Machine design course

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Machine design course

Transcript of Machine design course

  • 1. Mechanical Properties of Ductile Metallic Materials Lecture 1 Engineering 473 Machine Design
  • 2. Mechanical Properties (Static Strength Monotonic Elongation) y e u F ut S Ft S yt S et S 0.002 Stress P o = l 0 P/A P l l i = 0 0 l
  • 3. Mechanical Properties (Static Strength Nomenclature) y e u F ut S Ft S yt S et S 0.002 Stress = 0 P/A l l i = 0 0 l Subscripts y 0.2% offset yield u ultimate e elastic F fracture t tension c compression Syt & Sut are generally given in handbooks.
  • 4. Mechanical Properties (True Stress & True Strain) Logarithmic Strain l d d l l = i = = = P i A l o d ln o l l i l l True Stress u F F u True Stress Logarithmic Strain
  • 5. Mechanical Properties (Example Data) True Stress-Logarithmic Strain Curves for Several Metallic Materials H. Schwartzbart, W.F. Brown, Jr., Notch-Bar Tensile Properties of Various Materials and their Relation to the Unnotch Flow Curve and Notch Sharpness, Trans. ASM, 46, 998, 1954.
  • 6. Mechanical Properties (High Strain Rates) Stress-Strain Curves for Mild Steel at Room Temperatures at Various Rates of Strain Manjoine, M.J., Influence of Rate of Strain and Temperature on Yield Stresses of Mild Steel, Journal of Applied Mechanics, 11(A):211-218, December 1944.
  • 7. Mechanical Properties (High Strain Rates & High Temperatures) Experimental Data for 6061-T6 Aluminum Hoge, K.G., Influence of Strain Rate on Mechanical Properties of 6061-T6 Aluminum under Uniaxial and Biaxial States of Stress, Experimental Mechanics, 6:204-211, April 1966.
  • 8. Mechanical Properties (Monotonic Compression) y e u 0.002 Stress P l l i = 0 o = l 0 P/A ec S yc S uc S P 0 l
  • 9. Mechanical Properties (Work Hardening or Cold Working) Syt Syt
  • 10. Mechanical Properties (Reverse Loading) Monotonic Compression Curve Bauschingers Effect Yield stress in compression may decrease after an initial load application past the tension yield point. This phenomena is an important topic in plasticity theory.
  • 11. Mechanical Properties (Stress Controlled Cyclic Loading) Materials can demonstrate three characteristics: 1) cyclic hardening, 2) cyclic softening, and 3) cyclic strain accumulation (ratcheting). Skrzypek, J.J., Plasticity and Creep: Theory, Examples, and Problems, CRC Press, 1993, 130.
  • 12. Mechanical Properties (Strain Controlled Cyclic Loading) Materials can demonstrate two characteristics: 1) cyclic hardening and 2) cyclic softening. Skrzypek, J.J., Plasticity and Creep: Theory, Examples, and Problems, CRC Press, 1993, 130.
  • 13. Mechanical Properties (Creep) , T time Failure strain Primary Creep Secondary Creep Tertiary Creep Typical curves obtained from constant stress/temperature tests. Creep is most pronounced at high temperatures. It may also occur at room temperatures when the stress level is close to the yield strength.
  • 14. Summary The strength of ductile metallic materials is dependent on several parameters. 1. Load Direction (Tensile or Compressive) 2. Strain Rate (Slow or Fast) 3. Temperature (Hot or Cold) 4. Load History (Monotonic or Cyclic) 5. Fabrication Process (Next Class) Metals are complex materials when used throughout their total response envelope. Fortunately their elastic properties are most commonly used.
  • 15. Assignment Read pages 25-34 in Mott.
  • 16. Influence of Fabrication Processes on the Strength of Metals Lecture 2 Engineering 473 Machine Design
  • 17. Things that Affect Metal Strength The strength of ductile metallic materials is dependent on several parameters. 1. Load Direction (Tensile or Compressive) 2. Strain Rate (Slow or Fast) 3. Temperature (Hot or Cold) 4. Load History (Monotonic or Cyclic) 5. Fabrication Process
  • 18. Common Fabrication Processes Casting Sand Casting Investment Casting Shell Molding Powder-Metallurgy Hot-working Hot rolling Extrusion Forging Cold-working Heading Roll threading Spinning Stamping Heat Treatment Annealing Quenching Tempering Case Hardening
  • 19. Hot Working Hot working of metals is done for two reasons 1. Plastically mold the metal into the desired shape 2. Improve the properties of the metal as compared to the as-cast condition
  • 20. Microstructure Changes due to Hot Rolling The granular structure of the material is changed during hot rolling. Large coarse grain structure Smaller grains Allen, Fig. 16-14
  • 21. Hot Working Temperatures Material Temperature Range (oF) Aluminum Aluminum Alloys Beryllium Brass Cooper High Speed Steels Inconel Magnesium Alloys Monel Nickel Refractory Metals & Alloys Steel: Carbon Low Alloy Stainless Titanium Zinc Alloys 650-900 750-900 700-1300 1200-1475 1200-1650 1900-2200 1850-2350 400-750 1850-2150 1600-2300 1800-3000 1900-2400 1800-2300 1900-2200 1400-1800 425-550 Allen, Table 16-1
  • 22. Example of Microstructure Changes Low carbon cast steel (A)As cast (dendritic structure) (B)After hot rolling (reduced grain size) (C)After temper rolling (elongated grains) Directional Properties Allen, Fig, 16-18.
  • 23. Beneficial Effects of Hot Rolling Typical defects in cast metals which are minimized in hot worked metals 1. Large grain size (due to slow cooling) 2. Porosity (voids due to shrinkage) 3. Blow holes (due to gas evolution during solidification) 4. Segregation (due to limited solubility in the solid state) 5. Dirt and slag inclusions 6. Poor surface condition (due to oxides and scale) The strength of hot rolled metals is higher than cast metals. Allen, pg 508.
  • 24. Forging A hot working process Metal flows under high compressive stresses May be used with or without die cavity to obtain a specific shape A blacksmith uses a hammer and an anvil to forge metallic parts.
  • 25. Forged Workpiece The curvature on the sides of a forged product is due to friction between the ram and the workpiece. Allen, Fig. 16-19
  • 26. Directional Nature of Forged Material Properties Flow lines in upset forging of 1.5 dia. AISI 1045 Allen, Fig. 16-23 s