a sudden intense, blow • Toughness: • provides a measure of impact resistance; • ability of mat’l to absorb energy prior to failure • area under true stress - true strain curve • low toughness • clean break • brittle material • little to no plastic deformation • high toughness • significant plastic deformation • ductile material True stress, psi True strain, in/in Ductile Material True stress, psi True strain, in/in

  20. Impact Tests -Charpy and Izod • Heavy pendulum of mass, m, is dropped from a height, ho. • Pendulum swings through arc, strikes and breaks specimen, rebounds to height of hf. • Energy dissipated via elastic, plastic deformation and fracture • Potential Energy difference (ft-lb or Joules) read from impact tester. • Charpy: • Izod Pendulum Specimen (10 x 10 x 55 mm) Notch Pendulum Specimen (10 x 10 x 75 mm)

  21. Impact Tests - Izod and Charpy • Potential Energy Difference • ‘D U = U1 -U2 = mg (ho-hf) • Where: • ‘DU = potential energy difference • m = mass of pendulum • g = gravity • h0 = drop height • hf = rebound height • 1 ft -lb = 1.356 joules 1 2 ho hf

  22. Hardness Testing • Not a fundamental property • Provides quick, easy, cheap indication and comparative information regarding material's strength • Used as quality control technique • Widely used for steels • Many different types of hardness tests • Macrohardness • Brinell, Rockwell, etc. • “Destructive” test • Microhardness • Vickers, Knoop, etc. • “Non destructive” test

  23. Macrohardness Testing - Brinell P • Steel or tungsten carbide ball (10 mm dia) pressed against material • Load of 500, 1500 or 3000 kg applied for 5 - 10 seconds • Diameter of indention measured using microscope • BHN = P/ ((P D/2) x (D - (D2 - d2)1/2)) • Where: • P = applied load [kg] • D = diameter of ball [mm] • d = diameter of resultant penetration [mm] • BHN = Brinell Hardness Number (Pa) • Advantages: • measure hardness over large area, indifferent to small scale variations in structure; • simple and easy to conduct • used a lot for steels and irons. • Hardness Strength Relationship (for steels using 3000 kg load): • UTS (MPa) = 3.5 HB • UTS (psi) = 500 HB D d

  24. Macrohardness Testing - Rockwell • Indenter pressed on surface of material with a minor load, then major load • Difference in depth or penetration automatically measured => HR • Indenter Types and Scales • Superficial Hardness • Rockwell test conducted using light loads • Produces shallow indentions • Useful for evaluating surface treatments and thin materials P = 0 P = minor load P = major load

  25. Rockwell TestingDisadvantages and Advantages • Limitations: • not useful for mat’ls < 1/16 in thick; • not useful for mat’ls with rough surfaces; • not useful for non-homogeneous materials (e.g. gray cast iron) • composition and structure can greatly influence results; • Advantages: • provides direct hardness reading in a single step; • quick, easy to use; • provides for relatively small indentions that can be easily concealed or removed via finishing.

  26. Hardness Testing - Miscellaneous Methods • Vickers • Knoop • Sceleroscope: • diamond tipped indentor or hammer enclosed in a glass tube • hardness related to rebound of indentor • Mohs: • scratch resistance • Durometer: • measures hardness of rubbers, plastics and similar soft and elastic materials.

  27. Hardness Testing Precautions • Location • should be at least two indenter diameters from specimen edge • Thickness: • should be at least ten times the depth of penetration • Successive indentions: • should be far enough apart to not allow indentions to interact • Resultant Penetration Size: • should be large enough to give a representative hardness value for the bulk material • Surface Prep • not critical for Brinell • somewhat important for Rockwell • very important for tests having small indenter size (smaller the indenter size the more surface prep is required) • polishing surface provides more accurate results.

