html5-img
1 / 26

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 ) .

amanda-cherry
Télécharger la présentation

Strain Hardening, Ductile/Brittle Fractures

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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.

More Related