Mechanical Properties of Metals

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Mechanical Properties of Metals

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Mechanical Properties of Metals

  1. 1. Mechanical Properties of Metals
  2. 2. Mechanical Properties <ul><li>Stiffness - Elastic Modulus or Young’s Modulus (MPa) </li></ul><ul><li>Strength - Yield, Ultimate, Fracture, Proof, Offset Yield. Measured as stress (MPa) </li></ul><ul><li>Ductility - Measure of ability to deform plastically without fracture - Elongation, Area Reduction, Fracture Strain - (no units or mm/mm) </li></ul><ul><li>Toughness, Resilience - Measure of ability to absorb energy (J/m 3 ). </li></ul><ul><li>Hardness - Resistance to indentation/abrasion (Various scales, e.g.; Rockwell, Brinell, Vickers.) </li></ul>
  3. 3. Stress and Strain <ul><li>In a simplistic sense, stress may be thought of as Load/Area . </li></ul><ul><li>Similarly, strain is the deformation of the component/original length. </li></ul><ul><li>A stress may be direct , shear , or torsional - leading to corresponding deformations. </li></ul><ul><li>Stress cannot be measured directly, but deformation can be. </li></ul>
  4. 4. Direct Stress Examples Direct Stress - Tension Direct Stress - Compression Engineering Stress Engineering Strain
  5. 5. Tension Test Typical Universal Testing Machine Extensometer Measures  L Measures P
  6. 6. Modern Materials Testing System Hydraulic Wedge Grips Specimen Extensometer
  7. 7. ASTM Tension Test Specimen 2” Gauge Length A o =0.20 in 2 L o
  8. 8. Raw Data Obtained <ul><li>Elongation,  L (mm) </li></ul>Load, P (kN) Uniform Deformation Total Elongation Elastic Deformation X Maximum Load, P max Load, P f
  9. 9. Engineering Stress-Strain Curve Elongation 0.2% offset yield stress Proportional Limit E E (Ultimate) Engineering Strain, e =  L/Lo) Engineering Stress, S=P/Ao S y S u
  10. 10. Duke’s Quick Tip! <ul><li>Express Load in Newtons (N) and Area in mm 2 to get Stress in MPa. </li></ul><ul><li>Mechanical properties of metals are almost always given in MPa or ksi. </li></ul><ul><li>Imperial units: Load in kips (1000 lbf) & Area as in 2 gives Stress in ksi (kips/in 2 ) </li></ul><ul><li>1000 psi = 1 ksi = 6.89 MPa </li></ul>
  11. 11. Hooke’s Law Elastic Deformation <ul><li>Elastic deformation is not permanent; it means that when the load is removed, the part returns to its original shape and dimensions. </li></ul><ul><li>For most metals, the elastic region is linear. For some materials, including metals such as cast iron, polymers, and concrete, the elastic region is non-linear. </li></ul><ul><li>If the behavior is linear elastic, or nearly linear-elastic, Hooke’s Law may be applied: </li></ul><ul><li>Where E is the modulus of elasticity (MPa) </li></ul>
  12. 12. Modulus of Elasticity - Stiffness
  13. 13. Atomic Origin of Stiffness
  14. 14. Shear Stress and Strain shear stress,  = Shear Load / Area shear strain,  = angle of deformation (radians) shear modulus, G =  /  (elastic region) Shear Stress Shear Strain
  15. 15. Elastic Properties of Materials <ul><li>Poisson’s ratio: When a metal is strained in one direction, there are corresponding strains in all other directions. </li></ul><ul><li>For a uniaxial tension strain, the lateral strains are constrictive. </li></ul><ul><li>Conversely, for a uniaxial compressive strain, the lateral strains are expansive. </li></ul><ul><li>i.e.; the lateral strains are opposite in sign to the axial strain. </li></ul><ul><li>The ratio of lateral to axial strains is known as Poisson’s ratio,  . </li></ul>
  16. 16. Poisson’s Ratio,  For most metals, 0.25 <  < 0.35 in the elastic range Furthermore :
  17. 17. Plastic Deformation Stress Strain 0.002 0.002 0.002 S y S y S y Most Metals - Al, Cu Clad Al-Alloys Low carbon Steel Elastic Plastic Elastic Plastic Elastic Plastic
  18. 18. Microstructural Origins of Plasticity <ul><li>Slip, Climb and Slide of atoms in the crystal structure. </li></ul><ul><li>Slip and Climb occur at Dislocations and Slide occurs at Grain Boundaries. </li></ul> 
  19. 19. Elastic and Plastic Strain Stress Strain Plastic Elastic e e e p P Total Strain (e,S) The 0.2% offset yield stress is the stress that gives a plastic (permanent) strain of 0.002.
  20. 20. Elastic Recovery Strain Stress Loading Unloading Loading Unloading Reloading elastic strain Strain
  21. 21. Ductility - EL% & AR% <ul><li>El ongation </li></ul><ul><li>A rea R eduction </li></ul>L o A o L f A f
  22. 22. Ductile Vs Brittle Materials <ul><li>Only Ductile materials will exhibit necking . </li></ul><ul><li>Ductile if EL%>8% (approximately) </li></ul><ul><li>Brittle if EL% < 5% (approximately) </li></ul>Engineering Stress Engineering Strain
  23. 23. Toughness & Resilience <ul><li>Toughness: A measure of the ability of a material to absorb energy without fracture. (J/m 3 or N.mm/mm 3 = MPa) </li></ul><ul><li>Resilience: A measure of the ability of a material to absorb energy without plastic or permanent deformation. </li></ul><ul><li> (J/m 3 or N.mm/mm 3 = MPa) </li></ul><ul><li>Note: Both are determined as </li></ul><ul><li>energy/unit volume </li></ul>
  24. 24. Toughness, U t Engineering Strain, e =  L/Lo) Engineering Stress, S=P/Ao S u S y
  25. 25. Resilience, U r Engineering Strain, e =  L/Lo) Engineering Stress, S=P/Ao S u S y E e y
  26. 26. Typical Mechanical Properties Metals in annealed (soft) condition

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