  28. Bending • Normal stress distribution in bending • s = Mb Z/I • s = stress in bending • Mb = bending moment • Z measured from neutral axis • I = moment of inertia

  29. Bend Test For Brittle Materials

  30. Shear Loading • Translational mode of loading • Shear stress acting on shear plane • t = F / As • As = total area of shear planes • F = transmitted load • Can extend shear strength of material from tension test via: • to = s0 / (3)0.5 • Linear shear (translational shear) affected significantly by microstructural anisotropy and can require specialized tests

  31. Stress Concentrations • Irregular geometries => stress concentrations • Fillet • Radii • Notches • Holes • Simple relation for stress concentrations • smax = kt s a = (1 + 2a/b) s a • kt = stress concentration factor • a = dimensions of geometric irregularity perpendicular to hole • b = dimensions of geometric irregularity parallel to hole • Small cracks perpendicular to load, a >> b • Variety of kt developed through extensive experimentation and analysis

  32. Fracture Toughness • All materials contain flaws or defects • material defects (pores, cracks inclusions) • manufacturing defects (machining tool marks, arc strikes, contact damage) • design defects (abrupt section changes, excessively small fillet radii, holes) • Fractures initiate at defects • Defects have sharp geometries (a >> b) => high localized stresses => catastrophic failure • Unsteady crack growth occurs when elastic energy released by growth of defect exceeds energy required to form crack surfaces. • Design equation for stable crack growth: • K = Y s (p a ) 0.5 < Kc • K = stress intensity factor • Y = factor depending on geometry of crack relative to geometry of part • s= applied stress • A = crack length (defect size) • Kc = critical value = fracture toughness of material

  33. Fatigue • Materials under cyclic stress undergo progressive damage which lowers resistance to fracture • Fatigue failures count for 90% of all mechanical failures • Fatigue caused by simultaneous action of cyclic stress, tensile stress, plastic strain. • Plastic strain resulting from cyclic stress initiates crack, tensile stress promotes crack growth • Fatigue cracks typically initiate near or at “defects” that lie on or near the surface.

  34. Fatigue testing • Stressed based fatigue testing • Fatigue endurance limit = se: lower stress limit of S-N curve for which fracture does not occur ~ 10 7 cycles • Does not exist for all materials • Greatly affected by • Presence of stress risers: (small surface cracks, machining cracks, surface gouges) • Operating temperature (increase in temp => drop in fatigue resistance) • Environment (humidity, atmosphere, interaction with cycle frequency) • slow frequency - environment has more time to react • higher frequency - environment has less time to react • Residual Stresses (compressive residual stresses => increase fatigue life) • Strain based fatigue testing • Cycles to failure measured and plotted versus strain • Very useful in determining conditions for initiation fatigue • Used in designs where a major portion of the total life is exhausted in crack initiation phase of fatigue. • Fatigue crack growth rates are measured under conditions of cyclic stress intensity (DK) at subcritical levels (K < Kc)

  35. Fatigue Failure Beach or clamshell markings: Formed when load is changed during service or when loading is intermittent Striations: finer marks associated with position of crack tip after each cycle.

  36. I II III Fracture Strain, in/in de/dt eo Time, hours Creep • Higher temp + sapp < sys => cavitation, creep elongation and rupture of material • Tensile test subjected to constant load within high temp environment and measure elongation with time => creep curve • Creep rate: rate of elongation in second stage, de/dt • Time to rupture: total elapsed time • For design sf is the creep rupture strength

  37. Creep - Microstructural Mechanism • Dislocation Climb: • Movement of dislocation perpendicular to its slip plane by diffusion of atoms to or from the dislocation line • Dislocations escape from lattice imperfections, continue to slip and causes additional deformation of specimen even at low applied stress • Diffusion controlled phenomenon (therefore occurs more quickly at higher temperatures) • Arrhenius Relationship - Creep Rate • creep rate = K sn exp (Q c / R T) • Where: • R = gas constant • T = temp, K • c, K, n= material constants • Q = Activation energy related to self diffusion when dislocation climb is important

  38. Combination of applied stress plus corrosive environment => corrosion of part that would normally be resistant to corrosion Stress may be result of residual stresses Occurs for select metal environment pairs only such as High strength Al alloys in NaCl, seawater, water vapor Cu alloys (including brass) in ammonia, mercury salt solutions, amines, water Low carbon steel in NaOH, Nitrate solutions, acidic Hydrogen sulfide, seawater 400 and 300 series stainless steels in various environments etc,. Stress Corrosion Cracking (SCC)

  39. Physical Properties • Thermal Properties • heat capacity • thermal conductivity • thermal expansion • Electrical conductivity • Magnetic Response • Weight • Density • Melting/Boiling Point • Optical Properties

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