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Strain Hardening, Ductile/Brittle Fractures

Strain Hardening, Ductile/Brittle Fractures. UAA School of Engineering CE 334 - Properties of Materials Lecture # 6. Strain History. First Cycle: A structural element is loaded beyond the elastic range and experiences permanent set (  1 ) .

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Strain Hardening, Ductile/Brittle Fractures

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  1. Strain Hardening, Ductile/Brittle Fractures UAA School of Engineering CE 334 - Properties of Materials Lecture # 6

  2. Strain History • First Cycle:A structural element is loaded beyond the elastic range and experiences permanent set (1). • Second Cycle:The structural element is loaded to fracture. Experienced strain=- 1<  • Strain History:The final sketch shows the true “strain history” of the element. • How does pre-loading affect the results obtained from the second loading? 0 1 1 1 0 0 0

  3. What is Strain Hardening? How to select the unloading points in Lab2? • Strain history in plastic range:The history of previous loading and unloadingbeyond the yield stress. • Apparently lose ductility. Hardening due to strain • Distinguish with“Hardness”:Hardness isa measure of a material’s resistance to scratching or indentation.

  4. More Strain Hardening Mechanical Hysteresis:is a loading and unloading process beyond elastic range Energy dissipation:A loss of energy from the heat produced by internal friction as strain energy is dissipated during unloading.

  5. Effects of Strain Hardening • Loss of Ductility. • Decrease in Modulus of Toughness. • Apparent increase in Yield Strength. • Ultimate Tensile Strength is unaffected. • Modulus of Elasticity is unaffected. • Hardness increase ? ?

  6. Strain Hardening in Metal Processing • Hot-Working: milling, rolling: to its final shape • Cold-Working:A process of strain hardening at room temperature to deform the material beyond the elastic rangeto obtain a desired property. • Examples of cold-working:rolling, drawing, extruding, cutting, pulling, indenting…

  7. Purpose of Cold-Working • To make its final shape • To alter its structure and properties: Increase yield strength Decrease ductility

  8. Fracture • BrittleFracture • DuctileFracture

  9. Parameters Affecting Fracture • Load Rate • Nature of Loading • Triaxiality • Cyclic • Material • Temperature • Corrosion • Fabrication Cracks • Design Features • Notches • Holes • Fillets • Uneven surface Roughness

  10. Fracture Mechanics • A specialization within both Structural and • Mechanical Engineering. • The study of how structures fracture. • Difficult in mechanics and mathematics.

  11. Characteristics of Brittle Fracture in Tension • Underuniaxialtension loading, fracture occurs at90 degreeswith the axis of loading. • There is no plastic deformation (i.e. there is no necking). • The failure plane has a granular appearance.

  12. Mechanics of Brittle Material Fracture in Tension

  13. Mechanics of Brittle Material Fracture in Tension • Thetensilecomponent of stress “pulls” the crystal apart:  = [] • Shear strengthof the material isrelativelyhigher.  < [] • Fracture surfaceis orthogonal to the direction of maximum principle tensile stress.

  14. What isBrittle Failure ?

  15. Ductile Fracture

  16. Characteristics of Ductile Fracture • Necking in round specimens: • Asneckingoccurs, atri-axialstate of stress develops in the region of necking. This is mostpopular inroundspecimens. • Failure: Failure begins when micro-cracking causing a fibrous surface to develop. This is followed by a rapid fracture orientedat 45owith the axis of loading.

  17. Mechanics of Ductile Material Fracture in Tension

  18. Mechanics of Ductile Material Fracture in Tension • The SHEARcomponent of stress “shears” the crystal apart:  = [] < []Ok • Shearstrength of the material isrelatively lower. • Fracture surface is45o tothe direction ofmaximum principle tensile stress.

  19. What isDuctile Fracture ?

  20. Behavior Under Seismic Excitation (Inelastic Response) F Ground Disp. Time d Loading d dG F

  21. Behavior Under Seismic Excitation (Inelastic Response) F Ground Disp. Time d Unloading d Deformation Reversal dG F

  22. Behavior Under Seismic Excitation (Inelastic Response) F Ground Disp. Time d Reloading d dG F

  23. Definition of Ductility,m Stress or Force or Moment Strain or Displacement or Rotation du dy Hysteresis Curve

  24. Definition of Energy Dissipation,Q Stress or Force or Moment Area = Q = Energy Dissipated Units = Force x Displacement Strain or Displacement or Rotation

  25. Basic Earthquake Engineering Performance Objective (Theoretical) An adequate design is accomplished when a structure is dimensioned and detailed in such a way that the local ductility demands (energy dissipation demands) are smaller than their corresponding capacities.

  26. Bibliography • Durrant, Olani and Holiday, Brent, An Introduction to the Properties of Materials, Brigham Young University, 1980. • Shackelford, James F., Introduction to Material Science for Engineers, Macmillan Publishing Co., New York, 1985. The End! • Lab this week is the strain hardening lab.... Read it in advance. • Remember that the 1st lab write up is due at the start of the lab class.

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